* [PATCH 01/22] docs: add skeleton of XFS Data Structures and Algorithms book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
@ 2018-10-04 4:18 ` Darrick J. Wong
2018-10-04 4:18 ` [PATCH 03/22] docs: add XFS self-describing metadata integrity doc to DS&A book Darrick J. Wong
` (20 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:18 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Start adding the main TOC of the XFS data structures and algorithms
book. We'll add the individual sections in later patches.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
Documentation/conf.py | 2
.../filesystems/xfs-data-structures/about.rst | 123 ++++++++++++++++++++
.../filesystems/xfs-data-structures/auxiliary.rst | 4 +
.../filesystems/xfs-data-structures/dynamic.rst | 4 +
.../filesystems/xfs-data-structures/globals.rst | 4 +
.../filesystems/xfs-data-structures/index.rst | 15 ++
.../filesystems/xfs-data-structures/overview.rst | 44 +++++++
Documentation/index.rst | 1
8 files changed, 197 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/about.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/auxiliary.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/dynamic.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/globals.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/index.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/overview.rst
diff --git a/Documentation/conf.py b/Documentation/conf.py
index add6788bbb8c..fbf8f5dce7d9 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -383,6 +383,8 @@ latex_documents = [
'The kernel development community', 'manual'),
('admin-guide/xfs', 'xfs-admin-guide.tex',
'XFS Administration Guide', 'XFS Community', 'manual'),
+ ('filesystems/xfs-data-structures/index', 'xfs-data-structures.tex',
+ 'XFS Data Structures and Algorithms', 'XFS Community', 'manual'),
('filesystems/index', 'filesystems.tex', 'Linux Filesystems API',
'The kernel development community', 'manual'),
('filesystems/ext4/index', 'ext4.tex', 'ext4 Filesystem',
diff --git a/Documentation/filesystems/xfs-data-structures/about.rst b/Documentation/filesystems/xfs-data-structures/about.rst
new file mode 100644
index 000000000000..7df40b637e2e
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/about.rst
@@ -0,0 +1,123 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+About this Book
+===============
+
+XFS is a high performance filesystem which was designed to maximize
+parallel throughput and to scale up to extremely large 64-bit storage
+systems. Originally developed by SGI in October 1993 for IRIX, XFS can
+handle large files, large filesystems, many inodes, large directories,
+large file attributes, and large allocations. Filesystems are optimized
+for parallel access by splitting the storage device into semi-autonomous
+allocation groups. XFS employs branching trees (B+ trees) to facilitate
+fast searches of large lists; it also uses delayed extent-based
+allocation to improve data contiguity and IO performance.
+
+This document describes the on-disk layout of an XFS filesystem and how
+to use the debugging tools ``xfs_db`` and ``xfs_logprint`` to inspect
+the metadata structures. It also describes how on-disk metadata relates
+to the higher level design goals.
+
+This book’s source code is available in the Linux kernel git tree.
+Feedback should be sent to the XFS mailing list, currently at:
+``linux-xfs@vger.kernel.org``.
+
+ **Note**
+
+ All fields in XFS metadata structures are in big-endian byte order
+ except for log items which are formatted in host order.
+
+Copyright
+---------
+© Copyright 2006 Silicon Graphics Inc. All rights reserved. Permission is
+granted to copy, distribute, and/or modify this document under the terms of the
+Creative Commons Attribution-Share Alike, Version 3.0 or any later version
+published by the Creative Commons Corp. A copy of the license is available at
+http://creativecommons.org/licenses/by-sa/3.0/us/ .
+
+Change Log
+----------
+
+.. list-table::
+ :widths: 8 12 14 46
+ :header-rows: 1
+
+ * - Version
+ - Date
+ - Author
+ - Description
+
+ * - 0.1
+ - 2006
+ - Silicon Graphics, Inc.
+ - Initial Release
+
+ * - 1.0
+ - Fri Jul 03 2009
+ - Ryan Lerch
+ - Publican Conversion
+
+ * - 1.1
+ - March 2010
+ - Eric Sandeen
+ - Community Release
+
+ * - 1.99
+ - February 2014
+ - Dave Chinner
+ - AsciiDoc Conversion
+
+ * - 3.0
+ - October 2015
+ - Darrick J. Wong
+ - Miscellaneous fixes.
+ Add missing field definitions.
+ Add some missing xfs_db examples.
+ Add an overview of XFS.
+ Document the journal format.
+ Document the realtime device.
+
+ * - 3.1
+ - October 2015
+ - Darrick J. Wong
+ - Add v5 fields.
+ Discuss metadata integrity.
+ Document the free inode B+tree.
+ Create an index of magic numbers.
+ Document sparse inodes.
+
+ * - 3.14
+ - January 2016
+ - Darrick J. Wong
+ - Document disk format change testing.
+
+ * - 3.141
+ - June 2016
+ - Darrick J. Wong
+ - Document the reverse-mapping btree.
+ Move the b+tree info to a separate chapter.
+ Discuss overlapping interval b+trees.
+ Discuss new log items for atomic updates.
+ Document the reference-count btree.
+ Discuss block sharing, reflink, & deduplication.
+
+ * - 3.1415
+ - July 2016
+ - Darrick J. Wong
+ - Document the real-time reverse-mapping btree.
+
+ * - 3.14159
+ - June 2017
+ - Darrick J. Wong
+ - Add the metadump file format.
+
+ * - 3.141592
+ - May 2018
+ - Darrick J. Wong
+ - Incorporate Dave Chinner's log design document.
+ Incorporate Dave Chinner's self-describing metadata design document.
+
+ * - 4.20
+ - September 2018
+ - Darrick J. Wong
+ - Convert to RestructuredText and move to the kernel source tree.
diff --git a/Documentation/filesystems/xfs-data-structures/auxiliary.rst b/Documentation/filesystems/xfs-data-structures/auxiliary.rst
new file mode 100644
index 000000000000..d2fd2f88ad0e
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/auxiliary.rst
@@ -0,0 +1,4 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Auxiliary Data Structures
+=========================
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
new file mode 100644
index 000000000000..895c94e95889
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -0,0 +1,4 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Dynamic Allocated Structures
+============================
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
new file mode 100644
index 000000000000..3499e0fcd4a8
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -0,0 +1,4 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Global Structures
+=================
diff --git a/Documentation/filesystems/xfs-data-structures/index.rst b/Documentation/filesystems/xfs-data-structures/index.rst
new file mode 100644
index 000000000000..bd164f62c387
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/index.rst
@@ -0,0 +1,15 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+==================================
+XFS Data Structures and Algorithms
+==================================
+
+.. toctree::
+ :maxdepth: 5
+ :numbered:
+
+ about.rst
+ overview.rst
+ globals.rst
+ dynamic.rst
+ auxiliary.rst
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
new file mode 100644
index 000000000000..43b48f30f7e8
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -0,0 +1,44 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+High Level Design
+=================
+
+Overview
+--------
+
+XFS presents to users a standard Unix filesystem interface: a rooted tree of
+directories, files, symbolic links, and devices. All five of those entities
+are represented inside the filesystem by an index node, or "inode";
+each node is uniquely referenced by an inode number. Directories consist of
+(name, inode number) tuples and it is possible for multiple tuples to contain
+the same inode number. Data blocks are associated with files by means of a
+block map in each index node. It is also possible to attach (key, value)
+tuples to any index node; these are known as "extended attributes", which
+extend beyond the standard Unix file attributes.
+
+Internally, XFS filesystems are divided into a number of equally sized chunks
+called Allocation Groups. Each AG can almost be thought of as an individual
+filesystem that maintains its own space usage, index nodes, and other
+secondary metadata. Having multiple AGs allows XFS to handle most operations
+in parallel without degrading performance as the number of concurrent accesses
+increases. Each allocation group uses multiple B+trees to maintain bookkeeping
+records such as the locations of free blocks, the locations of allocated
+inodes, and the locations of free inodes.
+
+Files, symbolic links, and directories can have up to two block maps, or
+"forks", which associate filesystems blocks with a particular file or
+directory. The "attribute fork" tracks blocks used to store and index
+extended attributes, whereas the "data fork" tracks file data blocks,
+symbolic link targets, or directory blocks, depending on the type of the inode
+record. Both forks associate a logical offset with an extent of physical
+blocks, which makes sparse files and directories possible. Directory entries
+and extended attributes are contained inside a second-level data structure
+within the blocks that are mapped by the forks. This structure consists of
+variable-length directory or attribute records and possible a second B+tree to
+index these records.
+
+XFS employs a journalling log in which metadata changes are collected so that
+filesystem operations can be carried out atomically in the case of a crash.
+Furthermore, there is the concept of a real-time device wherein allocations
+are tracked more simply and in larger chunks to reduce jitter in allocation
+latency.
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 5db7e87c7cb1..4136d2274fa6 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -115,6 +115,7 @@ subprojects.
:maxdepth: 2
filesystems/ext4/index
+ filesystems/xfs-data-structures/index
Translations
------------
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 03/22] docs: add XFS self-describing metadata integrity doc to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
2018-10-04 4:18 ` [PATCH 01/22] docs: add skeleton of XFS Data Structures and Algorithms book Darrick J. Wong
@ 2018-10-04 4:18 ` Darrick J. Wong
2018-10-04 4:18 ` [PATCH 04/22] docs: add XFS delayed logging design " Darrick J. Wong
` (19 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:18 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/overview.rst | 2
.../self_describing_metadata.rst | 402 ++++++++++++++++++++
.../filesystems/xfs-self-describing-metadata.txt | 350 -----------------
3 files changed, 404 insertions(+), 350 deletions(-)
create mode 100644 Documentation/filesystems/xfs-data-structures/self_describing_metadata.rst
delete mode 100644 Documentation/filesystems/xfs-self-describing-metadata.txt
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index 43b48f30f7e8..8b3de9abcf39 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -42,3 +42,5 @@ filesystem operations can be carried out atomically in the case of a crash.
Furthermore, there is the concept of a real-time device wherein allocations
are tracked more simply and in larger chunks to reduce jitter in allocation
latency.
+
+.. include:: self_describing_metadata.rst
diff --git a/Documentation/filesystems/xfs-data-structures/self_describing_metadata.rst b/Documentation/filesystems/xfs-data-structures/self_describing_metadata.rst
new file mode 100644
index 000000000000..f9d41c76e1d5
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/self_describing_metadata.rst
@@ -0,0 +1,402 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Metadata Integrity
+------------------
+
+Introduction
+~~~~~~~~~~~~
+
+The largest scalability problem facing XFS is not one of algorithmic
+scalability, but of verification of the filesystem structure. Scalabilty of
+the structures and indexes on disk and the algorithms for iterating them are
+adequate for supporting PB scale filesystems with billions of inodes, however
+it is this very scalability that causes the verification problem.
+
+Almost all metadata on XFS is dynamically allocated. The only fixed location
+metadata is the allocation group headers (SB, AGF, AGFL and AGI), while all
+other metadata structures need to be discovered by walking the filesystem
+structure in different ways. While this is already done by userspace tools for
+validating and repairing the structure, there are limits to what they can
+verify, and this in turn limits the supportable size of an XFS filesystem.
+
+For example, it is entirely possible to manually use xfs\_db and a bit of
+scripting to analyse the structure of a 100TB filesystem when trying to
+determine the root cause of a corruption problem, but it is still mainly a
+manual task of verifying that things like single bit errors or misplaced
+writes weren’t the ultimate cause of a corruption event. It may take a few
+hours to a few days to perform such forensic analysis, so for at this scale
+root cause analysis is entirely possible.
+
+However, if we scale the filesystem up to 1PB, we now have 10x as much
+metadata to analyse and so that analysis blows out towards weeks/months of
+forensic work. Most of the analysis work is slow and tedious, so as the amount
+of analysis goes up, the more likely that the cause will be lost in the noise.
+Hence the primary concern for supporting PB scale filesystems is minimising
+the time and effort required for basic forensic analysis of the filesystem
+structure.
+
+Therefore, the version 5 disk format introduced larger headers for all
+metadata types, which enable the filesystem to check information being read
+from the disk more rigorously. Metadata integrity fields now include:
+
+- **Magic** numbers, to classify all types of metadata. This is unchanged
+ from v4.
+
+- A copy of the filesystem **UUID**, to confirm that a given disk block is
+ connected to the superblock.
+
+- The **owner**, to avoid accessing a piece of metadata which belongs to some
+ other part of the filesystem.
+
+- The filesystem **block number**, to detect misplaced writes.
+
+- The **log serial number** of the last write to this block, to avoid
+ replaying obsolete log entries.
+
+- A CRC32c **checksum** of the entire block, to detect minor corruption.
+
+Metadata integrity coverage has been extended to all metadata blocks in the
+filesystem, with the following notes:
+
+- Inodes can have multiple "owners" in the directory tree; therefore the
+ record contains the inode number instead of an owner or a block number.
+
+- Superblocks have no owners.
+
+- The disk quota file has no owner or block numbers.
+
+- Metadata owned by files list the inode number as the owner.
+
+- Per-AG data and B+tree blocks list the AG number as the owner.
+
+- Per-AG header sectors don’t list owners or block numbers, since they have
+ fixed locations.
+
+- Remote attribute blocks are not logged and therefore the LSN must be -1.
+
+This functionality enables XFS to decide that a block contents are so
+unexpected that it should stop immediately. Unfortunately checksums do not
+allow for automatic correction. Please keep regular backups, as always.
+
+Self Describing Metadata
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+One of the problems with the current metadata format is that apart from the
+magic number in the metadata block, we have no other way of identifying what
+it is supposed to be. We can’t even identify if it is the right place. Put
+simply, you can’t look at a single metadata block in isolation and say "yes,
+it is supposed to be there and the contents are valid".
+
+Hence most of the time spent on forensic analysis is spent doing basic
+verification of metadata values, looking for values that are in range (and
+hence not detected by automated verification checks) but are not correct.
+Finding and understanding how things like cross linked block lists (e.g.
+sibling pointers in a btree end up with loops in them) are the key to
+understanding what went wrong, but it is impossible to tell what order the
+blocks were linked into each other or written to disk after the fact.
+
+Hence we need to record more information into the metadata to allow us to
+quickly determine if the metadata is intact and can be ignored for the purpose
+of analysis. We can’t protect against every possible type of error, but we can
+ensure that common types of errors are easily detectable. Hence the concept of
+self describing metadata.
+
+The first, fundamental requirement of self describing metadata is that the
+metadata object contains some form of unique identifier in a well known
+location. This allows us to identify the expected contents of the block and
+hence parse and verify the metadata object. IF we can’t independently identify
+the type of metadata in the object, then the metadata doesn’t describe itself
+very well at all!
+
+Luckily, almost all XFS metadata has magic numbers embedded already - only the
+AGFL, remote symlinks and remote attribute blocks do not contain identifying
+magic numbers. Hence we can change the on-disk format of all these objects to
+add more identifying information and detect this simply by changing the magic
+numbers in the metadata objects. That is, if it has the current magic number,
+the metadata isn’t self identifying. If it contains a new magic number, it is
+self identifying and we can do much more expansive automated verification of
+the metadata object at runtime, during forensic analysis or repair.
+
+As a primary concern, self describing metadata needs some form of overall
+integrity checking. We cannot trust the metadata if we cannot verify that it
+has not been changed as a result of external influences. Hence we need some
+form of integrity check, and this is done by adding CRC32c validation to the
+metadata block. If we can verify the block contains the metadata it was
+intended to contain, a large amount of the manual verification work can be
+skipped.
+
+CRC32c was selected as metadata cannot be more than 64k in length in XFS and
+hence a 32 bit CRC is more than sufficient to detect multi-bit errors in
+metadata blocks. CRC32c is also now hardware accelerated on common CPUs so it
+is fast. So while CRC32c is not the strongest of possible integrity checks
+that could be used, it is more than sufficient for our needs and has
+relatively little overhead. Adding support for larger integrity fields and/or
+algorithms does really provide any extra value over CRC32c, but it does add a
+lot of complexity and so there is no provision for changing the integrity
+checking mechanism.
+
+Self describing metadata needs to contain enough information so that the
+metadata block can be verified as being in the correct place without needing
+to look at any other metadata. This means it needs to contain location
+information. Just adding a block number to the metadata is not sufficient to
+protect against mis-directed writes - a write might be misdirected to the
+wrong LUN and so be written to the "correct block" of the wrong filesystem.
+Hence location information must contain a filesystem identifier as well as a
+block number.
+
+Another key information point in forensic analysis is knowing who the metadata
+block belongs to. We already know the type, the location, that it is valid
+and/or corrupted, and how long ago that it was last modified. Knowing the
+owner of the block is important as it allows us to find other related metadata
+to determine the scope of the corruption. For example, if we have a extent
+btree object, we don’t know what inode it belongs to and hence have to walk
+the entire filesystem to find the owner of the block. Worse, the corruption
+could mean that no owner can be found (i.e. it’s an orphan block), and so
+without an owner field in the metadata we have no idea of the scope of the
+corruption. If we have an owner field in the metadata object, we can
+immediately do top down validation to determine the scope of the problem.
+
+Different types of metadata have different owner identifiers. For example,
+directory, attribute and extent tree blocks are all owned by an inode, whilst
+freespace btree blocks are owned by an allocation group. Hence the size and
+contents of the owner field are determined by the type of metadata object we
+are looking at. The owner information can also identify misplaced writes (e.g.
+freespace btree block written to the wrong AG).
+
+Self describing metadata also needs to contain some indication of when it was
+written to the filesystem. One of the key information points when doing
+forensic analysis is how recently the block was modified. Correlation of set
+of corrupted metadata blocks based on modification times is important as it
+can indicate whether the corruptions are related, whether there’s been
+multiple corruption events that lead to the eventual failure, and even whether
+there are corruptions present that the run-time verification is not detecting.
+
+For example, we can determine whether a metadata object is supposed to be free
+space or still allocated if it is still referenced by its owner by looking at
+when the free space btree block that contains the block was last written
+compared to when the metadata object itself was last written. If the free
+space block is more recent than the object and the object’s owner, then there
+is a very good chance that the block should have been removed from the owner.
+
+To provide this "written timestamp", each metadata block gets the Log Sequence
+Number (LSN) of the most recent transaction it was modified on written into
+it. This number will always increase over the life of the filesystem, and the
+only thing that resets it is running xfs\_repair on the filesystem. Further,
+by use of the LSN we can tell if the corrupted metadata all belonged to the
+same log checkpoint and hence have some idea of how much modification occurred
+between the first and last instance of corrupt metadata on disk and, further,
+how much modification occurred between the corruption being written and when
+it was detected.
+
+Runtime Validation
+~~~~~~~~~~~~~~~~~~
+
+Validation of self-describing metadata takes place at runtime in two places:
+
+- immediately after a successful read from disk
+
+- immediately prior to write IO submission
+
+The verification is completely stateless - it is done independently of the
+modification process, and seeks only to check that the metadata is what it
+says it is and that the metadata fields are within bounds and internally
+consistent. As such, we cannot catch all types of corruption that can occur
+within a block as there may be certain limitations that operational state
+enforces of the metadata, or there may be corruption of interblock
+relationships (e.g. corrupted sibling pointer lists). Hence we still need
+stateful checking in the main code body, but in general most of the per-field
+validation is handled by the verifiers.
+
+For read verification, the caller needs to specify the expected type of
+metadata that it should see, and the IO completion process verifies that the
+metadata object matches what was expected. If the verification process fails,
+then it marks the object being read as EFSCORRUPTED. The caller needs to catch
+this error (same as for IO errors), and if it needs to take special action due
+to a verification error it can do so by catching the EFSCORRUPTED error value.
+If we need more discrimination of error type at higher levels, we can define
+new error numbers for different errors as necessary.
+
+The first step in read verification is checking the magic number and
+determining whether CRC validating is necessary. If it is, the CRC32c is
+calculated and compared against the value stored in the object itself. Once
+this is validated, further checks are made against the location information,
+followed by extensive object specific metadata validation. If any of these
+checks fail, then the buffer is considered corrupt and the EFSCORRUPTED error
+is set appropriately.
+
+Write verification is the opposite of the read verification - first the object
+is extensively verified and if it is OK we then update the LSN from the last
+modification made to the object, After this, we calculate the CRC and insert
+it into the object. Once this is done the write IO is allowed to continue. If
+any error occurs during this process, the buffer is again marked with a
+EFSCORRUPTED error for the higher layers to catch.
+
+Structures
+~~~~~~~~~~
+
+A typical on-disk structure needs to contain the following information:
+
+.. code:: c
+
+ struct xfs_ondisk_hdr {
+ __be32 magic; /* magic number */
+ __be32 crc; /* CRC, not logged */
+ uuid_t uuid; /* filesystem identifier */
+ __be64 owner; /* parent object */
+ __be64 blkno; /* location on disk */
+ __be64 lsn; /* last modification in log, not logged */
+ };
+
+Depending on the metadata, this information may be part of a header structure
+separate to the metadata contents, or may be distributed through an existing
+structure. The latter occurs with metadata that already contains some of this
+information, such as the superblock and AG headers.
+
+Other metadata may have different formats for the information, but the same
+level of information is generally provided. For example:
+
+- short btree blocks have a 32 bit owner (ag number) and a 32 bit block
+ number for location. The two of these combined provide the same information
+ as @owner and @blkno in eh above structure, but using 8 bytes less space on
+ disk.
+
+- directory/attribute node blocks have a 16 bit magic number, and the header
+ that contains the magic number has other information in it as well. hence
+ the additional metadata headers change the overall format of the metadata.
+
+A typical buffer read verifier is structured as follows:
+
+.. code:: c
+
+ #define XFS_FOO_CRC_OFF offsetof(struct xfs_ondisk_hdr, crc)
+
+ static void
+ xfs_foo_read_verify(
+ struct xfs_buf *bp)
+ {
+ struct xfs_mount *mp = bp->b_target->bt_mount;
+
+ if ((xfs_sb_version_hascrc(&mp->m_sb) &&
+ !xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
+ XFS_FOO_CRC_OFF)) ||
+ !xfs_foo_verify(bp)) {
+ XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
+ xfs_buf_ioerror(bp, EFSCORRUPTED);
+ }
+ }
+
+The code ensures that the CRC is only checked if the filesystem has CRCs
+enabled by checking the superblock of the feature bit, and then if the CRC
+verifies OK (or is not needed) it verifies the actual contents of the block.
+
+The verifier function will take a couple of different forms, depending on
+whether the magic number can be used to determine the format of the block. In
+the case it can’t, the code is structured as follows:
+
+.. code:: c
+
+ static bool
+ xfs_foo_verify(
+ struct xfs_buf *bp)
+ {
+ struct xfs_mount *mp = bp->b_target->bt_mount;
+ struct xfs_ondisk_hdr *hdr = bp->b_addr;
+
+ if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
+ return false;
+
+ if (!xfs_sb_version_hascrc(&mp->m_sb)) {
+ if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
+ return false;
+ if (bp->b_bn != be64_to_cpu(hdr->blkno))
+ return false;
+ if (hdr->owner == 0)
+ return false;
+ }
+
+ /* object specific verification checks here */
+
+ return true;
+ }
+
+If there are different magic numbers for the different formats, the verifier
+will look like:
+
+.. code:: c
+
+ static bool
+ xfs_foo_verify(
+ struct xfs_buf *bp)
+ {
+ struct xfs_mount *mp = bp->b_target->bt_mount;
+ struct xfs_ondisk_hdr *hdr = bp->b_addr;
+
+ if (hdr->magic == cpu_to_be32(XFS_FOO_CRC_MAGIC)) {
+ if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
+ return false;
+ if (bp->b_bn != be64_to_cpu(hdr->blkno))
+ return false;
+ if (hdr->owner == 0)
+ return false;
+ } else if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
+ return false;
+
+ /* object specific verification checks here */
+
+ return true;
+ }
+
+Write verifiers are very similar to the read verifiers, they just do things in
+the opposite order to the read verifiers. A typical write verifier:
+
+.. code:: c
+
+ static void
+ xfs_foo_write_verify(
+ struct xfs_buf *bp)
+ {
+ struct xfs_mount *mp = bp->b_target->bt_mount;
+ struct xfs_buf_log_item *bip = bp->b_fspriv;
+
+ if (!xfs_foo_verify(bp)) {
+ XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
+ xfs_buf_ioerror(bp, EFSCORRUPTED);
+ return;
+ }
+
+ if (!xfs_sb_version_hascrc(&mp->m_sb))
+ return;
+
+
+ if (bip) {
+ struct xfs_ondisk_hdr *hdr = bp->b_addr;
+ hdr->lsn = cpu_to_be64(bip->bli_item.li_lsn);
+ }
+ xfs_update_cksum(bp->b_addr, BBTOB(bp->b_length), XFS_FOO_CRC_OFF);
+ }
+
+This will verify the internal structure of the metadata before we go any
+further, detecting corruptions that have occurred as the metadata has been
+modified in memory. If the metadata verifies OK, and CRCs are enabled, we then
+update the LSN field (when it was last modified) and calculate the CRC on the
+metadata. Once this is done, we can issue the IO.
+
+Inodes and Dquots
+~~~~~~~~~~~~~~~~~
+
+Inodes and dquots are special snowflakes. They have per-object CRC and
+self-identifiers, but they are packed so that there are multiple objects per
+buffer. Hence we do not use per-buffer verifiers to do the work of per-object
+verification and CRC calculations. The per-buffer verifiers simply perform
+basic identification of the buffer - that they contain inodes or dquots, and
+that there are magic numbers in all the expected spots. All further CRC and
+verification checks are done when each inode is read from or written back to
+the buffer.
+
+The structure of the verifiers and the identifiers checks is very similar to
+the buffer code described above. The only difference is where they are called.
+For example, inode read verification is done in xfs\_iread() when the inode is
+first read out of the buffer and the struct xfs\_inode is instantiated. The
+inode is already extensively verified during writeback in xfs\_iflush\_int, so
+the only addition here is to add the LSN and CRC to the inode as it is copied
+back into the buffer.
diff --git a/Documentation/filesystems/xfs-self-describing-metadata.txt b/Documentation/filesystems/xfs-self-describing-metadata.txt
deleted file mode 100644
index 05aa455163e3..000000000000
--- a/Documentation/filesystems/xfs-self-describing-metadata.txt
+++ /dev/null
@@ -1,350 +0,0 @@
-XFS Self Describing Metadata
-----------------------------
-
-Introduction
-------------
-
-The largest scalability problem facing XFS is not one of algorithmic
-scalability, but of verification of the filesystem structure. Scalabilty of the
-structures and indexes on disk and the algorithms for iterating them are
-adequate for supporting PB scale filesystems with billions of inodes, however it
-is this very scalability that causes the verification problem.
-
-Almost all metadata on XFS is dynamically allocated. The only fixed location
-metadata is the allocation group headers (SB, AGF, AGFL and AGI), while all
-other metadata structures need to be discovered by walking the filesystem
-structure in different ways. While this is already done by userspace tools for
-validating and repairing the structure, there are limits to what they can
-verify, and this in turn limits the supportable size of an XFS filesystem.
-
-For example, it is entirely possible to manually use xfs_db and a bit of
-scripting to analyse the structure of a 100TB filesystem when trying to
-determine the root cause of a corruption problem, but it is still mainly a
-manual task of verifying that things like single bit errors or misplaced writes
-weren't the ultimate cause of a corruption event. It may take a few hours to a
-few days to perform such forensic analysis, so for at this scale root cause
-analysis is entirely possible.
-
-However, if we scale the filesystem up to 1PB, we now have 10x as much metadata
-to analyse and so that analysis blows out towards weeks/months of forensic work.
-Most of the analysis work is slow and tedious, so as the amount of analysis goes
-up, the more likely that the cause will be lost in the noise. Hence the primary
-concern for supporting PB scale filesystems is minimising the time and effort
-required for basic forensic analysis of the filesystem structure.
-
-
-Self Describing Metadata
-------------------------
-
-One of the problems with the current metadata format is that apart from the
-magic number in the metadata block, we have no other way of identifying what it
-is supposed to be. We can't even identify if it is the right place. Put simply,
-you can't look at a single metadata block in isolation and say "yes, it is
-supposed to be there and the contents are valid".
-
-Hence most of the time spent on forensic analysis is spent doing basic
-verification of metadata values, looking for values that are in range (and hence
-not detected by automated verification checks) but are not correct. Finding and
-understanding how things like cross linked block lists (e.g. sibling
-pointers in a btree end up with loops in them) are the key to understanding what
-went wrong, but it is impossible to tell what order the blocks were linked into
-each other or written to disk after the fact.
-
-Hence we need to record more information into the metadata to allow us to
-quickly determine if the metadata is intact and can be ignored for the purpose
-of analysis. We can't protect against every possible type of error, but we can
-ensure that common types of errors are easily detectable. Hence the concept of
-self describing metadata.
-
-The first, fundamental requirement of self describing metadata is that the
-metadata object contains some form of unique identifier in a well known
-location. This allows us to identify the expected contents of the block and
-hence parse and verify the metadata object. IF we can't independently identify
-the type of metadata in the object, then the metadata doesn't describe itself
-very well at all!
-
-Luckily, almost all XFS metadata has magic numbers embedded already - only the
-AGFL, remote symlinks and remote attribute blocks do not contain identifying
-magic numbers. Hence we can change the on-disk format of all these objects to
-add more identifying information and detect this simply by changing the magic
-numbers in the metadata objects. That is, if it has the current magic number,
-the metadata isn't self identifying. If it contains a new magic number, it is
-self identifying and we can do much more expansive automated verification of the
-metadata object at runtime, during forensic analysis or repair.
-
-As a primary concern, self describing metadata needs some form of overall
-integrity checking. We cannot trust the metadata if we cannot verify that it has
-not been changed as a result of external influences. Hence we need some form of
-integrity check, and this is done by adding CRC32c validation to the metadata
-block. If we can verify the block contains the metadata it was intended to
-contain, a large amount of the manual verification work can be skipped.
-
-CRC32c was selected as metadata cannot be more than 64k in length in XFS and
-hence a 32 bit CRC is more than sufficient to detect multi-bit errors in
-metadata blocks. CRC32c is also now hardware accelerated on common CPUs so it is
-fast. So while CRC32c is not the strongest of possible integrity checks that
-could be used, it is more than sufficient for our needs and has relatively
-little overhead. Adding support for larger integrity fields and/or algorithms
-does really provide any extra value over CRC32c, but it does add a lot of
-complexity and so there is no provision for changing the integrity checking
-mechanism.
-
-Self describing metadata needs to contain enough information so that the
-metadata block can be verified as being in the correct place without needing to
-look at any other metadata. This means it needs to contain location information.
-Just adding a block number to the metadata is not sufficient to protect against
-mis-directed writes - a write might be misdirected to the wrong LUN and so be
-written to the "correct block" of the wrong filesystem. Hence location
-information must contain a filesystem identifier as well as a block number.
-
-Another key information point in forensic analysis is knowing who the metadata
-block belongs to. We already know the type, the location, that it is valid
-and/or corrupted, and how long ago that it was last modified. Knowing the owner
-of the block is important as it allows us to find other related metadata to
-determine the scope of the corruption. For example, if we have a extent btree
-object, we don't know what inode it belongs to and hence have to walk the entire
-filesystem to find the owner of the block. Worse, the corruption could mean that
-no owner can be found (i.e. it's an orphan block), and so without an owner field
-in the metadata we have no idea of the scope of the corruption. If we have an
-owner field in the metadata object, we can immediately do top down validation to
-determine the scope of the problem.
-
-Different types of metadata have different owner identifiers. For example,
-directory, attribute and extent tree blocks are all owned by an inode, whilst
-freespace btree blocks are owned by an allocation group. Hence the size and
-contents of the owner field are determined by the type of metadata object we are
-looking at. The owner information can also identify misplaced writes (e.g.
-freespace btree block written to the wrong AG).
-
-Self describing metadata also needs to contain some indication of when it was
-written to the filesystem. One of the key information points when doing forensic
-analysis is how recently the block was modified. Correlation of set of corrupted
-metadata blocks based on modification times is important as it can indicate
-whether the corruptions are related, whether there's been multiple corruption
-events that lead to the eventual failure, and even whether there are corruptions
-present that the run-time verification is not detecting.
-
-For example, we can determine whether a metadata object is supposed to be free
-space or still allocated if it is still referenced by its owner by looking at
-when the free space btree block that contains the block was last written
-compared to when the metadata object itself was last written. If the free space
-block is more recent than the object and the object's owner, then there is a
-very good chance that the block should have been removed from the owner.
-
-To provide this "written timestamp", each metadata block gets the Log Sequence
-Number (LSN) of the most recent transaction it was modified on written into it.
-This number will always increase over the life of the filesystem, and the only
-thing that resets it is running xfs_repair on the filesystem. Further, by use of
-the LSN we can tell if the corrupted metadata all belonged to the same log
-checkpoint and hence have some idea of how much modification occurred between
-the first and last instance of corrupt metadata on disk and, further, how much
-modification occurred between the corruption being written and when it was
-detected.
-
-Runtime Validation
-------------------
-
-Validation of self-describing metadata takes place at runtime in two places:
-
- - immediately after a successful read from disk
- - immediately prior to write IO submission
-
-The verification is completely stateless - it is done independently of the
-modification process, and seeks only to check that the metadata is what it says
-it is and that the metadata fields are within bounds and internally consistent.
-As such, we cannot catch all types of corruption that can occur within a block
-as there may be certain limitations that operational state enforces of the
-metadata, or there may be corruption of interblock relationships (e.g. corrupted
-sibling pointer lists). Hence we still need stateful checking in the main code
-body, but in general most of the per-field validation is handled by the
-verifiers.
-
-For read verification, the caller needs to specify the expected type of metadata
-that it should see, and the IO completion process verifies that the metadata
-object matches what was expected. If the verification process fails, then it
-marks the object being read as EFSCORRUPTED. The caller needs to catch this
-error (same as for IO errors), and if it needs to take special action due to a
-verification error it can do so by catching the EFSCORRUPTED error value. If we
-need more discrimination of error type at higher levels, we can define new
-error numbers for different errors as necessary.
-
-The first step in read verification is checking the magic number and determining
-whether CRC validating is necessary. If it is, the CRC32c is calculated and
-compared against the value stored in the object itself. Once this is validated,
-further checks are made against the location information, followed by extensive
-object specific metadata validation. If any of these checks fail, then the
-buffer is considered corrupt and the EFSCORRUPTED error is set appropriately.
-
-Write verification is the opposite of the read verification - first the object
-is extensively verified and if it is OK we then update the LSN from the last
-modification made to the object, After this, we calculate the CRC and insert it
-into the object. Once this is done the write IO is allowed to continue. If any
-error occurs during this process, the buffer is again marked with a EFSCORRUPTED
-error for the higher layers to catch.
-
-Structures
-----------
-
-A typical on-disk structure needs to contain the following information:
-
-struct xfs_ondisk_hdr {
- __be32 magic; /* magic number */
- __be32 crc; /* CRC, not logged */
- uuid_t uuid; /* filesystem identifier */
- __be64 owner; /* parent object */
- __be64 blkno; /* location on disk */
- __be64 lsn; /* last modification in log, not logged */
-};
-
-Depending on the metadata, this information may be part of a header structure
-separate to the metadata contents, or may be distributed through an existing
-structure. The latter occurs with metadata that already contains some of this
-information, such as the superblock and AG headers.
-
-Other metadata may have different formats for the information, but the same
-level of information is generally provided. For example:
-
- - short btree blocks have a 32 bit owner (ag number) and a 32 bit block
- number for location. The two of these combined provide the same
- information as @owner and @blkno in eh above structure, but using 8
- bytes less space on disk.
-
- - directory/attribute node blocks have a 16 bit magic number, and the
- header that contains the magic number has other information in it as
- well. hence the additional metadata headers change the overall format
- of the metadata.
-
-A typical buffer read verifier is structured as follows:
-
-#define XFS_FOO_CRC_OFF offsetof(struct xfs_ondisk_hdr, crc)
-
-static void
-xfs_foo_read_verify(
- struct xfs_buf *bp)
-{
- struct xfs_mount *mp = bp->b_target->bt_mount;
-
- if ((xfs_sb_version_hascrc(&mp->m_sb) &&
- !xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
- XFS_FOO_CRC_OFF)) ||
- !xfs_foo_verify(bp)) {
- XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
- xfs_buf_ioerror(bp, EFSCORRUPTED);
- }
-}
-
-The code ensures that the CRC is only checked if the filesystem has CRCs enabled
-by checking the superblock of the feature bit, and then if the CRC verifies OK
-(or is not needed) it verifies the actual contents of the block.
-
-The verifier function will take a couple of different forms, depending on
-whether the magic number can be used to determine the format of the block. In
-the case it can't, the code is structured as follows:
-
-static bool
-xfs_foo_verify(
- struct xfs_buf *bp)
-{
- struct xfs_mount *mp = bp->b_target->bt_mount;
- struct xfs_ondisk_hdr *hdr = bp->b_addr;
-
- if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
- return false;
-
- if (!xfs_sb_version_hascrc(&mp->m_sb)) {
- if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
- return false;
- if (bp->b_bn != be64_to_cpu(hdr->blkno))
- return false;
- if (hdr->owner == 0)
- return false;
- }
-
- /* object specific verification checks here */
-
- return true;
-}
-
-If there are different magic numbers for the different formats, the verifier
-will look like:
-
-static bool
-xfs_foo_verify(
- struct xfs_buf *bp)
-{
- struct xfs_mount *mp = bp->b_target->bt_mount;
- struct xfs_ondisk_hdr *hdr = bp->b_addr;
-
- if (hdr->magic == cpu_to_be32(XFS_FOO_CRC_MAGIC)) {
- if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
- return false;
- if (bp->b_bn != be64_to_cpu(hdr->blkno))
- return false;
- if (hdr->owner == 0)
- return false;
- } else if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
- return false;
-
- /* object specific verification checks here */
-
- return true;
-}
-
-Write verifiers are very similar to the read verifiers, they just do things in
-the opposite order to the read verifiers. A typical write verifier:
-
-static void
-xfs_foo_write_verify(
- struct xfs_buf *bp)
-{
- struct xfs_mount *mp = bp->b_target->bt_mount;
- struct xfs_buf_log_item *bip = bp->b_fspriv;
-
- if (!xfs_foo_verify(bp)) {
- XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
- xfs_buf_ioerror(bp, EFSCORRUPTED);
- return;
- }
-
- if (!xfs_sb_version_hascrc(&mp->m_sb))
- return;
-
-
- if (bip) {
- struct xfs_ondisk_hdr *hdr = bp->b_addr;
- hdr->lsn = cpu_to_be64(bip->bli_item.li_lsn);
- }
- xfs_update_cksum(bp->b_addr, BBTOB(bp->b_length), XFS_FOO_CRC_OFF);
-}
-
-This will verify the internal structure of the metadata before we go any
-further, detecting corruptions that have occurred as the metadata has been
-modified in memory. If the metadata verifies OK, and CRCs are enabled, we then
-update the LSN field (when it was last modified) and calculate the CRC on the
-metadata. Once this is done, we can issue the IO.
-
-Inodes and Dquots
------------------
-
-Inodes and dquots are special snowflakes. They have per-object CRC and
-self-identifiers, but they are packed so that there are multiple objects per
-buffer. Hence we do not use per-buffer verifiers to do the work of per-object
-verification and CRC calculations. The per-buffer verifiers simply perform basic
-identification of the buffer - that they contain inodes or dquots, and that
-there are magic numbers in all the expected spots. All further CRC and
-verification checks are done when each inode is read from or written back to the
-buffer.
-
-The structure of the verifiers and the identifiers checks is very similar to the
-buffer code described above. The only difference is where they are called. For
-example, inode read verification is done in xfs_iread() when the inode is first
-read out of the buffer and the struct xfs_inode is instantiated. The inode is
-already extensively verified during writeback in xfs_iflush_int, so the only
-addition here is to add the LSN and CRC to the inode as it is copied back into
-the buffer.
-
-XXX: inode unlinked list modification doesn't recalculate the inode CRC! None of
-the unlinked list modifications check or update CRCs, neither during unlink nor
-log recovery. So, it's gone unnoticed until now. This won't matter immediately -
-repair will probably complain about it - but it needs to be fixed.
-
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 04/22] docs: add XFS delayed logging design doc to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
2018-10-04 4:18 ` [PATCH 01/22] docs: add skeleton of XFS Data Structures and Algorithms book Darrick J. Wong
2018-10-04 4:18 ` [PATCH 03/22] docs: add XFS self-describing metadata integrity doc to DS&A book Darrick J. Wong
@ 2018-10-04 4:18 ` Darrick J. Wong
2018-10-04 4:18 ` [PATCH 05/22] docs: add XFS shared data block chapter " Darrick J. Wong
` (18 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:18 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/delayed_logging.rst | 828 ++++++++++++++++++++
.../filesystems/xfs-data-structures/overview.rst | 1
.../filesystems/xfs-delayed-logging-design.txt | 793 -------------------
3 files changed, 829 insertions(+), 793 deletions(-)
create mode 100644 Documentation/filesystems/xfs-data-structures/delayed_logging.rst
delete mode 100644 Documentation/filesystems/xfs-delayed-logging-design.txt
diff --git a/Documentation/filesystems/xfs-data-structures/delayed_logging.rst b/Documentation/filesystems/xfs-data-structures/delayed_logging.rst
new file mode 100644
index 000000000000..a4ae343e7556
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/delayed_logging.rst
@@ -0,0 +1,828 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Delayed Logging
+---------------
+
+Introduction to Re-logging in XFS
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+XFS logging is a combination of logical and physical logging. Some objects,
+such as inodes and dquots, are logged in logical format where the details
+logged are made up of the changes to in-core structures rather than on-disk
+structures. Other objects - typically buffers - have their physical changes
+logged. The reason for these differences is to reduce the amount of log space
+required for objects that are frequently logged. Some parts of inodes are more
+frequently logged than others, and inodes are typically more frequently logged
+than any other object (except maybe the superblock buffer) so keeping the
+amount of metadata logged low is of prime importance.
+
+The reason that this is such a concern is that XFS allows multiple separate
+modifications to a single object to be carried in the log at any given time.
+This allows the log to avoid needing to flush each change to disk before
+recording a new change to the object. XFS does this via a method called
+"re-logging". Conceptually, this is quite simple - all it requires is that any
+new change to the object is recorded with a **new copy** of all the existing
+changes in the new transaction that is written to the log.
+
+That is, if we have a sequence of changes A through to F, and the object was
+written to disk after change D, we would see in the log the following series
+of transactions, their contents and the log sequence number (LSN) of the
+transaction:
+
+::
+
+ Transaction Contents LSN
+ A A X
+ B A+B X+n
+ C A+B+C X+n+m
+ D A+B+C+D X+n+m+o
+ <object written to disk>
+ E E Y (> X+n+m+o)
+ F E+F Y+p
+
+In other words, each time an object is relogged, the new transaction contains
+the aggregation of all the previous changes currently held only in the log.
+
+This relogging technique also allows objects to be moved forward in the log so
+that an object being relogged does not prevent the tail of the log from ever
+moving forward. This can be seen in the table above by the changing
+(increasing) LSN of each subsequent transaction - the LSN is effectively a
+direct encoding of the location in the log of the transaction.
+
+This relogging is also used to implement long-running, multiple-commit
+transactions. These transaction are known as rolling transactions, and require
+a special log reservation known as a permanent transaction reservation. A
+typical example of a rolling transaction is the removal of extents from an
+inode which can only be done at a rate of two extents per transaction because
+of reservation size limitations. Hence a rolling extent removal transaction
+keeps relogging the inode and btree buffers as they get modified in each
+removal operation. This keeps them moving forward in the log as the operation
+progresses, ensuring that current operation never gets blocked by itself if
+the log wraps around.
+
+Hence it can be seen that the relogging operation is fundamental to the
+correct working of the XFS journalling subsystem. From the above description,
+most people should be able to see why the XFS metadata operations writes so
+much to the log - repeated operations to the same objects write the same
+changes to the log over and over again. Worse is the fact that objects tend to
+get dirtier as they get relogged, so each subsequent transaction is writing
+more metadata into the log.
+
+Another feature of the XFS transaction subsystem is that most transactions are
+asynchronous. That is, they don’t commit to disk until either a log buffer is
+filled (a log buffer can hold multiple transactions) or a synchronous
+operation forces the log buffers holding the transactions to disk. This means
+that XFS is doing aggregation of transactions in memory - batching them, if
+you like - to minimise the impact of the log IO on transaction throughput.
+
+The limitation on asynchronous transaction throughput is the number and size
+of log buffers made available by the log manager. By default there are 8 log
+buffers available and the size of each is 32kB - the size can be increased up
+to 256kB by use of a mount option.
+
+Effectively, this gives us the maximum bound of outstanding metadata changes
+that can be made to the filesystem at any point in time - if all the log
+buffers are full and under IO, then no more transactions can be committed
+until the current batch completes. It is now common for a single current CPU
+core to be to able to issue enough transactions to keep the log buffers full
+and under IO permanently. Hence the XFS journalling subsystem can be
+considered to be IO bound.
+
+Delayed Logging Concepts
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The key thing to note about the asynchronous logging combined with the
+relogging technique XFS uses is that we can be relogging changed objects
+multiple times before they are committed to disk in the log buffers. If we
+return to the previous relogging example, it is entirely possible that
+transactions A through D are committed to disk in the same log buffer.
+
+That is, a single log buffer may contain multiple copies of the same object,
+but only one of those copies needs to be there - the last one "D", as it
+contains all the changes from the previous changes. In other words, we have
+one necessary copy in the log buffer, and three stale copies that are simply
+wasting space. When we are doing repeated operations on the same set of
+objects, these "stale objects" can be over 90% of the space used in the log
+buffers. It is clear that reducing the number of stale objects written to the
+log would greatly reduce the amount of metadata we write to the log, and this
+is the fundamental goal of delayed logging.
+
+From a conceptual point of view, XFS is already doing relogging in memory
+(where memory == log buffer), only it is doing it extremely inefficiently. It
+is using logical to physical formatting to do the relogging because there is
+no infrastructure to keep track of logical changes in memory prior to
+physically formatting the changes in a transaction to the log buffer. Hence we
+cannot avoid accumulating stale objects in the log buffers.
+
+Delayed logging is the name we’ve given to keeping and tracking transactional
+changes to objects in memory outside the log buffer infrastructure. Because of
+the relogging concept fundamental to the XFS journalling subsystem, this is
+actually relatively easy to do - all the changes to logged items are already
+tracked in the current infrastructure. The big problem is how to accumulate
+them and get them to the log in a consistent, recoverable manner. Describing
+the problems and how they have been solved is the focus of this document.
+
+One of the key changes that delayed logging makes to the operation of the
+journalling subsystem is that it disassociates the amount of outstanding
+metadata changes from the size and number of log buffers available. In other
+words, instead of there only being a maximum of 2MB of transaction changes not
+written to the log at any point in time, there may be a much greater amount
+being accumulated in memory. Hence the potential for loss of metadata on a
+crash is much greater than for the existing logging mechanism.
+
+It should be noted that this does not change the guarantee that log recovery
+will result in a consistent filesystem. What it does mean is that as far as
+the recovered filesystem is concerned, there may be many thousands of
+transactions that simply did not occur as a result of the crash. This makes it
+even more important that applications that care about their data use fsync()
+where they need to ensure application level data integrity is maintained.
+
+It should be noted that delayed logging is not an innovative new concept that
+warrants rigorous proofs to determine whether it is correct or not. The method
+of accumulating changes in memory for some period before writing them to the
+log is used effectively in many filesystems including ext3 and ext4. Hence no
+time is spent in this document trying to convince the reader that the concept
+is sound. Instead it is simply considered a "solved problem" and as such
+implementing it in XFS is purely an exercise in software engineering.
+
+The fundamental requirements for delayed logging in XFS are simple:
+
+1. Reduce the amount of metadata written to the log by at least an order of
+ magnitude.
+
+2. Supply sufficient statistics to validate Requirement #1.
+
+3. Supply sufficient new tracing infrastructure to be able to debug problems
+ with the new code.
+
+4. No on-disk format change (metadata or log format).
+
+5. Enable and disable with a mount option.
+
+6. No performance regressions for synchronous transaction workloads.
+
+Delayed Logging Design
+~~~~~~~~~~~~~~~~~~~~~~
+
+Storing Changes
+^^^^^^^^^^^^^^^
+
+The problem with accumulating changes at a logical level (i.e. just using the
+existing log item dirty region tracking) is that when it comes to writing the
+changes to the log buffers, we need to ensure that the object we are
+formatting is not changing while we do this. This requires locking the object
+to prevent concurrent modification. Hence flushing the logical changes to the
+log would require us to lock every object, format them, and then unlock them
+again.
+
+This introduces lots of scope for deadlocks with transactions that are already
+running. For example, a transaction has object A locked and modified, but
+needs the delayed logging tracking lock to commit the transaction. However,
+the flushing thread has the delayed logging tracking lock already held, and is
+trying to get the lock on object A to flush it to the log buffer. This appears
+to be an unsolvable deadlock condition, and it was solving this problem that
+was the barrier to implementing delayed logging for so long.
+
+The solution is relatively simple - it just took a long time to recognise it.
+Put simply, the current logging code formats the changes to each item into an
+vector array that points to the changed regions in the item. The log write
+code simply copies the memory these vectors point to into the log buffer
+during transaction commit while the item is locked in the transaction. Instead
+of using the log buffer as the destination of the formatting code, we can use
+an allocated memory buffer big enough to fit the formatted vector.
+
+If we then copy the vector into the memory buffer and rewrite the vector to
+point to the memory buffer rather than the object itself, we now have a copy
+of the changes in a format that is compatible with the log buffer writing
+code. that does not require us to lock the item to access. This formatting and
+rewriting can all be done while the object is locked during transaction
+commit, resulting in a vector that is transactionally consistent and can be
+accessed without needing to lock the owning item.
+
+Hence we avoid the need to lock items when we need to flush outstanding
+asynchronous transactions to the log. The differences between the existing
+formatting method and the delayed logging formatting can be seen in the
+diagram below.
+
+Current format log vector:
+
+::
+
+ Object +---------------------------------------------+
+ Vector 1 +----+
+ Vector 2 +----+
+ Vector 3 +----------+
+
+After formatting:
+
+::
+
+ Log Buffer +-V1-+-V2-+----V3----+
+
+Delayed logging vector:
+
+::
+
+ Object +---------------------------------------------+
+ Vector 1 +----+
+ Vector 2 +----+
+ Vector 3 +----------+
+
+After formatting:
+
+::
+
+ Memory Buffer +-V1-+-V2-+----V3----+
+ Vector 1 +----+
+ Vector 2 +----+
+ Vector 3 +----------+
+
+The memory buffer and associated vector need to be passed as a single object,
+but still need to be associated with the parent object so if the object is
+relogged we can replace the current memory buffer with a new memory buffer
+that contains the latest changes.
+
+The reason for keeping the vector around after we’ve formatted the memory
+buffer is to support splitting vectors across log buffer boundaries correctly.
+If we don’t keep the vector around, we do not know where the region boundaries
+are in the item, so we’d need a new encapsulation method for regions in the
+log buffer writing (i.e. double encapsulation). This would be an on-disk
+format change and as such is not desirable. It also means we’d have to write
+the log region headers in the formatting stage, which is problematic as there
+is per region state that needs to be placed into the headers during the log
+write.
+
+Hence we need to keep the vector, but by attaching the memory buffer to it and
+rewriting the vector addresses to point at the memory buffer we end up with a
+self-describing object that can be passed to the log buffer write code to be
+handled in exactly the same manner as the existing log vectors are handled.
+Hence we avoid needing a new on-disk format to handle items that have been
+relogged in memory.
+
+Tracking Changes
+^^^^^^^^^^^^^^^^
+
+Now that we can record transactional changes in memory in a form that allows
+them to be used without limitations, we need to be able to track and
+accumulate them so that they can be written to the log at some later point in
+time. The log item is the natural place to store this vector and buffer, and
+also makes sense to be the object that is used to track committed objects as
+it will always exist once the object has been included in a transaction.
+
+The log item is already used to track the log items that have been written to
+the log but not yet written to disk. Such log items are considered "active"
+and as such are stored in the Active Item List (AIL) which is a LSN-ordered
+double linked list. Items are inserted into this list during log buffer IO
+completion, after which they are unpinned and can be written to disk. An
+object that is in the AIL can be relogged, which causes the object to be
+pinned again and then moved forward in the AIL when the log buffer IO
+completes for that transaction.
+
+Essentially, this shows that an item that is in the AIL can still be modified
+and relogged, so any tracking must be separate to the AIL infrastructure. As
+such, we cannot reuse the AIL list pointers for tracking committed items, nor
+can we store state in any field that is protected by the AIL lock. Hence the
+committed item tracking needs it’s own locks, lists and state fields in the
+log item.
+
+Similar to the AIL, tracking of committed items is done through a new list
+called the Committed Item List (CIL). The list tracks log items that have been
+committed and have formatted memory buffers attached to them. It tracks
+objects in transaction commit order, so when an object is relogged it is
+removed from it’s place in the list and re-inserted at the tail. This is
+entirely arbitrary and done to make it easy for debugging - the last items in
+the list are the ones that are most recently modified. Ordering of the CIL is
+not necessary for transactional integrity (as discussed in the next section)
+so the ordering is done for convenience/sanity of the developers.
+
+Checkpoints
+^^^^^^^^^^^
+
+When we have a log synchronisation event, commonly known as a "log force", all
+the items in the CIL must be written into the log via the log buffers. We need
+to write these items in the order that they exist in the CIL, and they need to
+be written as an atomic transaction. The need for all the objects to be
+written as an atomic transaction comes from the requirements of relogging and
+log replay - all the changes in all the objects in a given transaction must
+either be completely replayed during log recovery, or not replayed at all. If
+a transaction is not replayed because it is not complete in the log, then no
+later transactions should be replayed, either.
+
+To fulfill this requirement, we need to write the entire CIL in a single log
+transaction. Fortunately, the XFS log code has no fixed limit on the size of a
+transaction, nor does the log replay code. The only fundamental limit is that
+the transaction cannot be larger than just under half the size of the log. The
+reason for this limit is that to find the head and tail of the log, there must
+be at least one complete transaction in the log at any given time. If a
+transaction is larger than half the log, then there is the possibility that a
+crash during the write of a such a transaction could partially overwrite the
+only complete previous transaction in the log. This will result in a recovery
+failure and an inconsistent filesystem and hence we must enforce the maximum
+size of a checkpoint to be slightly less than a half the log.
+
+Apart from this size requirement, a checkpoint transaction looks no different
+to any other transaction - it contains a transaction header, a series of
+formatted log items and a commit record at the tail. From a recovery
+perspective, the checkpoint transaction is also no different - just a lot
+bigger with a lot more items in it. The worst case effect of this is that we
+might need to tune the recovery transaction object hash size.
+
+Because the checkpoint is just another transaction and all the changes to log
+items are stored as log vectors, we can use the existing log buffer writing
+code to write the changes into the log. To do this efficiently, we need to
+minimise the time we hold the CIL locked while writing the checkpoint
+transaction. The current log write code enables us to do this easily with the
+way it separates the writing of the transaction contents (the log vectors)
+from the transaction commit record, but tracking this requires us to have a
+per-checkpoint context that travels through the log write process through to
+checkpoint completion.
+
+Hence a checkpoint has a context that tracks the state of the current
+checkpoint from initiation to checkpoint completion. A new context is
+initiated at the same time a checkpoint transaction is started. That is, when
+we remove all the current items from the CIL during a checkpoint operation, we
+move all those changes into the current checkpoint context. We then initialise
+a new context and attach that to the CIL for aggregation of new transactions.
+
+This allows us to unlock the CIL immediately after transfer of all the
+committed items and effectively allow new transactions to be issued while we
+are formatting the checkpoint into the log. It also allows concurrent
+checkpoints to be written into the log buffers in the case of log force heavy
+workloads, just like the existing transaction commit code does. This, however,
+requires that we strictly order the commit records in the log so that
+checkpoint sequence order is maintained during log replay.
+
+To ensure that we can be writing an item into a checkpoint transaction at the
+same time another transaction modifies the item and inserts the log item into
+the new CIL, then checkpoint transaction commit code cannot use log items to
+store the list of log vectors that need to be written into the transaction.
+Hence log vectors need to be able to be chained together to allow them to be
+detached from the log items. That is, when the CIL is flushed the memory
+buffer and log vector attached to each log item needs to be attached to the
+checkpoint context so that the log item can be released. In diagrammatic form,
+the CIL would look like this before the flush:
+
+::
+
+ CIL Head
+ |
+ V
+ Log Item <-> log vector 1 -> memory buffer
+ | -> vector array
+ V
+ Log Item <-> log vector 2 -> memory buffer
+ | -> vector array
+ V
+ ......
+ |
+ V
+ Log Item <-> log vector N-1 -> memory buffer
+ | -> vector array
+ V
+ Log Item <-> log vector N -> memory buffer
+ -> vector array
+
+And after the flush the CIL head is empty, and the checkpoint context log
+vector list would look like:
+
+::
+
+ Checkpoint Context
+ |
+ V
+ log vector 1 -> memory buffer
+ | -> vector array
+ | -> Log Item
+ V
+ log vector 2 -> memory buffer
+ | -> vector array
+ | -> Log Item
+ V
+ ......
+ |
+ V
+ log vector N-1 -> memory buffer
+ | -> vector array
+ | -> Log Item
+ V
+ log vector N -> memory buffer
+ -> vector array
+ -> Log Item
+
+Once this transfer is done, the CIL can be unlocked and new transactions can
+start, while the checkpoint flush code works over the log vector chain to
+commit the checkpoint.
+
+Once the checkpoint is written into the log buffers, the checkpoint context is
+attached to the log buffer that the commit record was written to along with a
+completion callback. Log IO completion will call that callback, which can then
+run transaction committed processing for the log items (i.e. insert into AIL
+and unpin) in the log vector chain and then free the log vector chain and
+checkpoint context.
+
+Discussion Point: I am uncertain as to whether the log item is the most
+efficient way to track vectors, even though it seems like the natural way to
+do it. The fact that we walk the log items (in the CIL) just to chain the log
+vectors and break the link between the log item and the log vector means that
+we take a cache line hit for the log item list modification, then another for
+the log vector chaining. If we track by the log vectors, then we only need to
+break the link between the log item and the log vector, which means we should
+dirty only the log item cachelines. Normally I wouldn’t be concerned about one
+vs two dirty cachelines except for the fact I’ve seen upwards of 80,000 log
+vectors in one checkpoint transaction. I’d guess this is a "measure and
+compare" situation that can be done after a working and reviewed
+implementation is in the dev tree.
+
+Checkpoint Sequencing
+^^^^^^^^^^^^^^^^^^^^^
+
+One of the key aspects of the XFS transaction subsystem is that it tags
+committed transactions with the log sequence number of the transaction commit.
+This allows transactions to be issued asynchronously even though there may be
+future operations that cannot be completed until that transaction is fully
+committed to the log. In the rare case that a dependent operation occurs (e.g.
+re-using a freed metadata extent for a data extent), a special, optimised log
+force can be issued to force the dependent transaction to disk immediately.
+
+To do this, transactions need to record the LSN of the commit record of the
+transaction. This LSN comes directly from the log buffer the transaction is
+written into. While this works just fine for the existing transaction
+mechanism, it does not work for delayed logging because transactions are not
+written directly into the log buffers. Hence some other method of sequencing
+transactions is required.
+
+As discussed in the checkpoint section, delayed logging uses per-checkpoint
+contexts, and as such it is simple to assign a sequence number to each
+checkpoint. Because the switching of checkpoint contexts must be done
+atomically, it is simple to ensure that each new context has a monotonically
+increasing sequence number assigned to it without the need for an external
+atomic counter - we can just take the current context sequence number and add
+one to it for the new context.
+
+Then, instead of assigning a log buffer LSN to the transaction commit LSN
+during the commit, we can assign the current checkpoint sequence. This allows
+operations that track transactions that have not yet completed know what
+checkpoint sequence needs to be committed before they can continue. As a
+result, the code that forces the log to a specific LSN now needs to ensure
+that the log forces to a specific checkpoint.
+
+To ensure that we can do this, we need to track all the checkpoint contexts
+that are currently committing to the log. When we flush a checkpoint, the
+context gets added to a "committing" list which can be searched. When a
+checkpoint commit completes, it is removed from the committing list. Because
+the checkpoint context records the LSN of the commit record for the
+checkpoint, we can also wait on the log buffer that contains the commit
+record, thereby using the existing log force mechanisms to execute synchronous
+forces.
+
+It should be noted that the synchronous forces may need to be extended with
+mitigation algorithms similar to the current log buffer code to allow
+aggregation of multiple synchronous transactions if there are already
+synchronous transactions being flushed. Investigation of the performance of
+the current design is needed before making any decisions here.
+
+The main concern with log forces is to ensure that all the previous
+checkpoints are also committed to disk before the one we need to wait for.
+Therefore we need to check that all the prior contexts in the committing list
+are also complete before waiting on the one we need to complete. We do this
+synchronisation in the log force code so that we don’t need to wait anywhere
+else for such serialisation - it only matters when we do a log force.
+
+The only remaining complexity is that a log force now also has to handle the
+case where the forcing sequence number is the same as the current context.
+That is, we need to flush the CIL and potentially wait for it to complete.
+This is a simple addition to the existing log forcing code to check the
+sequence numbers and push if required. Indeed, placing the current sequence
+checkpoint flush in the log force code enables the current mechanism for
+issuing synchronous transactions to remain untouched (i.e. commit an
+asynchronous transaction, then force the log at the LSN of that transaction)
+and so the higher level code behaves the same regardless of whether delayed
+logging is being used or not.
+
+Checkpoint Log Space Accounting
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The big issue for a checkpoint transaction is the log space reservation for
+the transaction. We don’t know how big a checkpoint transaction is going to be
+ahead of time, nor how many log buffers it will take to write out, nor the
+number of split log vector regions are going to be used. We can track the
+amount of log space required as we add items to the commit item list, but we
+still need to reserve the space in the log for the checkpoint.
+
+A typical transaction reserves enough space in the log for the worst case
+space usage of the transaction. The reservation accounts for log record
+headers, transaction and region headers, headers for split regions, buffer
+tail padding, etc. as well as the actual space for all the changed metadata in
+the transaction. While some of this is fixed overhead, much of it is dependent
+on the size of the transaction and the number of regions being logged (the
+number of log vectors in the transaction).
+
+An example of the differences would be logging directory changes versus
+logging inode changes. If you modify lots of inode cores (e.g. chmod -R g+w
+\*), then there are lots of transactions that only contain an inode core and
+an inode log format structure. That is, two vectors totaling roughly 150
+bytes. If we modify 10,000 inodes, we have about 1.5MB of metadata to write in
+20,000 vectors. Each vector is 12 bytes, so the total to be logged is
+approximately 1.75MB. In comparison, if we are logging full directory buffers,
+they are typically 4KB each, so we in 1.5MB of directory buffers we’d have
+roughly 400 buffers and a buffer format structure for each buffer - roughly
+800 vectors or 1.51MB total space. From this, it should be obvious that a
+static log space reservation is not particularly flexible and is difficult to
+select the "optimal value" for all workloads.
+
+Further, if we are going to use a static reservation, which bit of the entire
+reservation does it cover? We account for space used by the transaction
+reservation by tracking the space currently used by the object in the CIL and
+then calculating the increase or decrease in space used as the object is
+relogged. This allows for a checkpoint reservation to only have to account for
+log buffer metadata used such as log header records.
+
+However, even using a static reservation for just the log metadata is
+problematic. Typically log record headers use at least 16KB of log space per
+1MB of log space consumed (512 bytes per 32k) and the reservation needs to be
+large enough to handle arbitrary sized checkpoint transactions. This
+reservation needs to be made before the checkpoint is started, and we need to
+be able to reserve the space without sleeping. For a 8MB checkpoint, we need a
+reservation of around 150KB, which is a non-trivial amount of space.
+
+A static reservation needs to manipulate the log grant counters - we can take
+a permanent reservation on the space, but we still need to make sure we
+refresh the write reservation (the actual space available to the transaction)
+after every checkpoint transaction completion. Unfortunately, if this space is
+not available when required, then the regrant code will sleep waiting for it.
+
+The problem with this is that it can lead to deadlocks as we may need to
+commit checkpoints to be able to free up log space (refer back to the
+description of rolling transactions for an example of this). Hence we **must**
+always have space available in the log if we are to use static reservations,
+and that is very difficult and complex to arrange. It is possible to do, but
+there is a simpler way.
+
+The simpler way of doing this is tracking the entire log space used by the
+items in the CIL and using this to dynamically calculate the amount of log
+space required by the log metadata. If this log metadata space changes as a
+result of a transaction commit inserting a new memory buffer into the CIL,
+then the difference in space required is removed from the transaction that
+causes the change. Transactions at this level will **always** have enough
+space available in their reservation for this as they have already reserved
+the maximal amount of log metadata space they require, and such a delta
+reservation will always be less than or equal to the maximal amount in the
+reservation.
+
+Hence we can grow the checkpoint transaction reservation dynamically as items
+are added to the CIL and avoid the need for reserving and regranting log space
+up front. This avoids deadlocks and removes a blocking point from the
+checkpoint flush code.
+
+As mentioned early, transactions can’t grow to more than half the size of the
+log. Hence as part of the reservation growing, we need to also check the size
+of the reservation against the maximum allowed transaction size. If we reach
+the maximum threshold, we need to push the CIL to the log. This is effectively
+a "background flush" and is done on demand. This is identical to a CIL push
+triggered by a log force, only that there is no waiting for the checkpoint
+commit to complete. This background push is checked and executed by
+transaction commit code.
+
+If the transaction subsystem goes idle while we still have items in the CIL,
+they will be flushed by the periodic log force issued by the xfssyncd. This
+log force will push the CIL to disk, and if the transaction subsystem stays
+idle, allow the idle log to be covered (effectively marked clean) in exactly
+the same manner that is done for the existing logging method. A discussion
+point is whether this log force needs to be done more frequently than the
+current rate which is once every 30s.
+
+Log Item Pinning
+^^^^^^^^^^^^^^^^
+
+Currently log items are pinned during transaction commit while the items are
+still locked. This happens just after the items are formatted, though it could
+be done any time before the items are unlocked. The result of this mechanism
+is that items get pinned once for every transaction that is committed to the
+log buffers. Hence items that are relogged in the log buffers will have a pin
+count for every outstanding transaction they were dirtied in. When each of
+these transactions is completed, they will unpin the item once. As a result,
+the item only becomes unpinned when all the transactions complete and there
+are no pending transactions. Thus the pinning and unpinning of a log item is
+symmetric as there is a 1:1 relationship with transaction commit and log item
+completion.
+
+For delayed logging, however, we have an asymmetric transaction commit to
+completion relationship. Every time an object is relogged in the CIL it goes
+through the commit process without a corresponding completion being
+registered. That is, we now have a many-to-one relationship between
+transaction commit and log item completion. The result of this is that pinning
+and unpinning of the log items becomes unbalanced if we retain the "pin on
+transaction commit, unpin on transaction completion" model.
+
+To keep pin/unpin symmetry, the algorithm needs to change to a "pin on
+insertion into the CIL, unpin on checkpoint completion". In other words, the
+pinning and unpinning becomes symmetric around a checkpoint context. We have
+to pin the object the first time it is inserted into the CIL - if it is
+already in the CIL during a transaction commit, then we do not pin it again.
+Because there can be multiple outstanding checkpoint contexts, we can still
+see elevated pin counts, but as each checkpoint completes the pin count will
+retain the correct value according to it’s context.
+
+Just to make matters more slightly more complex, this checkpoint level context
+for the pin count means that the pinning of an item must take place under the
+CIL commit/flush lock. If we pin the object outside this lock, we cannot
+guarantee which context the pin count is associated with. This is because of
+the fact pinning the item is dependent on whether the item is present in the
+current CIL or not. If we don’t pin the CIL first before we check and pin the
+object, we have a race with CIL being flushed between the check and the pin
+(or not pinning, as the case may be). Hence we must hold the CIL flush/commit
+lock to guarantee that we pin the items correctly.
+
+Concurrent Scalability
+^^^^^^^^^^^^^^^^^^^^^^
+
+A fundamental requirement for the CIL is that accesses through transaction
+commits must scale to many concurrent commits. The current transaction commit
+code does not break down even when there are transactions coming from 2048
+processors at once. The current transaction code does not go any faster than
+if there was only one CPU using it, but it does not slow down either.
+
+As a result, the delayed logging transaction commit code needs to be designed
+for concurrency from the ground up. It is obvious that there are serialisation
+points in the design - the three important ones are:
+
+1. Locking out new transaction commits while flushing the CIL
+
+2. Adding items to the CIL and updating item space accounting
+
+3. Checkpoint commit ordering
+
+Looking at the transaction commit and CIL flushing interactions, it is clear
+that we have a many-to-one interaction here. That is, the only restriction on
+the number of concurrent transactions that can be trying to commit at once is
+the amount of space available in the log for their reservations. The practical
+limit here is in the order of several hundred concurrent transactions for a
+128MB log, which means that it is generally one per CPU in a machine.
+
+The amount of time a transaction commit needs to hold out a flush is a
+relatively long period of time - the pinning of log items needs to be done
+while we are holding out a CIL flush, so at the moment that means it is held
+across the formatting of the objects into memory buffers (i.e. while memcpy()s
+are in progress). Ultimately a two pass algorithm where the formatting is done
+separately to the pinning of objects could be used to reduce the hold time of
+the transaction commit side.
+
+Because of the number of potential transaction commit side holders, the lock
+really needs to be a sleeping lock - if the CIL flush takes the lock, we do
+not want every other CPU in the machine spinning on the CIL lock. Given that
+flushing the CIL could involve walking a list of tens of thousands of log
+items, it will get held for a significant time and so spin contention is a
+significant concern. Preventing lots of CPUs spinning doing nothing is the
+main reason for choosing a sleeping lock even though nothing in either the
+transaction commit or CIL flush side sleeps with the lock held.
+
+It should also be noted that CIL flushing is also a relatively rare operation
+compared to transaction commit for asynchronous transaction workloads - only
+time will tell if using a read-write semaphore for exclusion will limit
+transaction commit concurrency due to cache line bouncing of the lock on the
+read side.
+
+The second serialisation point is on the transaction commit side where items
+are inserted into the CIL. Because transactions can enter this code
+concurrently, the CIL needs to be protected separately from the above
+commit/flush exclusion. It also needs to be an exclusive lock but it is only
+held for a very short time and so a spin lock is appropriate here. It is
+possible that this lock will become a contention point, but given the short
+hold time once per transaction I think that contention is unlikely.
+
+The final serialisation point is the checkpoint commit record ordering code
+that is run as part of the checkpoint commit and log force sequencing. The
+code path that triggers a CIL flush (i.e. whatever triggers the log force)
+will enter an ordering loop after writing all the log vectors into the log
+buffers but before writing the commit record. This loop walks the list of
+committing checkpoints and needs to block waiting for checkpoints to complete
+their commit record write. As a result it needs a lock and a wait variable.
+Log force sequencing also requires the same lock, list walk, and blocking
+mechanism to ensure completion of checkpoints.
+
+These two sequencing operations can use the mechanism even though the events
+they are waiting for are different. The checkpoint commit record sequencing
+needs to wait until checkpoint contexts contain a commit LSN (obtained through
+completion of a commit record write) while log force sequencing needs to wait
+until previous checkpoint contexts are removed from the committing list (i.e.
+they’ve completed). A simple wait variable and broadcast wakeups (thundering
+herds) has been used to implement these two serialisation queues. They use the
+same lock as the CIL, too. If we see too much contention on the CIL lock, or
+too many context switches as a result of the broadcast wakeups these
+operations can be put under a new spinlock and given separate wait lists to
+reduce lock contention and the number of processes woken by the wrong event.
+
+Lifecycle Changes
+^^^^^^^^^^^^^^^^^
+
+The existing log item life cycle is as follows:
+
+::
+
+ 1. Transaction allocate
+ 2. Transaction reserve
+ 3. Lock item
+ 4. Join item to transaction
+ If not already attached,
+ Allocate log item
+ Attach log item to owner item
+ Attach log item to transaction
+ 5. Modify item
+ Record modifications in log item
+ 6. Transaction commit
+ Pin item in memory
+ Format item into log buffer
+ Write commit LSN into transaction
+ Unlock item
+ Attach transaction to log buffer
+
+ <log buffer IO dispatched>
+ <log buffer IO completes>
+
+ 7. Transaction completion
+ Mark log item committed
+ Insert log item into AIL
+ Write commit LSN into log item
+ Unpin log item
+ 8. AIL traversal
+ Lock item
+ Mark log item clean
+ Flush item to disk
+
+ <item IO completion>
+
+ 9. Log item removed from AIL
+ Moves log tail
+ Item unlocked
+
+Essentially, steps 1-6 operate independently from step 7, which is also
+independent of steps 8-9. An item can be locked in steps 1-6 or steps 8-9 at
+the same time step 7 is occurring, but only steps 1-6 or 8-9 can occur at the
+same time. If the log item is in the AIL or between steps 6 and 7 and steps
+1-6 are re-entered, then the item is relogged. Only when steps 8-9 are entered
+and completed is the object considered clean.
+
+With delayed logging, there are new steps inserted into the life cycle:
+
+::
+
+ 1. Transaction allocate
+ 2. Transaction reserve
+ 3. Lock item
+ 4. Join item to transaction
+ If not already attached,
+ Allocate log item
+ Attach log item to owner item
+ Attach log item to transaction
+ 5. Modify item
+ Record modifications in log item
+ 6. Transaction commit
+ Pin item in memory if not pinned in CIL
+ Format item into log vector + buffer
+ Attach log vector and buffer to log item
+ Insert log item into CIL
+ Write CIL context sequence into transaction
+ Unlock item
+
+ <next log force>
+
+ 7. CIL push
+ lock CIL flush
+ Chain log vectors and buffers together
+ Remove items from CIL
+ unlock CIL flush
+ write log vectors into log
+ sequence commit records
+ attach checkpoint context to log buffer
+
+ <log buffer IO dispatched>
+ <log buffer IO completes>
+
+ 8. Checkpoint completion
+ Mark log item committed
+ Insert item into AIL
+ Write commit LSN into log item
+ Unpin log item
+ 9. AIL traversal
+ Lock item
+ Mark log item clean
+ Flush item to disk
+ <item IO completion>
+ 10. Log item removed from AIL
+ Moves log tail
+ Item unlocked
+
+From this, it can be seen that the only life cycle differences between the two
+logging methods are in the middle of the life cycle - they still have the same
+beginning and end and execution constraints. The only differences are in the
+committing of the log items to the log itself and the completion processing.
+Hence delayed logging should not introduce any constraints on log item
+behaviour, allocation or freeing that don’t already exist.
+
+As a result of this zero-impact "insertion" of delayed logging infrastructure
+and the design of the internal structures to avoid on disk format changes, we
+can basically switch between delayed logging and the existing mechanism with a
+mount option. Fundamentally, there is no reason why the log manager would not
+be able to swap methods automatically and transparently depending on load
+characteristics, but this should not be necessary if delayed logging works as
+designed.
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index 8b3de9abcf39..457e81c0eb40 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -44,3 +44,4 @@ are tracked more simply and in larger chunks to reduce jitter in allocation
latency.
.. include:: self_describing_metadata.rst
+.. include:: delayed_logging.rst
diff --git a/Documentation/filesystems/xfs-delayed-logging-design.txt b/Documentation/filesystems/xfs-delayed-logging-design.txt
deleted file mode 100644
index 2ce36439c09f..000000000000
--- a/Documentation/filesystems/xfs-delayed-logging-design.txt
+++ /dev/null
@@ -1,793 +0,0 @@
-XFS Delayed Logging Design
---------------------------
-
-Introduction to Re-logging in XFS
----------------------------------
-
-XFS logging is a combination of logical and physical logging. Some objects,
-such as inodes and dquots, are logged in logical format where the details
-logged are made up of the changes to in-core structures rather than on-disk
-structures. Other objects - typically buffers - have their physical changes
-logged. The reason for these differences is to reduce the amount of log space
-required for objects that are frequently logged. Some parts of inodes are more
-frequently logged than others, and inodes are typically more frequently logged
-than any other object (except maybe the superblock buffer) so keeping the
-amount of metadata logged low is of prime importance.
-
-The reason that this is such a concern is that XFS allows multiple separate
-modifications to a single object to be carried in the log at any given time.
-This allows the log to avoid needing to flush each change to disk before
-recording a new change to the object. XFS does this via a method called
-"re-logging". Conceptually, this is quite simple - all it requires is that any
-new change to the object is recorded with a *new copy* of all the existing
-changes in the new transaction that is written to the log.
-
-That is, if we have a sequence of changes A through to F, and the object was
-written to disk after change D, we would see in the log the following series
-of transactions, their contents and the log sequence number (LSN) of the
-transaction:
-
- Transaction Contents LSN
- A A X
- B A+B X+n
- C A+B+C X+n+m
- D A+B+C+D X+n+m+o
- <object written to disk>
- E E Y (> X+n+m+o)
- F E+F Yٍ+p
-
-In other words, each time an object is relogged, the new transaction contains
-the aggregation of all the previous changes currently held only in the log.
-
-This relogging technique also allows objects to be moved forward in the log so
-that an object being relogged does not prevent the tail of the log from ever
-moving forward. This can be seen in the table above by the changing
-(increasing) LSN of each subsequent transaction - the LSN is effectively a
-direct encoding of the location in the log of the transaction.
-
-This relogging is also used to implement long-running, multiple-commit
-transactions. These transaction are known as rolling transactions, and require
-a special log reservation known as a permanent transaction reservation. A
-typical example of a rolling transaction is the removal of extents from an
-inode which can only be done at a rate of two extents per transaction because
-of reservation size limitations. Hence a rolling extent removal transaction
-keeps relogging the inode and btree buffers as they get modified in each
-removal operation. This keeps them moving forward in the log as the operation
-progresses, ensuring that current operation never gets blocked by itself if the
-log wraps around.
-
-Hence it can be seen that the relogging operation is fundamental to the correct
-working of the XFS journalling subsystem. From the above description, most
-people should be able to see why the XFS metadata operations writes so much to
-the log - repeated operations to the same objects write the same changes to
-the log over and over again. Worse is the fact that objects tend to get
-dirtier as they get relogged, so each subsequent transaction is writing more
-metadata into the log.
-
-Another feature of the XFS transaction subsystem is that most transactions are
-asynchronous. That is, they don't commit to disk until either a log buffer is
-filled (a log buffer can hold multiple transactions) or a synchronous operation
-forces the log buffers holding the transactions to disk. This means that XFS is
-doing aggregation of transactions in memory - batching them, if you like - to
-minimise the impact of the log IO on transaction throughput.
-
-The limitation on asynchronous transaction throughput is the number and size of
-log buffers made available by the log manager. By default there are 8 log
-buffers available and the size of each is 32kB - the size can be increased up
-to 256kB by use of a mount option.
-
-Effectively, this gives us the maximum bound of outstanding metadata changes
-that can be made to the filesystem at any point in time - if all the log
-buffers are full and under IO, then no more transactions can be committed until
-the current batch completes. It is now common for a single current CPU core to
-be to able to issue enough transactions to keep the log buffers full and under
-IO permanently. Hence the XFS journalling subsystem can be considered to be IO
-bound.
-
-Delayed Logging: Concepts
--------------------------
-
-The key thing to note about the asynchronous logging combined with the
-relogging technique XFS uses is that we can be relogging changed objects
-multiple times before they are committed to disk in the log buffers. If we
-return to the previous relogging example, it is entirely possible that
-transactions A through D are committed to disk in the same log buffer.
-
-That is, a single log buffer may contain multiple copies of the same object,
-but only one of those copies needs to be there - the last one "D", as it
-contains all the changes from the previous changes. In other words, we have one
-necessary copy in the log buffer, and three stale copies that are simply
-wasting space. When we are doing repeated operations on the same set of
-objects, these "stale objects" can be over 90% of the space used in the log
-buffers. It is clear that reducing the number of stale objects written to the
-log would greatly reduce the amount of metadata we write to the log, and this
-is the fundamental goal of delayed logging.
-
-From a conceptual point of view, XFS is already doing relogging in memory (where
-memory == log buffer), only it is doing it extremely inefficiently. It is using
-logical to physical formatting to do the relogging because there is no
-infrastructure to keep track of logical changes in memory prior to physically
-formatting the changes in a transaction to the log buffer. Hence we cannot avoid
-accumulating stale objects in the log buffers.
-
-Delayed logging is the name we've given to keeping and tracking transactional
-changes to objects in memory outside the log buffer infrastructure. Because of
-the relogging concept fundamental to the XFS journalling subsystem, this is
-actually relatively easy to do - all the changes to logged items are already
-tracked in the current infrastructure. The big problem is how to accumulate
-them and get them to the log in a consistent, recoverable manner.
-Describing the problems and how they have been solved is the focus of this
-document.
-
-One of the key changes that delayed logging makes to the operation of the
-journalling subsystem is that it disassociates the amount of outstanding
-metadata changes from the size and number of log buffers available. In other
-words, instead of there only being a maximum of 2MB of transaction changes not
-written to the log at any point in time, there may be a much greater amount
-being accumulated in memory. Hence the potential for loss of metadata on a
-crash is much greater than for the existing logging mechanism.
-
-It should be noted that this does not change the guarantee that log recovery
-will result in a consistent filesystem. What it does mean is that as far as the
-recovered filesystem is concerned, there may be many thousands of transactions
-that simply did not occur as a result of the crash. This makes it even more
-important that applications that care about their data use fsync() where they
-need to ensure application level data integrity is maintained.
-
-It should be noted that delayed logging is not an innovative new concept that
-warrants rigorous proofs to determine whether it is correct or not. The method
-of accumulating changes in memory for some period before writing them to the
-log is used effectively in many filesystems including ext3 and ext4. Hence
-no time is spent in this document trying to convince the reader that the
-concept is sound. Instead it is simply considered a "solved problem" and as
-such implementing it in XFS is purely an exercise in software engineering.
-
-The fundamental requirements for delayed logging in XFS are simple:
-
- 1. Reduce the amount of metadata written to the log by at least
- an order of magnitude.
- 2. Supply sufficient statistics to validate Requirement #1.
- 3. Supply sufficient new tracing infrastructure to be able to debug
- problems with the new code.
- 4. No on-disk format change (metadata or log format).
- 5. Enable and disable with a mount option.
- 6. No performance regressions for synchronous transaction workloads.
-
-Delayed Logging: Design
------------------------
-
-Storing Changes
-
-The problem with accumulating changes at a logical level (i.e. just using the
-existing log item dirty region tracking) is that when it comes to writing the
-changes to the log buffers, we need to ensure that the object we are formatting
-is not changing while we do this. This requires locking the object to prevent
-concurrent modification. Hence flushing the logical changes to the log would
-require us to lock every object, format them, and then unlock them again.
-
-This introduces lots of scope for deadlocks with transactions that are already
-running. For example, a transaction has object A locked and modified, but needs
-the delayed logging tracking lock to commit the transaction. However, the
-flushing thread has the delayed logging tracking lock already held, and is
-trying to get the lock on object A to flush it to the log buffer. This appears
-to be an unsolvable deadlock condition, and it was solving this problem that
-was the barrier to implementing delayed logging for so long.
-
-The solution is relatively simple - it just took a long time to recognise it.
-Put simply, the current logging code formats the changes to each item into an
-vector array that points to the changed regions in the item. The log write code
-simply copies the memory these vectors point to into the log buffer during
-transaction commit while the item is locked in the transaction. Instead of
-using the log buffer as the destination of the formatting code, we can use an
-allocated memory buffer big enough to fit the formatted vector.
-
-If we then copy the vector into the memory buffer and rewrite the vector to
-point to the memory buffer rather than the object itself, we now have a copy of
-the changes in a format that is compatible with the log buffer writing code.
-that does not require us to lock the item to access. This formatting and
-rewriting can all be done while the object is locked during transaction commit,
-resulting in a vector that is transactionally consistent and can be accessed
-without needing to lock the owning item.
-
-Hence we avoid the need to lock items when we need to flush outstanding
-asynchronous transactions to the log. The differences between the existing
-formatting method and the delayed logging formatting can be seen in the
-diagram below.
-
-Current format log vector:
-
-Object +---------------------------------------------+
-Vector 1 +----+
-Vector 2 +----+
-Vector 3 +----------+
-
-After formatting:
-
-Log Buffer +-V1-+-V2-+----V3----+
-
-Delayed logging vector:
-
-Object +---------------------------------------------+
-Vector 1 +----+
-Vector 2 +----+
-Vector 3 +----------+
-
-After formatting:
-
-Memory Buffer +-V1-+-V2-+----V3----+
-Vector 1 +----+
-Vector 2 +----+
-Vector 3 +----------+
-
-The memory buffer and associated vector need to be passed as a single object,
-but still need to be associated with the parent object so if the object is
-relogged we can replace the current memory buffer with a new memory buffer that
-contains the latest changes.
-
-The reason for keeping the vector around after we've formatted the memory
-buffer is to support splitting vectors across log buffer boundaries correctly.
-If we don't keep the vector around, we do not know where the region boundaries
-are in the item, so we'd need a new encapsulation method for regions in the log
-buffer writing (i.e. double encapsulation). This would be an on-disk format
-change and as such is not desirable. It also means we'd have to write the log
-region headers in the formatting stage, which is problematic as there is per
-region state that needs to be placed into the headers during the log write.
-
-Hence we need to keep the vector, but by attaching the memory buffer to it and
-rewriting the vector addresses to point at the memory buffer we end up with a
-self-describing object that can be passed to the log buffer write code to be
-handled in exactly the same manner as the existing log vectors are handled.
-Hence we avoid needing a new on-disk format to handle items that have been
-relogged in memory.
-
-
-Tracking Changes
-
-Now that we can record transactional changes in memory in a form that allows
-them to be used without limitations, we need to be able to track and accumulate
-them so that they can be written to the log at some later point in time. The
-log item is the natural place to store this vector and buffer, and also makes sense
-to be the object that is used to track committed objects as it will always
-exist once the object has been included in a transaction.
-
-The log item is already used to track the log items that have been written to
-the log but not yet written to disk. Such log items are considered "active"
-and as such are stored in the Active Item List (AIL) which is a LSN-ordered
-double linked list. Items are inserted into this list during log buffer IO
-completion, after which they are unpinned and can be written to disk. An object
-that is in the AIL can be relogged, which causes the object to be pinned again
-and then moved forward in the AIL when the log buffer IO completes for that
-transaction.
-
-Essentially, this shows that an item that is in the AIL can still be modified
-and relogged, so any tracking must be separate to the AIL infrastructure. As
-such, we cannot reuse the AIL list pointers for tracking committed items, nor
-can we store state in any field that is protected by the AIL lock. Hence the
-committed item tracking needs it's own locks, lists and state fields in the log
-item.
-
-Similar to the AIL, tracking of committed items is done through a new list
-called the Committed Item List (CIL). The list tracks log items that have been
-committed and have formatted memory buffers attached to them. It tracks objects
-in transaction commit order, so when an object is relogged it is removed from
-it's place in the list and re-inserted at the tail. This is entirely arbitrary
-and done to make it easy for debugging - the last items in the list are the
-ones that are most recently modified. Ordering of the CIL is not necessary for
-transactional integrity (as discussed in the next section) so the ordering is
-done for convenience/sanity of the developers.
-
-
-Delayed Logging: Checkpoints
-
-When we have a log synchronisation event, commonly known as a "log force",
-all the items in the CIL must be written into the log via the log buffers.
-We need to write these items in the order that they exist in the CIL, and they
-need to be written as an atomic transaction. The need for all the objects to be
-written as an atomic transaction comes from the requirements of relogging and
-log replay - all the changes in all the objects in a given transaction must
-either be completely replayed during log recovery, or not replayed at all. If
-a transaction is not replayed because it is not complete in the log, then
-no later transactions should be replayed, either.
-
-To fulfill this requirement, we need to write the entire CIL in a single log
-transaction. Fortunately, the XFS log code has no fixed limit on the size of a
-transaction, nor does the log replay code. The only fundamental limit is that
-the transaction cannot be larger than just under half the size of the log. The
-reason for this limit is that to find the head and tail of the log, there must
-be at least one complete transaction in the log at any given time. If a
-transaction is larger than half the log, then there is the possibility that a
-crash during the write of a such a transaction could partially overwrite the
-only complete previous transaction in the log. This will result in a recovery
-failure and an inconsistent filesystem and hence we must enforce the maximum
-size of a checkpoint to be slightly less than a half the log.
-
-Apart from this size requirement, a checkpoint transaction looks no different
-to any other transaction - it contains a transaction header, a series of
-formatted log items and a commit record at the tail. From a recovery
-perspective, the checkpoint transaction is also no different - just a lot
-bigger with a lot more items in it. The worst case effect of this is that we
-might need to tune the recovery transaction object hash size.
-
-Because the checkpoint is just another transaction and all the changes to log
-items are stored as log vectors, we can use the existing log buffer writing
-code to write the changes into the log. To do this efficiently, we need to
-minimise the time we hold the CIL locked while writing the checkpoint
-transaction. The current log write code enables us to do this easily with the
-way it separates the writing of the transaction contents (the log vectors) from
-the transaction commit record, but tracking this requires us to have a
-per-checkpoint context that travels through the log write process through to
-checkpoint completion.
-
-Hence a checkpoint has a context that tracks the state of the current
-checkpoint from initiation to checkpoint completion. A new context is initiated
-at the same time a checkpoint transaction is started. That is, when we remove
-all the current items from the CIL during a checkpoint operation, we move all
-those changes into the current checkpoint context. We then initialise a new
-context and attach that to the CIL for aggregation of new transactions.
-
-This allows us to unlock the CIL immediately after transfer of all the
-committed items and effectively allow new transactions to be issued while we
-are formatting the checkpoint into the log. It also allows concurrent
-checkpoints to be written into the log buffers in the case of log force heavy
-workloads, just like the existing transaction commit code does. This, however,
-requires that we strictly order the commit records in the log so that
-checkpoint sequence order is maintained during log replay.
-
-To ensure that we can be writing an item into a checkpoint transaction at
-the same time another transaction modifies the item and inserts the log item
-into the new CIL, then checkpoint transaction commit code cannot use log items
-to store the list of log vectors that need to be written into the transaction.
-Hence log vectors need to be able to be chained together to allow them to be
-detached from the log items. That is, when the CIL is flushed the memory
-buffer and log vector attached to each log item needs to be attached to the
-checkpoint context so that the log item can be released. In diagrammatic form,
-the CIL would look like this before the flush:
-
- CIL Head
- |
- V
- Log Item <-> log vector 1 -> memory buffer
- | -> vector array
- V
- Log Item <-> log vector 2 -> memory buffer
- | -> vector array
- V
- ......
- |
- V
- Log Item <-> log vector N-1 -> memory buffer
- | -> vector array
- V
- Log Item <-> log vector N -> memory buffer
- -> vector array
-
-And after the flush the CIL head is empty, and the checkpoint context log
-vector list would look like:
-
- Checkpoint Context
- |
- V
- log vector 1 -> memory buffer
- | -> vector array
- | -> Log Item
- V
- log vector 2 -> memory buffer
- | -> vector array
- | -> Log Item
- V
- ......
- |
- V
- log vector N-1 -> memory buffer
- | -> vector array
- | -> Log Item
- V
- log vector N -> memory buffer
- -> vector array
- -> Log Item
-
-Once this transfer is done, the CIL can be unlocked and new transactions can
-start, while the checkpoint flush code works over the log vector chain to
-commit the checkpoint.
-
-Once the checkpoint is written into the log buffers, the checkpoint context is
-attached to the log buffer that the commit record was written to along with a
-completion callback. Log IO completion will call that callback, which can then
-run transaction committed processing for the log items (i.e. insert into AIL
-and unpin) in the log vector chain and then free the log vector chain and
-checkpoint context.
-
-Discussion Point: I am uncertain as to whether the log item is the most
-efficient way to track vectors, even though it seems like the natural way to do
-it. The fact that we walk the log items (in the CIL) just to chain the log
-vectors and break the link between the log item and the log vector means that
-we take a cache line hit for the log item list modification, then another for
-the log vector chaining. If we track by the log vectors, then we only need to
-break the link between the log item and the log vector, which means we should
-dirty only the log item cachelines. Normally I wouldn't be concerned about one
-vs two dirty cachelines except for the fact I've seen upwards of 80,000 log
-vectors in one checkpoint transaction. I'd guess this is a "measure and
-compare" situation that can be done after a working and reviewed implementation
-is in the dev tree....
-
-Delayed Logging: Checkpoint Sequencing
-
-One of the key aspects of the XFS transaction subsystem is that it tags
-committed transactions with the log sequence number of the transaction commit.
-This allows transactions to be issued asynchronously even though there may be
-future operations that cannot be completed until that transaction is fully
-committed to the log. In the rare case that a dependent operation occurs (e.g.
-re-using a freed metadata extent for a data extent), a special, optimised log
-force can be issued to force the dependent transaction to disk immediately.
-
-To do this, transactions need to record the LSN of the commit record of the
-transaction. This LSN comes directly from the log buffer the transaction is
-written into. While this works just fine for the existing transaction
-mechanism, it does not work for delayed logging because transactions are not
-written directly into the log buffers. Hence some other method of sequencing
-transactions is required.
-
-As discussed in the checkpoint section, delayed logging uses per-checkpoint
-contexts, and as such it is simple to assign a sequence number to each
-checkpoint. Because the switching of checkpoint contexts must be done
-atomically, it is simple to ensure that each new context has a monotonically
-increasing sequence number assigned to it without the need for an external
-atomic counter - we can just take the current context sequence number and add
-one to it for the new context.
-
-Then, instead of assigning a log buffer LSN to the transaction commit LSN
-during the commit, we can assign the current checkpoint sequence. This allows
-operations that track transactions that have not yet completed know what
-checkpoint sequence needs to be committed before they can continue. As a
-result, the code that forces the log to a specific LSN now needs to ensure that
-the log forces to a specific checkpoint.
-
-To ensure that we can do this, we need to track all the checkpoint contexts
-that are currently committing to the log. When we flush a checkpoint, the
-context gets added to a "committing" list which can be searched. When a
-checkpoint commit completes, it is removed from the committing list. Because
-the checkpoint context records the LSN of the commit record for the checkpoint,
-we can also wait on the log buffer that contains the commit record, thereby
-using the existing log force mechanisms to execute synchronous forces.
-
-It should be noted that the synchronous forces may need to be extended with
-mitigation algorithms similar to the current log buffer code to allow
-aggregation of multiple synchronous transactions if there are already
-synchronous transactions being flushed. Investigation of the performance of the
-current design is needed before making any decisions here.
-
-The main concern with log forces is to ensure that all the previous checkpoints
-are also committed to disk before the one we need to wait for. Therefore we
-need to check that all the prior contexts in the committing list are also
-complete before waiting on the one we need to complete. We do this
-synchronisation in the log force code so that we don't need to wait anywhere
-else for such serialisation - it only matters when we do a log force.
-
-The only remaining complexity is that a log force now also has to handle the
-case where the forcing sequence number is the same as the current context. That
-is, we need to flush the CIL and potentially wait for it to complete. This is a
-simple addition to the existing log forcing code to check the sequence numbers
-and push if required. Indeed, placing the current sequence checkpoint flush in
-the log force code enables the current mechanism for issuing synchronous
-transactions to remain untouched (i.e. commit an asynchronous transaction, then
-force the log at the LSN of that transaction) and so the higher level code
-behaves the same regardless of whether delayed logging is being used or not.
-
-Delayed Logging: Checkpoint Log Space Accounting
-
-The big issue for a checkpoint transaction is the log space reservation for the
-transaction. We don't know how big a checkpoint transaction is going to be
-ahead of time, nor how many log buffers it will take to write out, nor the
-number of split log vector regions are going to be used. We can track the
-amount of log space required as we add items to the commit item list, but we
-still need to reserve the space in the log for the checkpoint.
-
-A typical transaction reserves enough space in the log for the worst case space
-usage of the transaction. The reservation accounts for log record headers,
-transaction and region headers, headers for split regions, buffer tail padding,
-etc. as well as the actual space for all the changed metadata in the
-transaction. While some of this is fixed overhead, much of it is dependent on
-the size of the transaction and the number of regions being logged (the number
-of log vectors in the transaction).
-
-An example of the differences would be logging directory changes versus logging
-inode changes. If you modify lots of inode cores (e.g. chmod -R g+w *), then
-there are lots of transactions that only contain an inode core and an inode log
-format structure. That is, two vectors totaling roughly 150 bytes. If we modify
-10,000 inodes, we have about 1.5MB of metadata to write in 20,000 vectors. Each
-vector is 12 bytes, so the total to be logged is approximately 1.75MB. In
-comparison, if we are logging full directory buffers, they are typically 4KB
-each, so we in 1.5MB of directory buffers we'd have roughly 400 buffers and a
-buffer format structure for each buffer - roughly 800 vectors or 1.51MB total
-space. From this, it should be obvious that a static log space reservation is
-not particularly flexible and is difficult to select the "optimal value" for
-all workloads.
-
-Further, if we are going to use a static reservation, which bit of the entire
-reservation does it cover? We account for space used by the transaction
-reservation by tracking the space currently used by the object in the CIL and
-then calculating the increase or decrease in space used as the object is
-relogged. This allows for a checkpoint reservation to only have to account for
-log buffer metadata used such as log header records.
-
-However, even using a static reservation for just the log metadata is
-problematic. Typically log record headers use at least 16KB of log space per
-1MB of log space consumed (512 bytes per 32k) and the reservation needs to be
-large enough to handle arbitrary sized checkpoint transactions. This
-reservation needs to be made before the checkpoint is started, and we need to
-be able to reserve the space without sleeping. For a 8MB checkpoint, we need a
-reservation of around 150KB, which is a non-trivial amount of space.
-
-A static reservation needs to manipulate the log grant counters - we can take a
-permanent reservation on the space, but we still need to make sure we refresh
-the write reservation (the actual space available to the transaction) after
-every checkpoint transaction completion. Unfortunately, if this space is not
-available when required, then the regrant code will sleep waiting for it.
-
-The problem with this is that it can lead to deadlocks as we may need to commit
-checkpoints to be able to free up log space (refer back to the description of
-rolling transactions for an example of this). Hence we *must* always have
-space available in the log if we are to use static reservations, and that is
-very difficult and complex to arrange. It is possible to do, but there is a
-simpler way.
-
-The simpler way of doing this is tracking the entire log space used by the
-items in the CIL and using this to dynamically calculate the amount of log
-space required by the log metadata. If this log metadata space changes as a
-result of a transaction commit inserting a new memory buffer into the CIL, then
-the difference in space required is removed from the transaction that causes
-the change. Transactions at this level will *always* have enough space
-available in their reservation for this as they have already reserved the
-maximal amount of log metadata space they require, and such a delta reservation
-will always be less than or equal to the maximal amount in the reservation.
-
-Hence we can grow the checkpoint transaction reservation dynamically as items
-are added to the CIL and avoid the need for reserving and regranting log space
-up front. This avoids deadlocks and removes a blocking point from the
-checkpoint flush code.
-
-As mentioned early, transactions can't grow to more than half the size of the
-log. Hence as part of the reservation growing, we need to also check the size
-of the reservation against the maximum allowed transaction size. If we reach
-the maximum threshold, we need to push the CIL to the log. This is effectively
-a "background flush" and is done on demand. This is identical to
-a CIL push triggered by a log force, only that there is no waiting for the
-checkpoint commit to complete. This background push is checked and executed by
-transaction commit code.
-
-If the transaction subsystem goes idle while we still have items in the CIL,
-they will be flushed by the periodic log force issued by the xfssyncd. This log
-force will push the CIL to disk, and if the transaction subsystem stays idle,
-allow the idle log to be covered (effectively marked clean) in exactly the same
-manner that is done for the existing logging method. A discussion point is
-whether this log force needs to be done more frequently than the current rate
-which is once every 30s.
-
-
-Delayed Logging: Log Item Pinning
-
-Currently log items are pinned during transaction commit while the items are
-still locked. This happens just after the items are formatted, though it could
-be done any time before the items are unlocked. The result of this mechanism is
-that items get pinned once for every transaction that is committed to the log
-buffers. Hence items that are relogged in the log buffers will have a pin count
-for every outstanding transaction they were dirtied in. When each of these
-transactions is completed, they will unpin the item once. As a result, the item
-only becomes unpinned when all the transactions complete and there are no
-pending transactions. Thus the pinning and unpinning of a log item is symmetric
-as there is a 1:1 relationship with transaction commit and log item completion.
-
-For delayed logging, however, we have an asymmetric transaction commit to
-completion relationship. Every time an object is relogged in the CIL it goes
-through the commit process without a corresponding completion being registered.
-That is, we now have a many-to-one relationship between transaction commit and
-log item completion. The result of this is that pinning and unpinning of the
-log items becomes unbalanced if we retain the "pin on transaction commit, unpin
-on transaction completion" model.
-
-To keep pin/unpin symmetry, the algorithm needs to change to a "pin on
-insertion into the CIL, unpin on checkpoint completion". In other words, the
-pinning and unpinning becomes symmetric around a checkpoint context. We have to
-pin the object the first time it is inserted into the CIL - if it is already in
-the CIL during a transaction commit, then we do not pin it again. Because there
-can be multiple outstanding checkpoint contexts, we can still see elevated pin
-counts, but as each checkpoint completes the pin count will retain the correct
-value according to it's context.
-
-Just to make matters more slightly more complex, this checkpoint level context
-for the pin count means that the pinning of an item must take place under the
-CIL commit/flush lock. If we pin the object outside this lock, we cannot
-guarantee which context the pin count is associated with. This is because of
-the fact pinning the item is dependent on whether the item is present in the
-current CIL or not. If we don't pin the CIL first before we check and pin the
-object, we have a race with CIL being flushed between the check and the pin
-(or not pinning, as the case may be). Hence we must hold the CIL flush/commit
-lock to guarantee that we pin the items correctly.
-
-Delayed Logging: Concurrent Scalability
-
-A fundamental requirement for the CIL is that accesses through transaction
-commits must scale to many concurrent commits. The current transaction commit
-code does not break down even when there are transactions coming from 2048
-processors at once. The current transaction code does not go any faster than if
-there was only one CPU using it, but it does not slow down either.
-
-As a result, the delayed logging transaction commit code needs to be designed
-for concurrency from the ground up. It is obvious that there are serialisation
-points in the design - the three important ones are:
-
- 1. Locking out new transaction commits while flushing the CIL
- 2. Adding items to the CIL and updating item space accounting
- 3. Checkpoint commit ordering
-
-Looking at the transaction commit and CIL flushing interactions, it is clear
-that we have a many-to-one interaction here. That is, the only restriction on
-the number of concurrent transactions that can be trying to commit at once is
-the amount of space available in the log for their reservations. The practical
-limit here is in the order of several hundred concurrent transactions for a
-128MB log, which means that it is generally one per CPU in a machine.
-
-The amount of time a transaction commit needs to hold out a flush is a
-relatively long period of time - the pinning of log items needs to be done
-while we are holding out a CIL flush, so at the moment that means it is held
-across the formatting of the objects into memory buffers (i.e. while memcpy()s
-are in progress). Ultimately a two pass algorithm where the formatting is done
-separately to the pinning of objects could be used to reduce the hold time of
-the transaction commit side.
-
-Because of the number of potential transaction commit side holders, the lock
-really needs to be a sleeping lock - if the CIL flush takes the lock, we do not
-want every other CPU in the machine spinning on the CIL lock. Given that
-flushing the CIL could involve walking a list of tens of thousands of log
-items, it will get held for a significant time and so spin contention is a
-significant concern. Preventing lots of CPUs spinning doing nothing is the
-main reason for choosing a sleeping lock even though nothing in either the
-transaction commit or CIL flush side sleeps with the lock held.
-
-It should also be noted that CIL flushing is also a relatively rare operation
-compared to transaction commit for asynchronous transaction workloads - only
-time will tell if using a read-write semaphore for exclusion will limit
-transaction commit concurrency due to cache line bouncing of the lock on the
-read side.
-
-The second serialisation point is on the transaction commit side where items
-are inserted into the CIL. Because transactions can enter this code
-concurrently, the CIL needs to be protected separately from the above
-commit/flush exclusion. It also needs to be an exclusive lock but it is only
-held for a very short time and so a spin lock is appropriate here. It is
-possible that this lock will become a contention point, but given the short
-hold time once per transaction I think that contention is unlikely.
-
-The final serialisation point is the checkpoint commit record ordering code
-that is run as part of the checkpoint commit and log force sequencing. The code
-path that triggers a CIL flush (i.e. whatever triggers the log force) will enter
-an ordering loop after writing all the log vectors into the log buffers but
-before writing the commit record. This loop walks the list of committing
-checkpoints and needs to block waiting for checkpoints to complete their commit
-record write. As a result it needs a lock and a wait variable. Log force
-sequencing also requires the same lock, list walk, and blocking mechanism to
-ensure completion of checkpoints.
-
-These two sequencing operations can use the mechanism even though the
-events they are waiting for are different. The checkpoint commit record
-sequencing needs to wait until checkpoint contexts contain a commit LSN
-(obtained through completion of a commit record write) while log force
-sequencing needs to wait until previous checkpoint contexts are removed from
-the committing list (i.e. they've completed). A simple wait variable and
-broadcast wakeups (thundering herds) has been used to implement these two
-serialisation queues. They use the same lock as the CIL, too. If we see too
-much contention on the CIL lock, or too many context switches as a result of
-the broadcast wakeups these operations can be put under a new spinlock and
-given separate wait lists to reduce lock contention and the number of processes
-woken by the wrong event.
-
-
-Lifecycle Changes
-
-The existing log item life cycle is as follows:
-
- 1. Transaction allocate
- 2. Transaction reserve
- 3. Lock item
- 4. Join item to transaction
- If not already attached,
- Allocate log item
- Attach log item to owner item
- Attach log item to transaction
- 5. Modify item
- Record modifications in log item
- 6. Transaction commit
- Pin item in memory
- Format item into log buffer
- Write commit LSN into transaction
- Unlock item
- Attach transaction to log buffer
-
- <log buffer IO dispatched>
- <log buffer IO completes>
-
- 7. Transaction completion
- Mark log item committed
- Insert log item into AIL
- Write commit LSN into log item
- Unpin log item
- 8. AIL traversal
- Lock item
- Mark log item clean
- Flush item to disk
-
- <item IO completion>
-
- 9. Log item removed from AIL
- Moves log tail
- Item unlocked
-
-Essentially, steps 1-6 operate independently from step 7, which is also
-independent of steps 8-9. An item can be locked in steps 1-6 or steps 8-9
-at the same time step 7 is occurring, but only steps 1-6 or 8-9 can occur
-at the same time. If the log item is in the AIL or between steps 6 and 7
-and steps 1-6 are re-entered, then the item is relogged. Only when steps 8-9
-are entered and completed is the object considered clean.
-
-With delayed logging, there are new steps inserted into the life cycle:
-
- 1. Transaction allocate
- 2. Transaction reserve
- 3. Lock item
- 4. Join item to transaction
- If not already attached,
- Allocate log item
- Attach log item to owner item
- Attach log item to transaction
- 5. Modify item
- Record modifications in log item
- 6. Transaction commit
- Pin item in memory if not pinned in CIL
- Format item into log vector + buffer
- Attach log vector and buffer to log item
- Insert log item into CIL
- Write CIL context sequence into transaction
- Unlock item
-
- <next log force>
-
- 7. CIL push
- lock CIL flush
- Chain log vectors and buffers together
- Remove items from CIL
- unlock CIL flush
- write log vectors into log
- sequence commit records
- attach checkpoint context to log buffer
-
- <log buffer IO dispatched>
- <log buffer IO completes>
-
- 8. Checkpoint completion
- Mark log item committed
- Insert item into AIL
- Write commit LSN into log item
- Unpin log item
- 9. AIL traversal
- Lock item
- Mark log item clean
- Flush item to disk
- <item IO completion>
- 10. Log item removed from AIL
- Moves log tail
- Item unlocked
-
-From this, it can be seen that the only life cycle differences between the two
-logging methods are in the middle of the life cycle - they still have the same
-beginning and end and execution constraints. The only differences are in the
-committing of the log items to the log itself and the completion processing.
-Hence delayed logging should not introduce any constraints on log item
-behaviour, allocation or freeing that don't already exist.
-
-As a result of this zero-impact "insertion" of delayed logging infrastructure
-and the design of the internal structures to avoid on disk format changes, we
-can basically switch between delayed logging and the existing mechanism with a
-mount option. Fundamentally, there is no reason why the log manager would not
-be able to swap methods automatically and transparently depending on load
-characteristics, but this should not be necessary if delayed logging works as
-designed.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 05/22] docs: add XFS shared data block chapter to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (2 preceding siblings ...)
2018-10-04 4:18 ` [PATCH 04/22] docs: add XFS delayed logging design " Darrick J. Wong
@ 2018-10-04 4:18 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 06/22] docs: add XFS online repair " Darrick J. Wong
` (17 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:18 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/overview.rst | 1
.../filesystems/xfs-data-structures/reflink.rst | 43 ++++++++++++++++++++
2 files changed, 44 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/reflink.rst
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index 457e81c0eb40..d8d668ec6097 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -45,3 +45,4 @@ latency.
.. include:: self_describing_metadata.rst
.. include:: delayed_logging.rst
+.. include:: reflink.rst
diff --git a/Documentation/filesystems/xfs-data-structures/reflink.rst b/Documentation/filesystems/xfs-data-structures/reflink.rst
new file mode 100644
index 000000000000..653b3def7e6e
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/reflink.rst
@@ -0,0 +1,43 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Sharing Data Blocks
+-------------------
+
+On a traditional filesystem, there is a 1:1 mapping between a logical block
+offset in a file and a physical block on disk, which is to say that physical
+blocks are not shared. However, there exist various use cases for being able
+to share blocks between files — deduplicating files saves space on archival
+systems; creating space-efficient clones of disk images for virtual machines
+and containers facilitates efficient datacenters; and deferring the payment of
+the allocation cost of a file system tree copy as long as possible makes
+regular work faster. In all of these cases, a write to one of the shared
+copies **must** not affect the other shared copies, which means that writes to
+shared blocks must employ a copy-on-write strategy. Sharing blocks in this
+manner is commonly referred to as "reflinking".
+
+XFS implements block sharing in a fairly straightforward manner. All existing
+data fork structures remain unchanged, save for the addition of a
+per-allocation group `reference count B+tree <#reference-count-b-tree>`__. This
+data structure tracks reference counts for all shared physical blocks, with a
+few rules to maintain compatibility with existing code: If a block is free, it
+will be tracked in the free space B+trees. If a block is owned by a single
+file, it appears in neither the free space nor the reference count B+trees. If
+a block is shared, it will appear in the reference count B+tree with a
+reference count >= 2. The first two cases are established precedent in XFS, so
+the third case is the only behavioral change.
+
+When a filesystem block is shared, the block mapping in the destination file
+is updated to point to that filesystem block and the reference count B+tree
+records are updated to reflect the increased reference count. If a shared
+block is written, a new block will be allocated, the dirty data written to
+this new block, and the file’s block mapping updated to point to the new
+block. If a shared block is unmapped, the reference count records are updated
+to reflect the decreased reference count and the block is also freed if its
+reference count becomes zero. This enables users to create space efficient
+clones of disk images and to copy filesystem subtrees quickly, using the
+standard Linux coreutils packages.
+
+Deduplication employs the same mechanism to share blocks and copy them at
+write time. However, the kernel confirms that the contents of both files are
+identical before updating the destination file’s mapping. This enables XFS to
+be used by userspace deduplication programs such as duperemove.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 06/22] docs: add XFS online repair chapter to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (3 preceding siblings ...)
2018-10-04 4:18 ` [PATCH 05/22] docs: add XFS shared data block chapter " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 07/22] docs: add XFS common types and magic numbers " Darrick J. Wong
` (16 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/overview.rst | 1
.../xfs-data-structures/reconstruction.rst | 68 ++++++++++++++++++++
2 files changed, 69 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/reconstruction.rst
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index d8d668ec6097..b1b3f711638b 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -46,3 +46,4 @@ latency.
.. include:: self_describing_metadata.rst
.. include:: delayed_logging.rst
.. include:: reflink.rst
+.. include:: reconstruction.rst
diff --git a/Documentation/filesystems/xfs-data-structures/reconstruction.rst b/Documentation/filesystems/xfs-data-structures/reconstruction.rst
new file mode 100644
index 000000000000..10a7a728c50c
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/reconstruction.rst
@@ -0,0 +1,68 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Metadata Reconstruction
+-----------------------
+
+ **Note**
+
+ This is a theoretical discussion of how reconstruction could work; none of
+ this is implemented as of 2018.
+
+A simple UNIX filesystem can be thought of in terms of a directed acyclic
+graph. To a first approximation, there exists a root directory node, which
+points to other nodes. Those other nodes can themselves be directories or they
+can be files. Each file, in turn, points to data blocks.
+
+XFS adds a few more details to this picture:
+
+- The real root(s) of an XFS filesystem are the allocation group headers
+ (superblock, AGF, AGI, AGFL).
+
+- Each allocation group’s headers point to various per-AG B+trees (free
+ space, inode, free inodes, free list, etc.)
+
+- The free space B+trees point to unused extents;
+
+- The inode B+trees point to blocks containing inode chunks;
+
+- All superblocks point to the root directory and the log;
+
+- Hardlinks mean that multiple directories can point to a single file node;
+
+- File data block pointers are indexed by file offset;
+
+- Files and directories can have a second collection of pointers to data
+ blocks which contain extended attributes;
+
+- Large directories require multiple data blocks to store all the
+ subpointers;
+
+- Still larger directories use high-offset data blocks to store a B+tree of
+ hashes to directory entries;
+
+- Large extended attribute forks similarly use high-offset data blocks to
+ store a B+tree of hashes to attribute keys; and
+
+- Symbolic links can point to data blocks.
+
+The beauty of this massive graph structure is that under normal circumstances,
+everything known to the filesystem is discoverable (access controls
+notwithstanding) from the root. The major weakness of this structure of course
+is that breaking a edge in the graph can render entire subtrees inaccessible.
+xfs\_repair “recovers” from broken directories by scanning for unlinked inodes
+and connecting them to /lost+found, but this isn’t sufficiently general to
+recover from breaks in other parts of the graph structure. Wouldn’t it be
+useful to have back pointers as a secondary data structure? The current repair
+strategy is to reconstruct whatever can be rebuilt, but to scrap anything that
+doesn’t check out.
+
+The `reverse-mapping B+tree <#reverse-mapping-b-tree>`__ fills in part of the
+puzzle. Since it contains copies of every entry in each inode’s data and
+attribute forks, we can fix a corrupted block map with these records.
+Furthermore, if the inode B+trees become corrupt, it is possible to visit all
+inode chunks using the reverse-mapping data. Should XFS ever gain the ability
+to store parent directory information in each inode, it also becomes possible
+to resurrect damaged directory trees, which should reduce the complaints about
+inodes ending up in /lost+found. Everything else in the per-AG primary
+metadata can already be reconstructed via xfs\_repair. Hopefully,
+reconstruction will not turn out to be a fool’s errand.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 07/22] docs: add XFS common types and magic numbers to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (4 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 06/22] docs: add XFS online repair " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 08/22] docs: add XFS testing chapter to the " Darrick J. Wong
` (15 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/common_types.rst | 61 ++++
.../filesystems/xfs-data-structures/magic.rst | 277 ++++++++++++++++++++
.../filesystems/xfs-data-structures/overview.rst | 2
3 files changed, 340 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/common_types.rst
create mode 100644 Documentation/filesystems/xfs-data-structures/magic.rst
diff --git a/Documentation/filesystems/xfs-data-structures/common_types.rst b/Documentation/filesystems/xfs-data-structures/common_types.rst
new file mode 100644
index 000000000000..63de847924c6
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/common_types.rst
@@ -0,0 +1,61 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Common XFS Types
+----------------
+
+All the following XFS types can be found in xfs\_types.h. NULL values are
+always -1 on disk (ie. all bits for the value set to one).
+
+**xfs\_ino\_t**
+ Unsigned 64 bit absolute `inode number <#inode-numbers>`__.
+
+**xfs\_off\_t**
+ Signed 64 bit file offset.
+
+**xfs\_daddr\_t**
+ Signed 64 bit disk address (sectors).
+
+**xfs\_agnumber\_t**
+ Unsigned 32 bit `AG number <#allocation-groups>`__.
+
+**xfs\_agblock\_t**
+ Unsigned 32 bit AG relative block number.
+
+**xfs\_extlen\_t**
+ Unsigned 32 bit `extent <#data-extents>`__ length in blocks.
+
+**xfs\_extnum\_t**
+ Signed 32 bit number of extents in a data fork.
+
+**xfs\_aextnum\_t**
+ Signed 16 bit number of extents in an attribute fork.
+
+**xfs\_dablk\_t**
+ Unsigned 32 bit block number for `directories <#directories>`__ and
+ `extended attributes <#extended-attributes>`__.
+
+**xfs\_dahash\_t**
+ Unsigned 32 bit hash of a directory file name or extended attribute name.
+
+**xfs\_fsblock\_t**
+ Unsigned 64 bit filesystem block number combining `AG
+ number <#allocation-groups>`__ and block offset into the AG.
+
+**xfs\_rfsblock\_t**
+ Unsigned 64 bit raw filesystem block number.
+
+**xfs\_rtblock\_t**
+ Unsigned 64 bit extent number in the `real-time <#real-time-devices>`__
+ sub-volume.
+
+**xfs\_fileoff\_t**
+ Unsigned 64 bit block offset into a file.
+
+**xfs\_filblks\_t**
+ Unsigned 64 bit block count for a file.
+
+**uuid\_t**
+ 16-byte universally unique identifier (UUID).
+
+**xfs\_fsize\_t**
+ Signed 64 bit byte size of a file.
diff --git a/Documentation/filesystems/xfs-data-structures/magic.rst b/Documentation/filesystems/xfs-data-structures/magic.rst
new file mode 100644
index 000000000000..f5e57581645d
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/magic.rst
@@ -0,0 +1,277 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Magic Numbers
+-------------
+
+These are the magic numbers that are known to XFS, along with links to the
+relevant chapters. Magic numbers tend to have consistent locations:
+
+- 32-bit magic numbers are always at offset zero in the block.
+
+- 16-bit magic numbers for the directory and attribute B+tree are at offset
+ eight.
+
+- The quota magic number is at offset zero.
+
+- The inode magic is at the beginning of each inode.
+
+.. list-table::
+ :widths: 28 12 8 34
+ :header-rows: 1
+
+ * - Flag
+ - Hexadecimal
+ - ASCII
+ - Data structure
+ * - XFS_SB_MAGIC
+ - 0x58465342
+ - XFSB
+ - `Superblock <#superblocks>`__
+ * - XFS_AGF_MAGIC
+ - 0x58414746
+ - XAGF
+ - `Free Space <#ag-free-space-block>`__
+ * - XFS_AGI_MAGIC
+ - 0x58414749
+ - XAGI
+ - `Inode Information <#inode-information>`__
+ * - XFS_AGFL_MAGIC
+ - 0x5841464c
+ - XAFL
+ - `Free Space List <#ag-free-list>`__, v5 only
+ * - XFS_DINODE_MAGIC
+ - 0x494e
+ - IN
+ - `Inodes <#inode-core>`__
+ * - XFS_DQUOT_MAGIC
+ - 0x4451
+ - DQ
+ - `Quota Inodes <#quota-inodes>`__
+ * - XFS_SYMLINK_MAGIC
+ - 0x58534c4d
+ - XSLM
+ - `Symbolic Links <#extent-symbolic-links>`__
+ * - XFS_ABTB_MAGIC
+ - 0x41425442
+ - ABTB
+ - `Free Space by Block B+tree <#ag-free-space-b-trees>`__
+ * - XFS_ABTB_CRC_MAGIC
+ - 0x41423342
+ - AB3B
+ - `Free Space by Block B+tree <#ag-free-space-b-trees>`__, v5 only
+ * - XFS_ABTC_MAGIC
+ - 0x41425443
+ - ABTC
+ - `Free Space by Size B+tree <#ag-free-space-b-trees>`__
+ * - XFS_ABTC_CRC_MAGIC
+ - 0x41423343
+ - AB3C
+ - `Free Space by Size B+tree <#ag-free-space-b-trees>`__, v5 only
+ * - XFS_IBT_MAGIC
+ - 0x49414254
+ - IABT
+ - `Inode B+tree <#inode-b-trees>`__
+ * - XFS_IBT_CRC_MAGIC
+ - 0x49414233
+ - IAB3
+ - `Inode B+tree <#inode-b-trees>`__, v5 only
+ * - XFS_FIBT_MAGIC
+ - 0x46494254
+ - FIBT
+ - `Free Inode B+tree <#inode-b-trees>`__
+ * - XFS_FIBT_CRC_MAGIC
+ - 0x46494233
+ - FIB3
+ - `Free Inode B+tree <#inode-b-trees>`__, v5 only
+ * - XFS_BMAP_MAGIC
+ - 0x424d4150
+ - BMAP
+ - `B+Tree Extent List <#b-tree-extent-list>`__
+ * - XFS_BMAP_CRC_MAGIC
+ - 0x424d4133
+ - BMA3
+ - `B+Tree Extent List <#b-tree-extent-list>`__, v5 only
+ * - XLOG_HEADER_MAGIC_NUM
+ - 0xfeedbabe
+ -
+ - `Log Records <#log-records>`__
+ * - XFS_DA_NODE_MAGIC
+ - 0xfebe
+ -
+ - `Directory/Attribute Node <#directory-attribute-internal-node>`__
+ * - XFS_DA3_NODE_MAGIC
+ - 0x3ebe
+ -
+ - `Directory/Attribute Node <#directory-attribute-internal-node>`__, v5 only
+ * - XFS_DIR2_BLOCK_MAGIC
+ - 0x58443242
+ - XD2B
+ - `Block Directory Data <#block-directories>`__
+ * - XFS_DIR3_BLOCK_MAGIC
+ - 0x58444233
+ - XDB3
+ - `Block Directory Data <#block-directories>`__, v5 only
+ * - XFS_DIR2_DATA_MAGIC
+ - 0x58443244
+ - XD2D
+ - `Leaf Directory Data <#leaf-directories>`__
+ * - XFS_DIR3_DATA_MAGIC
+ - 0x58444433
+ - XDD3
+ - `Leaf Directory Data <#leaf-directories>`__, v5 only
+ * - XFS_DIR2_LEAF1_MAGIC
+ - 0xd2f1
+ -
+ - `Leaf Directory <#leaf-directories>`__
+ * - XFS_DIR3_LEAF1_MAGIC
+ - 0x3df1
+ -
+ - `Leaf Directory <#leaf-directories>`__, v5 only
+ * - XFS_DIR2_LEAFN_MAGIC
+ - 0xd2ff
+ -
+ - `Node Directory <#node-directories>`__
+ * - XFS_DIR3_LEAFN_MAGIC
+ - 0x3dff
+ -
+ - `Node Directory <#node-directories>`__, v5 only
+ * - XFS_DIR2_FREE_MAGIC
+ - 0x58443246
+ - XD2F
+ - `Node Directory Free Space <#node-directories>`__
+ * - XFS_DIR3_FREE_MAGIC
+ - 0x58444633
+ - XDF3
+ - `Node Directory Free Space <#node-directories>`__, v5 only
+ * - XFS_ATTR_LEAF_MAGIC
+ - 0xfbee
+ -
+ - `Leaf Attribute <#leaf-attributes>`__
+ * - XFS_ATTR3_LEAF_MAGIC
+ - 0x3bee
+ -
+ - `Leaf Attribute <#leaf-attributes>`__, v5 only
+ * - XFS_ATTR3_RMT_MAGIC
+ - 0x5841524d
+ - XARM
+ - `Remote Attribute Value <#remote-attribute-values>`__, v5 only
+ * - XFS_RMAP_CRC_MAGIC
+ - 0x524d4233
+ - RMB3
+ - `Reverse Mapping B+tree <#reverse-mapping-b-tree>`__, v5 only
+ * - XFS_RTRMAP_CRC_MAGIC
+ - 0x4d415052
+ - MAPR
+ - `Real-Time Reverse Mapping B+tree <#real-time-reverse-mapping-b-tree>`__, v5 only
+ * - XFS_REFC_CRC_MAGIC
+ - 0x52334643
+ - R3FC
+ - `Reference Count B+tree <#reference-count-b-tree>`__, v5 only
+ * - XFS_MD_MAGIC
+ - 0x5846534d
+ - XFSM
+ - `Metadata Dumps <#metadata-dumps>`__
+
+The magic numbers for log items are at offset zero in each log item, but items
+are not aligned to blocks.
+
+.. list-table::
+ :widths: 24 12 8 36
+ :header-rows: 1
+
+ * - Flag
+ - Hexadecimal
+ - ASCII
+ - Data structure
+ * - XFS_TRANS_HEADER_MAGIC
+ - 0x5452414e
+ - TRAN
+ - `Log Transactions <#transaction-headers>`__
+ * - XFS_LI_EFI
+ - 0x1236
+ -
+ - `Extent Freeing Intent Log Item <#intent-to-free-an-extent>`__
+ * - XFS_LI_EFD
+ - 0x1237
+ -
+ - `Extent Freeing Done Log Item <#completion-of-intent-to-free-an-extent>`__
+ * - XFS_LI_IUNLINK
+ - 0x1238
+ -
+ - Unknown?
+ * - XFS_LI_INODE
+ - 0x123b
+ -
+ - `Inode Updates Log Item <#inode-updates>`__
+ * - XFS_LI_BUF
+ - 0x123c
+ -
+ - `Buffer Writes Log Item <#buffer-log-item>`__
+ * - XFS_LI_DQUOT
+ - 0x123d
+ -
+ - `Update Quota Log Item <#quota-update-data-log-item>`__
+ * - XFS_LI_QUOTAOFF
+ - 0x123e
+ -
+ - `Quota Off Log Item <#disable-quota-log-item>`__
+ * - XFS_LI_ICREATE
+ - 0x123f
+ -
+ - `Inode Creation Log Item <#inode-creation-log-item>`__
+ * - XFS_LI_RUI
+ - 0x1240
+ -
+ - `Reverse Mapping Update Intent <#reverse-mapping-updates-intent>`__
+ * - XFS_LI_RUD
+ - 0x1241
+ -
+ - `Reverse Mapping Update Done <#completion-of-reverse-mapping-updates>`__
+ * - XFS_LI_CUI
+ - 0x1242
+ -
+ - `Reference Count Update Intent <#reference-count-updates-intent>`__
+ * - XFS_LI_CUD
+ - 0x1243
+ -
+ - `Reference Count Update Done <#completion-of-reference-count-updates>`__
+ * - XFS_LI_BUI
+ - 0x1244
+ -
+ - `File Block Mapping Update Intent <#file-block-mapping-intent>`__
+ * - XFS_LI_BUD
+ - 0x1245
+ -
+ - `File Block Mapping Update Done <#completion-of-file-block-mapping-updates>`__
+
+Theoretical Limits
+------------------
+
+XFS can create really big filesystems!
+
++---------------------+---------------------+---------------------+---------------------+
+| Item | 1KiB blocks | 4KiB blocks | 64KiB blocks |
++=====================+=====================+=====================+=====================+
+| Blocks | 2\ :sup:`52` | 2\ :sup:`52` | 2\ :sup:`52` |
++---------------------+---------------------+---------------------+---------------------+
+| Inodes | 2\ :sup:`63` | 2\ :sup:`63` | 2\ :sup:`64` |
++---------------------+---------------------+---------------------+---------------------+
+| Allocation Groups | 2\ :sup:`32` | 2\ :sup:`32` | 2\ :sup:`32` |
++---------------------+---------------------+---------------------+---------------------+
+| File System Size | 8EiB | 8EiB | 8EiB |
++---------------------+---------------------+---------------------+---------------------+
+| Blocks per AG | 2\ :sup:`31` | 2\ :sup:`31` | 2\ :sup:`31` |
++---------------------+---------------------+---------------------+---------------------+
+| Inodes per AG | 2\ :sup:`32` | 2\ :sup:`32` | 2\ :sup:`32` |
++---------------------+---------------------+---------------------+---------------------+
+| Max AG Size | 2TiB | 8TiB | 128TiB |
++---------------------+---------------------+---------------------+---------------------+
+| Blocks Per File | 2\ :sup:`54` | 2\ :sup:`54` | 2\ :sup:`54` |
++---------------------+---------------------+---------------------+---------------------+
+| File Size | 8EiB | 8EiB | 8EiB |
++---------------------+---------------------+---------------------+---------------------+
+| Max Dir Size | 32GiB | 32GiB | 32GiB |
++---------------------+---------------------+---------------------+---------------------+
+
+Linux doesn’t suppport files or devices larger than 8EiB, so the block
+limitations are largely ignorable.
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index b1b3f711638b..23eb71d65c93 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -47,3 +47,5 @@ latency.
.. include:: delayed_logging.rst
.. include:: reflink.rst
.. include:: reconstruction.rst
+.. include:: common_types.rst
+.. include:: magic.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 08/22] docs: add XFS testing chapter to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (5 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 07/22] docs: add XFS common types and magic numbers " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 09/22] docs: add XFS btrees " Darrick J. Wong
` (14 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/overview.rst | 1 +
.../filesystems/xfs-data-structures/testing.rst | 25 ++++++++++++++++++++
2 files changed, 26 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/testing.rst
diff --git a/Documentation/filesystems/xfs-data-structures/overview.rst b/Documentation/filesystems/xfs-data-structures/overview.rst
index 23eb71d65c93..d6711dc653d8 100644
--- a/Documentation/filesystems/xfs-data-structures/overview.rst
+++ b/Documentation/filesystems/xfs-data-structures/overview.rst
@@ -49,3 +49,4 @@ latency.
.. include:: reconstruction.rst
.. include:: common_types.rst
.. include:: magic.rst
+.. include:: testing.rst
diff --git a/Documentation/filesystems/xfs-data-structures/testing.rst b/Documentation/filesystems/xfs-data-structures/testing.rst
new file mode 100644
index 000000000000..3d3386854408
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/testing.rst
@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Testing Filesystem Changes
+--------------------------
+
+People put a lot of trust in filesystems to preserve their data in a reliable
+fashion. To that end, it is very important that users and developers have
+access to a suite of regression tests that can be used to prove correct
+operation of any given filesystem code, or to analyze failures to fix problems
+found in the code. The XFS regression test suite, xfstests, is hosted at
+``git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git``. Most tests apply to
+filesystems in general, but the suite also contains tests for features
+specific to each filesystem.
+
+When fixing bugs, it is important to provide a testcase exposing the bug so
+that the developers can avoid a future re-occurrence of the regression.
+Furthermore, if you’re developing a new user-visible feature for XFS, please
+help the rest of the development community to sustain and maintain the whole
+codebase by providing generous test coverage to check its behavior.
+
+When altering, adding, or removing an on-disk data structure, please remember
+to update both the in-kernel structure size checks in xfs\_ondisk.h and to
+ensure that your changes are reflected in xfstest xfs/122. These regression
+tests enable us to detect compiler bugs, alignment problems, and anything else
+that might result in the creation of incompatible filesystem images.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 09/22] docs: add XFS btrees to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (6 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 08/22] docs: add XFS testing chapter to the " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 10/22] docs: add XFS dir/attr btree structure " Darrick J. Wong
` (13 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/btrees.rst | 197 ++++++++++++++++++++
.../filesystems/xfs-data-structures/globals.rst | 2
2 files changed, 199 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/btrees.rst
diff --git a/Documentation/filesystems/xfs-data-structures/btrees.rst b/Documentation/filesystems/xfs-data-structures/btrees.rst
new file mode 100644
index 000000000000..e343f71b37f6
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/btrees.rst
@@ -0,0 +1,197 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Fixed Length Record B+trees
+---------------------------
+
+XFS uses b+trees to index all metadata records. This well known data structure
+is used to provide efficient random and sequential access to metadata records
+while minimizing seek times. There are two btree formats: a short format for
+records pertaining to a single allocation group, since all block pointers in
+an AG are 32-bits in size; and a long format for records pertaining to a file,
+since file data can have 64-bit block offsets. Each b+tree block is either a
+leaf node containing records, or an internal node containing keys and pointers
+to other b+tree blocks. The tree consists of a root block which may point to
+some number of other blocks; blocks in the bottom level of the b+tree contains
+only records.
+
+Leaf blocks of both types of b+trees have the same general format: a header
+describing the data in the block, and an array of records. The specific header
+formats are given in the next two sections, and the record format is provided
+by the b+tree client itself. The generic b+tree code does not have any
+specific knowledge of the record format.
+
+::
+
+ +--------+------------+------------+
+ | header | record | records... |
+ +--------+------------+------------+
+
+Internal node blocks of both types of b+trees also have the same general
+format: a header describing the data in the block, an array of keys, and an
+array of pointers. Each pointer may be associated with one or two keys. The
+first key uniquely identifies the first record accessible via the leftmost
+path down the branch of the tree.
+
+If the records in a b+tree are indexed by an interval, then a range of keys
+can uniquely identify a single record. For example, if a record covers blocks
+12-16, then any one of the keys 12, 13, 14, 15, or 16 return the same record.
+In this case, the key for the record describing "12-16" is 12. If none of the
+records overlap, we only need to store one key.
+
+This is the format of a standard b+tree node:
+
+::
+
+ +--------+---------+---------+---------+---------+
+ | header | key | keys... | ptr | ptrs... |
+ +--------+---------+---------+---------+---------+
+
+If the b+tree records do not overlap, performing a b+tree lookup is simple.
+Start with the root. If it is a leaf block, perform a binary search of the
+records until we find the record with a lower key than our search key. If the
+block is a node block, perform a binary search of the keys until we find a key
+lower than our search key, then follow the pointer to the next block. Repeat
+until we find a record.
+
+However, if b+tree records contain intervals and are allowed to overlap, the
+internal nodes of the b+tree become larger:
+
+::
+
+ +--------+---------+----------+---------+-------------+---------+---------+
+ | header | low key | high key | low key | high key... | ptr | ptrs... |
+ +--------+---------+----------+---------+-------------+---------+---------+
+
+The low keys are exactly the same as the keys in the non-overlapping b+tree.
+High keys, however, are a little different. Recall that a record with a key
+consisting of an interval can be referenced by a number of keys. Since the low
+key of a record indexes the low end of that key range, the high key indexes
+the high end of the key range. Returning to the example above, the high key
+for the record describing "12-16" is 16. The high key recorded in a b+tree
+node is the largest of the high keys of all records accessible under the
+subtree rooted by the pointer. For a level 1 node, this is the largest high
+key in the pointed-to leaf node; for any other node, this is the largest of
+the high keys in the pointed-to node.
+
+Nodes and leaves use the same magic numbers.
+
+Short Format B+trees
+~~~~~~~~~~~~~~~~~~~~
+
+Each allocation group uses a "short format" B+tree to index various
+information about the allocation group. The structure is called short format
+because all block pointers are AG block numbers. The trees use the following
+header:
+
+.. code:: c
+
+ struct xfs_btree_sblock {
+ __be32 bb_magic;
+ __be16 bb_level;
+ __be16 bb_numrecs;
+ __be32 bb_leftsib;
+ __be32 bb_rightsib;
+
+ /* version 5 filesystem fields start here */
+ __be64 bb_blkno;
+ __be64 bb_lsn;
+ uuid_t bb_uuid;
+ __be32 bb_owner;
+ __le32 bb_crc;
+ };
+
+**bb\_magic**
+ Specifies the magic number for the per-AG B+tree block.
+
+**bb\_level**
+ The level of the tree in which this block is found. If this value is 0,
+ this is a leaf block and contains records; otherwise, it is a node block
+ and contains keys and pointers. Level values increase towards the root.
+
+**bb\_numrecs**
+ Number of records in this block.
+
+**bb\_leftsib**
+ AG block number of the left sibling of this B+tree node.
+
+**bb\_rightsib**
+ AG block number of the right sibling of this B+tree node.
+
+**bb\_blkno**
+ FS block number of this B+tree block.
+
+**bb\_lsn**
+ Log sequence number of the last write to this block.
+
+**bb\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**bb\_owner**
+ The AG number that this B+tree block ought to be in.
+
+**bb\_crc**
+ Checksum of the B+tree block.
+
+Long Format B+trees
+~~~~~~~~~~~~~~~~~~~
+
+Long format B+trees are similar to short format B+trees, except that their
+block pointers are 64-bit filesystem block numbers instead of 32-bit AG block
+numbers. Because of this, long format b+trees can be (and usually are) rooted
+in an inode’s data or attribute fork. The nodes and leaves of this B+tree use
+the xfs\_btree\_lblock declaration:
+
+.. code:: c
+
+ struct xfs_btree_lblock {
+ __be32 bb_magic;
+ __be16 bb_level;
+ __be16 bb_numrecs;
+ __be64 bb_leftsib;
+ __be64 bb_rightsib;
+
+ /* version 5 filesystem fields start here */
+ __be64 bb_blkno;
+ __be64 bb_lsn;
+ uuid_t bb_uuid;
+ __be64 bb_owner;
+ __le32 bb_crc;
+ __be32 bb_pad;
+ };
+
+**bb\_magic**
+ Specifies the magic number for the btree block.
+
+**bb\_level**
+ The level of the tree in which this block is found. If this value is 0,
+ this is a leaf block and contains records; otherwise, it is a node block
+ and contains keys and pointers.
+
+**bb\_numrecs**
+ Number of records in this block.
+
+**bb\_leftsib**
+ FS block number of the left sibling of this B+tree node.
+
+**bb\_rightsib**
+ FS block number of the right sibling of this B+tree node.
+
+**bb\_blkno**
+ FS block number of this B+tree block.
+
+**bb\_lsn**
+ Log sequence number of the last write to this block.
+
+**bb\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**bb\_owner**
+ The AG number that this B+tree block ought to be in.
+
+**bb\_crc**
+ Checksum of the B+tree block.
+
+**bb\_pad**
+ Pads the structure to 64 bytes.
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index 3499e0fcd4a8..8a2173908b0e 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -2,3 +2,5 @@
Global Structures
=================
+
+.. include:: btrees.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 10/22] docs: add XFS dir/attr btree structure to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (7 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 09/22] docs: add XFS btrees " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 11/22] docs: add XFS allocation group metadata " Darrick J. Wong
` (12 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/dabtrees.rst | 221 ++++++++++++++++++++
.../filesystems/xfs-data-structures/globals.rst | 1
2 files changed, 222 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/dabtrees.rst
diff --git a/Documentation/filesystems/xfs-data-structures/dabtrees.rst b/Documentation/filesystems/xfs-data-structures/dabtrees.rst
new file mode 100644
index 000000000000..9daac6295941
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/dabtrees.rst
@@ -0,0 +1,221 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Variable Length Record B+trees
+------------------------------
+
+Directories and extended attributes are implemented as a simple key-value
+record store inside the blocks pointed to by the data or attribute fork of a
+file. Blocks referenced by either data structure are block offsets of an inode
+fork, not physical blocks.
+
+Directory and attribute data are stored as a linear array of variable-length
+records in the low blocks of a fork. Both data types share the property that
+record keys and record values are both arbitrary and unique sequences of
+bytes. See the respective sections about `directories <#directories>`__ or
+`attributes <#extended-attributes>`__ for more information about the exact
+record formats.
+
+The dir/attr b+tree (or "dabtree"), if present, computes a hash of the record
+key to produce the b+tree key, and b+tree keys are used to index the fork
+block in which the record may be found. Unlike the fixed-length b+trees, the
+variable length b+trees can index the same key multiple times. B+tree
+keypointers and records both take this format:
+
+::
+
+ +---------+--------------+
+ | hashval | before_block |
+ +---------+--------------+
+
+The "before block" is the block offset in the inode fork of the block in which
+we can find the record whose hashed key is "hashval". The hash function is as
+follows:
+
+.. code:: c
+
+ #define rol32(x,y) (((x) << (y)) | ((x) >> (32 - (y))))
+
+ xfs_dahash_t
+ xfs_da_hashname(const uint8_t *name, int namelen)
+ {
+ xfs_dahash_t hash;
+
+ /*
+ * Do four characters at a time as long as we can.
+ */
+ for (hash = 0; namelen >= 4; namelen -= 4, name += 4)
+ hash = (name[0] << 21) ^ (name[1] << 14) ^ (name[2] << 7) ^
+ (name[3] << 0) ^ rol32(hash, 7 * 4);
+
+ /*
+ * Now do the rest of the characters.
+ */
+ switch (namelen) {
+ case 3:
+ return (name[0] << 14) ^ (name[1] << 7) ^ (name[2] << 0) ^
+ rol32(hash, 7 * 3);
+ case 2:
+ return (name[0] << 7) ^ (name[1] << 0) ^ rol32(hash, 7 * 2);
+ case 1:
+ return (name[0] << 0) ^ rol32(hash, 7 * 1);
+ default: /* case 0: */
+ return hash;
+ }
+ }
+
+.. _directory-attribute-block-header:
+
+Block Headers
+~~~~~~~~~~~~~
+
+- Tree nodes, leaf and node `directories <#directories>`__, and leaf and node
+ `extended attributes <#extended-attributes>`__ use the xfs\_da\_blkinfo\_t
+ filesystem block header. The structure appears as follows:
+
+.. code:: c
+
+ typedef struct xfs_da_blkinfo {
+ __be32 forw;
+ __be32 back;
+ __be16 magic;
+ __be16 pad;
+ } xfs_da_blkinfo_t;
+
+**forw**
+ Logical block offset of the previous B+tree block at this level.
+
+**back**
+ Logical block offset of the next B+tree block at this level.
+
+**magic**
+ Magic number for this directory/attribute block.
+
+**pad**
+ Padding to maintain alignment.
+
+- On a v5 filesystem, the leaves use the struct xfs\_da3\_blkinfo\_t
+ filesystem block header. This header is used in the same place as
+ xfs\_da\_blkinfo\_t:
+
+.. code:: c
+
+ struct xfs_da3_blkinfo {
+ /* these values are inside xfs_da_blkinfo */
+ __be32 forw;
+ __be32 back;
+ __be16 magic;
+ __be16 pad;
+
+ __be32 crc;
+ __be64 blkno;
+ __be64 lsn;
+ uuid_t uuid;
+ __be64 owner;
+ };
+
+**forw**
+ Logical block offset of the previous B+tree block at this level.
+
+**back**
+ Logical block offset of the next B+tree block at this level.
+
+**magic**
+ Magic number for this directory/attribute block.
+
+**pad**
+ Padding to maintain alignment.
+
+**crc**
+ Checksum of the directory/attribute block.
+
+**blkno**
+ Block number of this directory/attribute block.
+
+**lsn**
+ Log sequence number of the last write to this block.
+
+**uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**owner**
+ The inode number that this directory/attribute block belongs to.
+
+.. _directory-attribute-internal-node:
+
+Internal Nodes
+~~~~~~~~~~~~~~
+
+The nodes of a dabtree have the following format:
+
+.. code:: c
+
+ typedef struct xfs_da_intnode {
+ struct xfs_da_node_hdr {
+ xfs_da_blkinfo_t info;
+ __uint16_t count;
+ __uint16_t level;
+ } hdr;
+ struct xfs_da_node_entry {
+ xfs_dahash_t hashval;
+ xfs_dablk_t before;
+ } btree[1];
+ } xfs_da_intnode_t;
+
+**info**
+ Directory/attribute block info. The magic number is XFS\_DA\_NODE\_MAGIC
+ (0xfebe).
+
+**count**
+ Number of node entries in this block.
+
+**level**
+ The level of this block in the B+tree. Levels start at 1 for blocks that
+ point to directory or attribute data blocks and increase towards the root.
+
+**hashval**
+ The hash value of a particular record.
+
+**before**
+ The directory/attribute logical block containing all entries up to the
+ corresponding hash value.
+
+ - On a v5 filesystem, the directory/attribute node blocks have the
+ following structure:
+
+.. code:: c
+
+ struct xfs_da3_intnode {
+ struct xfs_da3_node_hdr {
+ struct xfs_da3_blkinfo info;
+ __uint16_t count;
+ __uint16_t level;
+ __uint32_t pad32;
+ } hdr;
+ struct xfs_da_node_entry {
+ xfs_dahash_t hashval;
+ xfs_dablk_t before;
+ } btree[1];
+ };
+
+**info**
+ Directory/attribute block info. The magic number is XFS\_DA3\_NODE\_MAGIC
+ (0x3ebe).
+
+**count**
+ Number of node entries in this block.
+
+**level**
+ The level of this block in the B+tree. Levels start at 1 for blocks that
+ point to directory or attribute data blocks, and increase towards the
+ root.
+
+**pad32**
+ Padding to maintain alignment.
+
+**hashval**
+ The hash value of a particular record.
+
+**before**
+ The directory/attribute logical block containing all entries up to the
+ corresponding hash value.
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index 8a2173908b0e..546968699a56 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -4,3 +4,4 @@ Global Structures
=================
.. include:: btrees.rst
+.. include:: dabtrees.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 11/22] docs: add XFS allocation group metadata to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (8 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 10/22] docs: add XFS dir/attr btree structure " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 12/22] docs: add XFS reverse mapping structures " Darrick J. Wong
` (11 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/allocation_groups.rst | 1381 ++++++++++++++++++++
.../filesystems/xfs-data-structures/globals.rst | 1
2 files changed, 1382 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/allocation_groups.rst
diff --git a/Documentation/filesystems/xfs-data-structures/allocation_groups.rst b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
new file mode 100644
index 000000000000..30d169ab5cc5
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
@@ -0,0 +1,1381 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Allocation Groups
+-----------------
+
+As mentioned earlier, XFS filesystems are divided into a number of equally
+sized chunks called Allocation Groups. Each AG can almost be thought of as an
+individual filesystem that maintains its own space usage. Each AG can be up to
+one terabyte in size (512 bytes × 2\ :sup:`31`), regardless of the underlying
+device’s sector size.
+
+Each AG has the following characteristics:
+
+- A super block describing overall filesystem info
+
+- Free space management
+
+- Inode allocation and tracking
+
+- Reverse block-mapping index (optional)
+
+- Data block reference count index (optional)
+
+Having multiple AGs allows XFS to handle most operations in parallel without
+degrading performance as the number of concurrent accesses increases.
+
+The only global information maintained by the first AG (primary) is free space
+across the filesystem and total inode counts. If the
+XFS\_SB\_VERSION2\_LAZYSBCOUNTBIT flag is set in the superblock, these are
+only updated on-disk when the filesystem is cleanly unmounted (umount or
+shutdown).
+
+Immediately after a mkfs.xfs, the primary AG has the following disk layout;
+the subsequent AGs do not have any inodes allocated:
+
+.. figure:: images/6.png
+ :alt: Allocation group layout
+
+ Allocation group layout
+
+Each of these structures are expanded upon in the following sections.
+
+Superblocks
+~~~~~~~~~~~
+
+Each AG starts with a superblock. The first one, in AG 0, is the primary
+superblock which stores aggregate AG information. Secondary superblocks are
+only used by xfs\_repair when the primary superblock has been corrupted. A
+superblock is one sector in length.
+
+The superblock is defined by the following structure. The description of each
+field follows.
+
+.. code:: c
+
+ struct xfs_sb
+ {
+ __uint32_t sb_magicnum;
+ __uint32_t sb_blocksize;
+ xfs_rfsblock_t sb_dblocks;
+ xfs_rfsblock_t sb_rblocks;
+ xfs_rtblock_t sb_rextents;
+ uuid_t sb_uuid;
+ xfs_fsblock_t sb_logstart;
+ xfs_ino_t sb_rootino;
+ xfs_ino_t sb_rbmino;
+ xfs_ino_t sb_rsumino;
+ xfs_agblock_t sb_rextsize;
+ xfs_agblock_t sb_agblocks;
+ xfs_agnumber_t sb_agcount;
+ xfs_extlen_t sb_rbmblocks;
+ xfs_extlen_t sb_logblocks;
+ __uint16_t sb_versionnum;
+ __uint16_t sb_sectsize;
+ __uint16_t sb_inodesize;
+ __uint16_t sb_inopblock;
+ char sb_fname[12];
+ __uint8_t sb_blocklog;
+ __uint8_t sb_sectlog;
+ __uint8_t sb_inodelog;
+ __uint8_t sb_inopblog;
+ __uint8_t sb_agblklog;
+ __uint8_t sb_rextslog;
+ __uint8_t sb_inprogress;
+ __uint8_t sb_imax_pct;
+ __uint64_t sb_icount;
+ __uint64_t sb_ifree;
+ __uint64_t sb_fdblocks;
+ __uint64_t sb_frextents;
+ xfs_ino_t sb_uquotino;
+ xfs_ino_t sb_gquotino;
+ __uint16_t sb_qflags;
+ __uint8_t sb_flags;
+ __uint8_t sb_shared_vn;
+ xfs_extlen_t sb_inoalignmt;
+ __uint32_t sb_unit;
+ __uint32_t sb_width;
+ __uint8_t sb_dirblklog;
+ __uint8_t sb_logsectlog;
+ __uint16_t sb_logsectsize;
+ __uint32_t sb_logsunit;
+ __uint32_t sb_features2;
+ __uint32_t sb_bad_features2;
+
+ /* version 5 superblock fields start here */
+ __uint32_t sb_features_compat;
+ __uint32_t sb_features_ro_compat;
+ __uint32_t sb_features_incompat;
+ __uint32_t sb_features_log_incompat;
+
+ __uint32_t sb_crc;
+ xfs_extlen_t sb_spino_align;
+
+ xfs_ino_t sb_pquotino;
+ xfs_lsn_t sb_lsn;
+ uuid_t sb_meta_uuid;
+ xfs_ino_t sb_rrmapino;
+ };
+
+**sb\_magicnum**
+ Identifies the filesystem. Its value is XFS\_SB\_MAGIC "XFSB"
+ (0x58465342).
+
+**sb\_blocksize**
+ The size of a basic unit of space allocation in bytes. Typically, this is
+ 4096 (4KB) but can range from 512 to 65536 bytes.
+
+**sb\_dblocks**
+ Total number of blocks available for data and metadata on the filesystem.
+
+**sb\_rblocks**
+ Number blocks in the real-time disk device. Refer to `real-time
+ sub-volumes <#real-time-devices>`__ for more information.
+
+**sb\_rextents**
+ Number of extents on the real-time device.
+
+**sb\_uuid**
+ UUID (Universally Unique ID) for the filesystem. Filesystems can be
+ mounted by the UUID instead of device name.
+
+**sb\_logstart**
+ First block number for the journaling log if the log is internal (ie. not
+ on a separate disk device). For an external log device, this will be zero
+ (the log will also start on the first block on the log device). The
+ identity of the log devices is not recorded in the filesystem, but the
+ UUIDs of the filesystem and the log device are compared to prevent
+ corruption.
+
+**sb\_rootino**
+ Root inode number for the filesystem. Normally, the root inode is at the
+ start of the first possible inode chunk in AG 0. This is 128 when using a
+ 4KB block size.
+
+**sb\_rbmino**
+ Bitmap inode for real-time extents.
+
+**sb\_rsumino**
+ Summary inode for real-time bitmap.
+
+**sb\_rextsize**
+ Realtime extent size in blocks.
+
+**sb\_agblocks**
+ Size of each AG in blocks. For the actual size of the last AG, refer to
+ the `free space <#ag-free-space-management>`__ agf\_length value.
+
+**sb\_agcount**
+ Number of AGs in the filesystem.
+
+**sb\_rbmblocks**
+ Number of real-time bitmap blocks.
+
+**sb\_logblocks**
+ Number of blocks for the journaling log.
+
+**sb\_versionnum**
+ Filesystem version number. This is a bitmask specifying the features
+ enabled when creating the filesystem. Any disk checking tools or drivers
+ that do not recognize any set bits must not operate upon the filesystem.
+ Most of the flags indicate features introduced over time. If the value of
+ the lower nibble is >= 4, the higher bits indicate feature flags as
+ follows:
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_SB_VERSION_ATTRBIT
+ - Set if any inode have extended attributes. If this bit is set; the
+ XFS_SB_VERSION2_ATTR2BIT is not set; and the ``attr2`` mount flag is not
+ specified, the ``di_forkoff`` inode field will not be dynamically
+ adjusted. See the section about `extended attribute versions
+ <#extended-attribute-versions>`__ for more information.
+
+ * - XFS_SB_VERSION_NLINKBIT
+ - Set if any inodes use 32-bit di_nlink values.
+
+ * - XFS_SB_VERSION_QUOTABIT
+ - Quotas are enabled on the filesystem. This also brings in the various
+ quota fields in the superblock.
+
+ * - XFS_SB_VERSION_ALIGNBIT
+ - Set if sb_inoalignmt is used.
+
+ * - XFS_SB_VERSION_DALIGNBIT
+ - Set if sb_unit and sb_width are used.
+
+ * - XFS_SB_VERSION_SHAREDBIT
+ - Set if sb_shared_vn is used.
+
+ * - XFS_SB_VERSION_LOGV2BIT
+ - Version 2 journaling logs are used.
+
+ * - XFS_SB_VERSION_SECTORBIT
+ - Set if sb_sectsize is not 512.
+
+ * - XFS_SB_VERSION_EXTFLGBIT
+ - Unwritten extents are used. This is always set.
+
+ * - XFS_SB_VERSION_DIRV2BIT
+ - Version 2 directories are used. This is always set.
+
+ * - XFS_SB_VERSION_MOREBITSBIT
+ - Set if the sb_features2 field in the superblock contains more flags.
+
+Table: Version 4 Superblock version flags
+
+If the lower nibble of this value is 5, then this is a v5 filesystem; the
+XFS\_SB\_VERSION2\_CRCBIT feature must be set in sb\_features2.
+
+**sb\_sectsize**
+ Specifies the underlying disk sector size in bytes. Typically this is 512
+ or 4096 bytes. This determines the minimum I/O alignment, especially for
+ direct I/O.
+
+**sb\_inodesize**
+ Size of the inode in bytes. The default is 256 (2 inodes per standard
+ sector) but can be made as large as 2048 bytes when creating the
+ filesystem. On a v5 filesystem, the default and minimum inode size are
+ both 512 bytes.
+
+**sb\_inopblock**
+ Number of inodes per block. This is equivalent to sb\_blocksize /
+ sb\_inodesize.
+
+**sb\_fname[12]**
+ Name for the filesystem. This value can be used in the mount command.
+
+**sb\_blocklog**
+ log\ :sub:`2` value of sb\_blocksize. In other terms, sb\_blocksize =
+ 2^sb\_blocklog^.
+
+**sb\_sectlog**
+ log\ :sub:`2` value of sb\_sectsize.
+
+**sb\_inodelog**
+ log\ :sub:`2` value of sb\_inodesize.
+
+**sb\_inopblog**
+ log\ :sub:`2` value of sb\_inopblock.
+
+**sb\_agblklog**
+ log\ :sub:`2` value of sb\_agblocks (rounded up). This value is used to
+ generate inode numbers and absolute block numbers defined in extent maps.
+
+**sb\_rextslog**
+ log\ :sub:`2` value of sb\_rextents.
+
+**sb\_inprogress**
+ Flag specifying that the filesystem is being created.
+
+**sb\_imax\_pct**
+ Maximum percentage of filesystem space that can be used for inodes. The
+ default value is 5%.
+
+**sb\_icount**
+ Global count for number inodes allocated on the filesystem. This is only
+ maintained in the first superblock.
+
+**sb\_ifree**
+ Global count of free inodes on the filesystem. This is only maintained in
+ the first superblock.
+
+**sb\_fdblocks**
+ Global count of free data blocks on the filesystem. This is only
+ maintained in the first superblock.
+
+**sb\_frextents**
+ Global count of free real-time extents on the filesystem. This is only
+ maintained in the first superblock.
+
+**sb\_uquotino**
+ Inode for user quotas. This and the following two quota fields only apply
+ if XFS\_SB\_VERSION\_QUOTABIT flag is set in sb\_versionnum. Refer to
+ `quota inodes <#quota-inodes>`__ for more information
+
+**sb\_gquotino**
+ Inode for group or project quotas. Group and Project quotas cannot be used
+ at the same time.
+
+**sb\_qflags**
+ Quota flags. It can be a combination of the following flags:
+
+.. list-table::
+ :widths: 20 60
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_UQUOTA_ACCT
+ - User quota accounting is enabled.
+
+ * - XFS_UQUOTA_ENFD
+ - User quotas are enforced.
+
+ * - XFS_UQUOTA_CHKD
+ - User quotas have been checked.
+
+ * - XFS_PQUOTA_ACCT
+ - Project quota accounting is enabled.
+
+ * - XFS_OQUOTA_ENFD
+ - Other (group/project) quotas are enforced.
+
+ * - XFS_OQUOTA_CHKD
+ - Other (group/project) quotas have been checked.
+
+ * - XFS_GQUOTA_ACCT
+ - Group quota accounting is enabled.
+
+ * - XFS_GQUOTA_ENFD
+ - Group quotas are enforced.
+
+ * - XFS_GQUOTA_CHKD
+ - Group quotas have been checked.
+
+ * - XFS_PQUOTA_ENFD
+ - Project quotas are enforced.
+
+ * - XFS_PQUOTA_CHKD
+ - Project quotas have been checked.
+
+Table: Superblock quota flags
+
+**sb\_flags**
+ Miscellaneous flags.
+
+.. list-table::
+ :widths: 20 60
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_SBF_READONLY
+ - Only read-only mounts allowed.
+
+Table: Superblock flags
+
+**sb\_shared\_vn**
+ Reserved and must be zero ("vn" stands for version number).
+
+**sb\_inoalignmt**
+ Inode chunk alignment in fsblocks. Prior to v5, the default value provided
+ for inode chunks to have an 8KiB alignment. Starting with v5, the default
+ value scales with the multiple of the inode size over 256 bytes.
+ Concretely, this means an alignment of 16KiB for 512-byte inodes, 32KiB
+ for 1024-byte inodes, etc. If sparse inodes are enabled, the ir\_startino
+ field of each inode B+tree record must be aligned to this block
+ granularity, even if the inode given by ir\_startino itself is sparse.
+
+**sb\_unit**
+ Underlying stripe or raid unit in blocks.
+
+**sb\_width**
+ Underlying stripe or raid width in blocks.
+
+**sb\_dirblklog**
+ log\ :sub:`2` multiplier that determines the granularity of directory
+ block allocations in fsblocks.
+
+**sb\_logsectlog**
+ log\ :sub:`2` value of the log subvolume’s sector size. This is only used
+ if the journaling log is on a separate disk device (i.e. not internal).
+
+**sb\_logsectsize**
+ The log’s sector size in bytes if the filesystem uses an external log
+ device.
+
+**sb\_logsunit**
+ The log device’s stripe or raid unit size. This only applies to version 2
+ logs XFS\_SB\_VERSION\_LOGV2BIT is set in sb\_versionnum.
+
+**sb\_features2**
+ Additional version flags if XFS\_SB\_VERSION\_MOREBITSBIT is set in
+ sb\_versionnum. The currently defined additional features include:
+
+.. list-table::
+ :widths: 32 48
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_SB_VERSION2_LAZYSBCOUNTBIT
+ - Lazy global counters. Making a filesystem with this bit set can improve
+ performance. The global free space and inode counts are only updated in
+ the primary superblock when the filesystem is cleanly unmounted.
+
+ * - XFS_SB_VERSION2_ATTR2BIT
+ - Extended attributes version 2. Making a filesystem with this optimises
+ the inode layout of extended attributes. If this bit is set and the
+ +noattr2+ mount flag is not specified, the +di_forkoff+ inode field will
+ be dynamically adjusted. See the section about `extended attribute
+ versions <#extended-attribute-versions>`__ for more information.
+
+ * - XFS_SB_VERSION2_PARENTBIT
+ - Parent pointers. All inodes must have an extended attribute that points
+ back to its parent inode. The primary purpose for this information is
+ in backup systems. This feature bit refers to the IRIX parent pointer
+ implementation.
+
+ * - XFS_SB_VERSION2_PROJID32BIT
+ - 32-bit Project ID. Inodes can be associated with a project ID number,
+ which can be used to enforce disk space usage quotas for a particular
+ group of directories. This flag indicates that project IDs can be 32
+ bits in size.
+
+ * - XFS_SB_VERSION2_CRCBIT
+ - Metadata checksumming. All metadata blocks have an extended header
+ containing the block checksum, a copy of the metadata UUID, the log
+ sequence number of the last update to prevent stale replays, and a back
+ pointer to the owner of the block. This feature must be and can only be
+ set if the lowest nibble of ``sb_versionnum`` is set to 5.
+
+ * - XFS_SB_VERSION2_FTYPE
+ - Directory file type. Each directory entry records the type of the inode
+ to which the entry points. This speeds up directory iteration by
+ removing the need to load every inode into memory.
+
+Table: Extended Version 4 Superblock flags
+
+**sb\_bad\_features2**
+ This field mirrors sb\_features2, due to past 64-bit alignment errors.
+
+**sb\_features\_compat**
+ Read-write compatible feature flags. The kernel can still read and write
+ this FS even if it doesn’t understand the flag. Currently, there are no
+ valid flags.
+
+**sb\_features\_ro\_compat**
+ Read-only compatible feature flags. The kernel can still read this FS even
+ if it doesn’t understand the flag.
+
+.. list-table::
+ :widths: 32 48
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_SB_FEAT_RO_COMPAT_FINOBT
+ - Free inode B+tree. Each allocation group contains a B+tree to track
+ inode chunks containing free inodes. This is a performance optimization
+ to reduce the time required to allocate inodes.
+
+ * - XFS_SB_FEAT_RO_COMPAT_RMAPBT
+ - Reverse mapping B+tree. Each allocation group contains a B+tree
+ containing records mapping AG blocks to their owners. See the section
+ about `online repairs <#metadata-reconstruction>`__ for more details.
+
+ * - XFS_SB_FEAT_RO_COMPAT_REFLINK
+ - Reference count B+tree. Each allocation group contains a B+tree to
+ track the reference counts of AG blocks. This enables files to share
+ data blocks safely. See the section about `reflink and deduplication
+ <#sharing-data-blocks>`__ for more details.
+
+Table: Extended Version 5 Superblock Read-Only compatibility flags
+
+**sb\_features\_incompat**
+ Read-write incompatible feature flags. The kernel cannot read or write
+ this FS if it doesn’t understand the flag.
+
+.. list-table::
+ :widths: 32 48
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_SB_FEAT_INCOMPAT_FTYPE
+ - Directory file type. Each directory entry tracks the type of the inode
+ to which the entry points. This is a performance optimization to remove
+ the need to load every inode into memory to iterate a directory.
+
+ * - XFS_SB_FEAT_INCOMPAT_SPINODES
+ - Sparse inodes. This feature relaxes the requirement to allocate inodes
+ in chunks of 64. When the free space is heavily fragmented, there might
+ exist plenty of free space but not enough contiguous free space to
+ allocate a new inode chunk. With this feature, the user can continue to
+ create files until all free space is exhausted.
+
+ Unused space in the inode B+tree records are used to track which parts
+ of the inode chunk are not inodes.
+
+ See the chapter on `sparse inodes <#sparse-inodes>`__ for more
+ information.
+
+ * - XFS_SB_FEAT_INCOMPAT_META_UUID
+ - Metadata UUID. The UUID stamped into each metadata block must match the
+ value in ``sb_meta_uuid``. This enables the administrator to change
+ ``sb_uuid`` at will without having to rewrite the entire filesystem.
+
+Table: Extended Version 5 Superblock Read-Write incompatibility flags
+
+**sb\_features\_log\_incompat**
+ Read-write incompatible feature flags for the log. The kernel cannot read
+ or write this FS log if it doesn’t understand the flag. Currently, no
+ flags are defined.
+
+**sb\_crc**
+ Superblock checksum.
+
+**sb\_spino\_align**
+ Sparse inode alignment, in fsblocks. Each chunk of inodes referenced by a
+ sparse inode B+tree record must be aligned to this block granularity.
+
+**sb\_pquotino**
+ Project quota inode.
+
+**sb\_lsn**
+ Log sequence number of the last superblock update.
+
+**sb\_meta\_uuid**
+ If the XFS\_SB\_FEAT\_INCOMPAT\_META\_UUID feature is set, then the UUID
+ field in all metadata blocks must match this UUID. If not, the block
+ header UUID field must match sb\_uuid.
+
+**sb\_rrmapino**
+ If the XFS\_SB\_FEAT\_RO\_COMPAT\_RMAPBT feature is set and a real-time
+ device is present (sb\_rblocks > 0), this field points to an inode that
+ contains the root to the `Real-Time Reverse Mapping B+tree
+ <#real-time-reverse-mapping-b-tree>`__. This field is zero otherwise.
+
+xfs\_db Superblock Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A filesystem is made on a single disk with the following command:
+
+::
+
+ # mkfs.xfs -i attr=2 -n size=16384 -f /dev/sda7
+ meta-data=/dev/sda7 isize=256 agcount=16, agsize=3923122 blks
+ = sectsz=512 attr=2
+ data = bsize=4096 blocks=62769952, imaxpct=25
+ = sunit=0 swidth=0 blks, unwritten=1
+ naming =version 2 bsize=16384
+ log =internal log bsize=4096 blocks=30649, version=1
+ = sectsz=512 sunit=0 blks
+ realtime =none extsz=65536 blocks=0, rtextents=0
+
+And in xfs\_db, inspecting the superblock:
+
+::
+
+ xfs_db> sb
+ xfs_db> p
+ magicnum = 0x58465342
+ blocksize = 4096
+ dblocks = 62769952
+ rblocks = 0
+ rextents = 0
+ uuid = 32b24036-6931-45b4-b68c-cd5e7d9a1ca5
+ logstart = 33554436
+ rootino = 128
+ rbmino = 129
+ rsumino = 130
+ rextsize = 16
+ agblocks = 3923122
+ agcount = 16
+ rbmblocks = 0
+ logblocks = 30649
+ versionnum = 0xb084
+ sectsize = 512
+ inodesize = 256
+ inopblock = 16
+ fname = "\000\000\000\000\000\000\000\000\000\000\000\000"
+ blocklog = 12
+ sectlog = 9
+ inodelog = 8
+ inopblog = 4
+ agblklog = 22
+ rextslog = 0
+ inprogress = 0
+ imax_pct = 25
+ icount = 64
+ ifree = 61
+ fdblocks = 62739235
+ frextents = 0
+ uquotino = 0
+ gquotino = 0
+ qflags = 0
+ flags = 0
+ shared_vn = 0
+ inoalignmt = 2
+ unit = 0
+ width = 0
+ dirblklog = 2
+ logsectlog = 0
+ logsectsize = 0
+ logsunit = 0
+ features2 = 8
+
+AG Free Space Management
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The XFS filesystem tracks free space in an allocation group using two B+trees.
+One B+tree tracks space by block number, the second by the size of the free
+space block. This scheme allows XFS to find quickly free space near a given
+block or of a given size.
+
+All block numbers, indexes, and counts are AG relative.
+
+AG Free Space Block
+^^^^^^^^^^^^^^^^^^^
+
+The second sector in an AG contains the information about the two free space
+B+trees and associated free space information for the AG. The "AG Free
+Space Block" also knows as the AGF, uses the following structure:
+
+.. code:: c
+
+ struct xfs_agf {
+ __be32 agf_magicnum;
+ __be32 agf_versionnum;
+ __be32 agf_seqno;
+ __be32 agf_length;
+ __be32 agf_roots[XFS_BTNUM_AGF];
+ __be32 agf_levels[XFS_BTNUM_AGF];
+ __be32 agf_flfirst;
+ __be32 agf_fllast;
+ __be32 agf_flcount;
+ __be32 agf_freeblks;
+ __be32 agf_longest;
+ __be32 agf_btreeblks;
+
+ /* version 5 filesystem fields start here */
+ uuid_t agf_uuid;
+ __be32 agf_rmap_blocks;
+ __be32 agf_refcount_blocks;
+ __be32 agf_refcount_root;
+ __be32 agf_refcount_level;
+ __be64 agf_spare64[14];
+
+ /* unlogged fields, written during buffer writeback. */
+ __be64 agf_lsn;
+ __be32 agf_crc;
+ __be32 agf_spare2;
+ };
+
+The rest of the bytes in the sector are zeroed. XFS\_BTNUM\_AGF is set to 3:
+index 0 for the free space B+tree indexed by block number; index 1 for the
+free space B+tree indexed by extent size; and index 2 for the reverse-mapping
+B+tree.
+
+**agf\_magicnum**
+ Specifies the magic number for the AGF sector: "XAGF" (0x58414746).
+
+**agf\_versionnum**
+ Set to XFS\_AGF\_VERSION which is currently 1.
+
+**agf\_seqno**
+ Specifies the AG number for the sector.
+
+**agf\_length**
+ Specifies the size of the AG in filesystem blocks. For all AGs except the
+ last, this must be equal to the superblock’s sb\_agblocks value. For the
+ last AG, this could be less than the sb\_agblocks value. It is this value
+ that should be used to determine the size of the AG.
+
+**agf\_roots**
+ Specifies the block number for the root of the two free space B+trees and
+ the reverse-mapping B+tree, if enabled.
+
+**agf\_levels**
+ Specifies the level or depth of the two free space B+trees and the
+ reverse-mapping B+tree, if enabled. For a fresh AG, this value will be
+ one, and the "roots" will point to a single leaf of level 0.
+
+**agf\_flfirst**
+ Specifies the index of the first "free list" block. Free lists are
+ covered in more detail later on.
+
+**agf\_fllast**
+ Specifies the index of the last "free list" block.
+
+**agf\_flcount**
+ Specifies the number of blocks in the "free list".
+
+**agf\_freeblks**
+ Specifies the current number of free blocks in the AG.
+
+**agf\_longest**
+ Specifies the number of blocks of longest contiguous free space in the AG.
+
+**agf\_btreeblks**
+ Specifies the number of blocks used for the free space B+trees. This is
+ only used if the XFS\_SB\_VERSION2\_LAZYSBCOUNTBIT bit is set in
+ sb\_features2.
+
+**agf\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**agf\_rmap\_blocks**
+ The size of the reverse mapping B+tree in this allocation group, in
+ blocks.
+
+**agf\_refcount\_blocks**
+ The size of the reference count B+tree in this allocation group, in
+ blocks.
+
+**agf\_refcount\_root**
+ Block number for the root of the reference count B+tree, if enabled.
+
+**agf\_refcount\_level**
+ Depth of the reference count B+tree, if enabled.
+
+**agf\_spare64**
+ Empty space in the logged part of the AGF sector, for use for future
+ features.
+
+**agf\_lsn**
+ Log sequence number of the last AGF write.
+
+**agf\_crc**
+ Checksum of the AGF sector.
+
+**agf\_spare2**
+ Empty space in the unlogged part of the AGF sector.
+
+AG Free Space B+trees
+^^^^^^^^^^^^^^^^^^^^^
+
+The two Free Space B+trees store a sorted array of block offset and block
+counts in the leaves of the B+tree. The first B+tree is sorted by the offset,
+the second by the count or size.
+
+Leaf nodes contain a sorted array of offset/count pairs which are also used
+for node keys:
+
+.. code:: c
+
+ struct xfs_alloc_rec {
+ __be32 ar_startblock;
+ __be32 ar_blockcount;
+ };
+
+**ar\_startblock**
+ AG block number of the start of the free space.
+
+**ar\_blockcount**
+ Length of the free space.
+
+Node pointers are an AG relative block pointer:
+
+.. code:: c
+
+ typedef __be32 xfs_alloc_ptr_t;
+
+- As the free space tracking is AG relative, all the block numbers are only
+ 32-bits.
+
+- The bb\_magic value depends on the B+tree: "ABTB" (0x41425442) for the block
+ offset B+tree, "ABTC" (0x41425443) for the block count B+tree. On a v5
+ filesystem, these are "AB3B" (0x41423342) and "AB3C" (0x41423343),
+ respectively.
+
+- The xfs\_btree\_sblock\_t header is used for intermediate B+tree node as
+ well as the leaves.
+
+- For a typical 4KB filesystem block size, the offset for the
+ xfs\_alloc\_ptr\_t array would be 0xab0 (2736 decimal).
+
+- There are a series of macros in xfs\_btree.h for deriving the offsets,
+ counts, maximums, etc for the B+trees used in XFS.
+
+The following diagram shows a single level B+tree which consists of one leaf:
+
+.. figure:: images/15a.png
+ :alt: Freespace B+tree with one leaf.
+
+ Freespace B+tree with one leaf.
+
+With the intermediate nodes, the associated leaf pointers are stored in a
+separate array about two thirds into the block. The following diagram
+illustrates a 2-level B+tree for a free space B+tree:
+
+.. figure:: images/15b.png
+ :alt: Multi-level freespace B+tree.
+
+ Multi-level freespace B+tree.
+
+AG Free List
+^^^^^^^^^^^^
+
+The AG Free List is located in the 4\ :sup:`th` sector of each AG and is known
+as the AGFL. It is an array of AG relative block pointers for reserved space
+for growing the free space B+trees. This space cannot be used for general user
+data including inodes, data, directories and extended attributes.
+
+With a freshly made filesystem, 4 blocks are reserved immediately after the
+free space B+tree root blocks (blocks 4 to 7). As they are used up as the free
+space fragments, additional blocks will be reserved from the AG and added to
+the free list array. This size may increase as features are added.
+
+As the free list array is located within a single sector, a typical device
+will have space for 128 elements in the array (512 bytes per sector, 4 bytes
+per AG relative block pointer). The actual size can be determined by using the
+XFS\_AGFL\_SIZE macro.
+
+Active elements in the array are specified by the `AGF’s
+<#ag-free-space-block>`__ agf\_flfirst, agf\_fllast and agf\_flcount values.
+The array is managed as a circular list.
+
+On a v5 filesystem, the following header precedes the free list entries:
+
+.. code:: c
+
+ struct xfs_agfl {
+ __be32 agfl_magicnum;
+ __be32 agfl_seqno;
+ uuid_t agfl_uuid;
+ __be64 agfl_lsn;
+ __be32 agfl_crc;
+ };
+
+**agfl\_magicnum**
+ Specifies the magic number for the AGFL sector: "XAFL" (0x5841464c).
+
+**agfl\_seqno**
+ Specifies the AG number for the sector.
+
+**agfl\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**agfl\_lsn**
+ Log sequence number of the last AGFL write.
+
+**agfl\_crc**
+ Checksum of the AGFL sector.
+
+On a v4 filesystem there is no header; the array of free block numbers begins
+at the beginning of the sector.
+
+.. figure:: images/16.png
+ :alt: AG Free List layout
+
+ AG Free List layout
+
+The presence of these reserved blocks guarantees that the free space B+trees
+can be updated if any blocks are freed by extent changes in a full AG.
+
+xfs\_db AGF Example
+"""""""""""""""""""
+
+These examples are derived from an AG that has been deliberately fragmented.
+The AGF:
+
+::
+
+ xfs_db> agf 0
+ xfs_db> p
+ magicnum = 0x58414746
+ versionnum = 1
+ seqno = 0
+ length = 3923122
+ bnoroot = 7
+ cntroot = 83343
+ bnolevel = 2
+ cntlevel = 2
+ flfirst = 22
+ fllast = 27
+ flcount = 6
+ freeblks = 3654234
+ longest = 3384327
+ btreeblks = 0
+
+In the AGFL, the active elements are from 22 to 27 inclusive which are
+obtained from the flfirst and fllast values from the agf in the previous
+example:
+
+::
+
+ xfs_db> agfl 0
+ xfs_db> p
+ bno[0-127] = 0:4 1:5 2:6 3:7 4:83342 5:83343 6:83344 7:83345 8:83346 9:83347
+ 10:4 11:5 12:80205 13:80780 14:81496 15:81766 16:83346 17:4 18:5
+ 19:80205 20:82449 21:81496 22:81766 23:82455 24:80780 25:5
+ 26:80205 27:83344
+
+The root block of the free space B+tree sorted by block offset is found in the
+AGF’s bnoroot value:
+
+::
+
+ xfs_db> fsblock 7
+ xfs_db> type bnobt
+ xfs_db> p
+ magic = 0x41425442
+ level = 1
+ numrecs = 4
+ leftsib = null
+ rightsib = null
+ keys[1-4] = [startblock,blockcount]
+ 1:[12,16] 2:[184586,3] 3:[225579,1] 4:[511629,1]
+ ptrs[1-4] = 1:2 2:83347 3:6 4:4
+
+Blocks 2, 83347, 6 and 4 contain the leaves for the free space B+tree by
+starting block. Block 2 would contain offsets 12 up to but not including
+184586 while block 4 would have all offsets from 511629 to the end of the AG.
+
+The root block of the free space B+tree sorted by block count is found in the
+AGF’s cntroot value:
+
+::
+
+ xfs_db> fsblock 83343
+ xfs_db> type cntbt
+ xfs_db> p
+ magic = 0x41425443
+ level = 1
+ numrecs = 4
+ leftsib = null
+ rightsib = null
+ keys[1-4] = [blockcount,startblock]
+ 1:[1,81496] 2:[1,511729] 3:[3,191875] 4:[6,184595]
+ ptrs[1-4] = 1:3 2:83345 3:83342 4:83346
+
+The leaf in block 3, in this example, would only contain single block counts.
+The offsets are sorted in ascending order if the block count is the same.
+
+Inspecting the leaf in block 83346, we can see the largest block at the end:
+
+::
+
+ xfs_db> fsblock 83346
+ xfs_db> type cntbt
+ xfs_db> p
+ magic = 0x41425443
+ level = 0
+ numrecs = 344
+ leftsib = 83342
+ rightsib = null
+ recs[1-344] = [startblock,blockcount]
+ 1:[184595,6] 2:[187573,6] 3:[187776,6]
+ ...
+ 342:[513712,755] 343:[230317,258229] 344:[538795,3384327]
+
+The longest block count (3384327) must be the same as the AGF’s longest value.
+
+AG Inode Management
+~~~~~~~~~~~~~~~~~~~
+
+Inode Numbers
+^^^^^^^^^^^^^
+
+Inode numbers in XFS come in two forms: AG relative and absolute.
+
+AG relative inode numbers always fit within 32 bits. The number of bits
+actually used is determined by the sum of the `superblock’s <#superblocks>`__
+sb\_inoplog and sb\_agblklog values. Relative inode numbers are found within
+the AG’s inode structures.
+
+Absolute inode numbers include the AG number in the high bits, above the bits
+used for the AG relative inode number. Absolute inode numbers are found in
+`directory <#directories>`__ entries and the superblock.
+
+.. figure:: images/18.png
+ :alt: Inode number formats
+
+ Inode number formats
+
+Inode Information
+^^^^^^^^^^^^^^^^^
+
+Each AG manages its own inodes. The third sector in the AG contains
+information about the AG’s inodes and is known as the AGI.
+
+The AGI uses the following structure:
+
+.. code:: c
+
+ struct xfs_agi {
+ __be32 agi_magicnum;
+ __be32 agi_versionnum;
+ __be32 agi_seqno
+ __be32 agi_length;
+ __be32 agi_count;
+ __be32 agi_root;
+ __be32 agi_level;
+ __be32 agi_freecount;
+ __be32 agi_newino;
+ __be32 agi_dirino;
+ __be32 agi_unlinked[64];
+
+ /*
+ * v5 filesystem fields start here; this marks the end of logging region 1
+ * and start of logging region 2.
+ */
+ uuid_t agi_uuid;
+ __be32 agi_crc;
+ __be32 agi_pad32;
+ __be64 agi_lsn;
+
+ __be32 agi_free_root;
+ __be32 agi_free_level;
+ }
+
+**agi\_magicnum**
+ Specifies the magic number for the AGI sector: "XAGI" (0x58414749).
+
+**agi\_versionnum**
+ Set to XFS\_AGI\_VERSION which is currently 1.
+
+**agi\_seqno**
+ Specifies the AG number for the sector.
+
+**agi\_length**
+ Specifies the size of the AG in filesystem blocks.
+
+**agi\_count**
+ Specifies the number of inodes allocated for the AG.
+
+**agi\_root**
+ Specifies the block number in the AG containing the root of the inode
+ B+tree.
+
+**agi\_level**
+ Specifies the number of levels in the inode B+tree.
+
+**agi\_freecount**
+ Specifies the number of free inodes in the AG.
+
+**agi\_newino**
+ Specifies AG-relative inode number of the most recently allocated chunk.
+
+**agi\_dirino**
+ Deprecated and not used, this is always set to NULL (-1).
+
+**agi\_unlinked[64]**
+ Hash table of unlinked (deleted) inodes that are still being referenced.
+ Refer to `unlinked list pointers <#unlinked-pointer>`__ for more
+ information.
+
+**agi\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**agi\_crc**
+ Checksum of the AGI sector.
+
+**agi\_pad32**
+ Padding field, otherwise unused.
+
+**agi\_lsn**
+ Log sequence number of the last write to this block.
+
+**agi\_free\_root**
+ Specifies the block number in the AG containing the root of the free inode
+ B+tree.
+
+**agi\_free\_level**
+ Specifies the number of levels in the free inode B+tree.
+
+Inode B+trees
+~~~~~~~~~~~~~
+
+Inodes are traditionally allocated in chunks of 64, and a B+tree is used to
+track these chunks of inodes as they are allocated and freed. The block
+containing root of the B+tree is defined by the AGI’s agi\_root value. If the
+XFS\_SB\_FEAT\_RO\_COMPAT\_FINOBT feature is enabled, a second B+tree is used
+to track the chunks containing free inodes; this is an optimization to speed
+up inode allocation.
+
+The B+tree header for the nodes and leaves use the xfs\_btree\_sblock
+structure which is the same as the header used in the `AGF
+B+trees <#ag-free-space-b-trees>`__.
+
+The magic number of the inode B+tree is "IABT" (0x49414254). On a v5
+filesystem, the magic number is "IAB3" (0x49414233).
+
+The magic number of the free inode B+tree is "FIBT" (0x46494254). On a v5
+filesystem, the magic number is "FIB3" (0x46494254).
+
+Leaves contain an array of the following structure:
+
+.. code:: c
+
+ struct xfs_inobt_rec {
+ __be32 ir_startino;
+ __be32 ir_freecount;
+ __be64 ir_free;
+ };
+
+**ir\_startino**
+ The lowest-numbered inode in this chunk.
+
+**ir\_freecount**
+ Number of free inodes in this chunk.
+
+**ir\_free**
+ A 64 element bitmap showing which inodes in this chunk are free.
+
+Nodes contain key/pointer pairs using the following types:
+
+.. code:: c
+
+ struct xfs_inobt_key {
+ __be32 ir_startino;
+ };
+ typedef __be32 xfs_inobt_ptr_t;
+
+The following diagram illustrates a single level inode B+tree:
+
+.. figure:: images/20a.png
+ :alt: Single Level inode B+tree
+
+ Single Level inode B+tree
+
+And a 2-level inode B+tree:
+
+.. figure:: images/20b.png
+ :alt: Multi-Level inode B+tree
+
+ Multi-Level inode B+tree
+
+xfs\_db AGI Example
+^^^^^^^^^^^^^^^^^^^
+
+This is an AGI of a freshly populated filesystem:
+
+::
+
+ xfs_db> agi 0
+ xfs_db> p
+ magicnum = 0x58414749
+ versionnum = 1
+ seqno = 0
+ length = 825457
+ count = 5440
+ root = 3
+ level = 1
+ freecount = 9
+ newino = 5792
+ dirino = null
+ unlinked[0-63] =
+ uuid = 3dfa1e5c-5a5f-4ca2-829a-000e453600fe
+ lsn = 0x1000032c2
+ crc = 0x14cb7e5c (correct)
+ free_root = 4
+ free_level = 1
+
+From this example, we see that the inode B+tree is rooted at AG block 3 and
+that the free inode B+tree is rooted at AG block 4. Let’s look at the inode
+B+tree:
+
+::
+
+ xfs_db> addr root
+ xfs_db> p
+ magic = 0x49414233
+ level = 0
+ numrecs = 85
+ leftsib = null
+ rightsib = null
+ bno = 24
+ lsn = 0x1000032c2
+ uuid = 3dfa1e5c-5a5f-4ca2-829a-000e453600fe
+ owner = 0
+ crc = 0x768f9592 (correct)
+ recs[1-85] = [startino,freecount,free]
+ 1:[96,0,0] 2:[160,0,0] 3:[224,0,0] 4:[288,0,0]
+ 5:[352,0,0] 6:[416,0,0] 7:[480,0,0] 8:[544,0,0]
+ 9:[608,0,0] 10:[672,0,0] 11:[736,0,0] 12:[800,0,0]
+ ...
+ 85:[5792,9,0xff80000000000000]
+
+Most of the inode chunks on this filesystem are totally full, since the free
+value is zero. This means that we ought to expect inode 160 to be linked
+somewhere in the directory structure. However, notice that 0xff80000000000000
+in record 85 — this means that we would expect inode 5856 to be free. Moving
+on to the free inode B+tree, we see that this is indeed the case:
+
+::
+
+ xfs_db> addr free_root
+ xfs_db> p
+ magic = 0x46494233
+ level = 0
+ numrecs = 1
+ leftsib = null
+ rightsib = null
+ bno = 32
+ lsn = 0x1000032c2
+ uuid = 3dfa1e5c-5a5f-4ca2-829a-000e453600fe
+ owner = 0
+ crc = 0x338af88a (correct)
+ recs[1] = [startino,freecount,free] 1:[5792,9,0xff80000000000000]
+
+Observe also that the AGI’s agi\_newino points to this chunk, which has never
+been fully allocated.
+
+Sparse Inodes
+^^^^^^^^^^^^^
+
+As mentioned in the previous section, XFS allocates inodes in chunks of 64. If
+there are no free extents large enough to hold a full chunk of 64 inodes, the
+inode allocation fails and XFS claims to have run out of space. On a
+filesystem with highly fragmented free space, this can lead to out of space
+errors long before the filesystem runs out of free blocks.
+
+The sparse inode feature tracks inode chunks in the inode B+tree as if they
+were full chunks but uses some previously unused bits in the freecount field
+to track which parts of the inode chunk are not allocated for use as inodes.
+This allows XFS to allocate inodes one block at a time if absolutely
+necessary.
+
+The inode and free inode B+trees operate in the same manner as they do without
+the sparse inode feature; the B+tree header for the nodes and leaves use the
+xfs\_btree\_sblock structure which is the same as the header used in the `AGF
+B+trees <#ag-free-space-b-trees>`__.
+
+It is theoretically possible for a sparse inode B+tree record to reference
+multiple non-contiguous inode chunks.
+
+Leaves contain an array of the following structure:
+
+.. code:: c
+
+ struct xfs_inobt_rec {
+ __be32 ir_startino;
+ __be16 ir_holemask;
+ __u8 ir_count;
+ __u8 ir_freecount;
+ __be64 ir_free;
+ };
+
+**ir\_startino**
+ The lowest-numbered inode in this chunk, rounded down to the nearest
+ multiple of 64, even if the start of this chunk is sparse.
+
+**ir\_holemask**
+ A 16 element bitmap showing which parts of the chunk are not allocated to
+ inodes. Each bit represents four inodes; if a bit is marked here, the
+ corresponding bits in ir\_free must also be marked.
+
+**ir\_count**
+ Number of inodes allocated to this chunk.
+
+**ir\_freecount**
+ Number of free inodes in this chunk.
+
+**ir\_free**
+ A 64 element bitmap showing which inodes in this chunk are not available
+ for allocation.
+
+xfs\_db Sparse Inode AGI Example
+""""""""""""""""""""""""""""""""
+
+This example derives from an AG that has been deliberately fragmented. The
+inode B+tree:
+
+::
+
+ xfs_db> agi 0
+ xfs_db> p
+ magicnum = 0x58414749
+ versionnum = 1
+ seqno = 0
+ length = 6400
+ count = 10432
+ root = 2381
+ level = 2
+ freecount = 0
+ newino = 14912
+ dirino = null
+ unlinked[0-63] =
+ uuid = b9b4623b-f678-4d48-8ce7-ce08950e3cd6
+ lsn = 0x600000ac4
+ crc = 0xef550dbc (correct)
+ free_root = 4
+ free_level = 1
+
+This AGI was formatted on a v5 filesystem; notice the extra v5 fields. So far
+everything else looks much the same as always.
+
+::
+
+ xfs_db> addr root
+ magic = 0x49414233
+ level = 1
+ numrecs = 2
+ leftsib = null
+ rightsib = null
+ bno = 19048
+ lsn = 0x50000192b
+ uuid = b9b4623b-f678-4d48-8ce7-ce08950e3cd6
+ owner = 0
+ crc = 0xd98cd2ca (correct)
+ keys[1-2] = [startino] 1:[128] 2:[35136]
+ ptrs[1-2] = 1:3 2:2380
+ xfs_db> addr ptrs[1]
+ xfs_db> p
+ magic = 0x49414233
+ level = 0
+ numrecs = 159
+ leftsib = null
+ rightsib = 2380
+ bno = 24
+ lsn = 0x600000ac4
+ uuid = b9b4623b-f678-4d48-8ce7-ce08950e3cd6
+ owner = 0
+ crc = 0x836768a6 (correct)
+ recs[1-159] = [startino,holemask,count,freecount,free]
+ 1:[128,0,64,0,0]
+ 2:[14912,0xff,32,0,0xffffffff]
+ 3:[15040,0,64,0,0]
+ 4:[15168,0xff00,32,0,0xffffffff00000000]
+ 5:[15296,0,64,0,0]
+ 6:[15424,0xff,32,0,0xffffffff]
+ 7:[15552,0,64,0,0]
+ 8:[15680,0xff00,32,0,0xffffffff00000000]
+ 9:[15808,0,64,0,0]
+ 10:[15936,0xff,32,0,0xffffffff]
+
+Here we see the difference in the inode B+tree records. For example, in record
+2, we see that the holemask has a value of 0xff. This means that the first
+sixteen inodes in this chunk record do not actually map to inode blocks; the
+first inode in this chunk is actually inode 14944:
+
+::
+
+ xfs_db> inode 14912
+ Metadata corruption detected at block 0x3a40/0x2000
+ ...
+ Metadata CRC error detected for ino 14912
+ xfs_db> p core.magic
+ core.magic = 0
+ xfs_db> inode 14944
+ xfs_db> p core.magic
+ core.magic = 0x494e
+
+The chunk record also indicates that this chunk has 32 inodes, and that the
+missing inodes are also "free".
+
+Real-time Devices
+~~~~~~~~~~~~~~~~~
+
+The performance of the standard XFS allocator varies depending on the internal
+state of the various metadata indices enabled on the filesystem. For
+applications which need to minimize the jitter of allocation latency, XFS
+supports the notion of a "real-time device". This is a special device
+separate from the regular filesystem where extent allocations are tracked with
+a bitmap and free space is indexed with a two-dimensional array. If an inode
+is flagged with XFS\_DIFLAG\_REALTIME, its data will live on the real time
+device. The metadata for real time devices is discussed in the section about
+`real time inodes <#real-time-inodes>`__.
+
+By placing the real time device (and the journal) on separate high-performance
+storage devices, it is possible to reduce most of the unpredictability in I/O
+response times that come from metadata operations.
+
+None of the XFS per-AG B+trees are involved with real time files. It is not
+possible for real time files to share data blocks.
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index 546968699a56..c91b1d24d6e7 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -5,3 +5,4 @@ Global Structures
.. include:: btrees.rst
.. include:: dabtrees.rst
+.. include:: allocation_groups.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 12/22] docs: add XFS reverse mapping structures to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (9 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 11/22] docs: add XFS allocation group metadata " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 13/22] docs: add XFS refcount btree structure to " Darrick J. Wong
` (10 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/allocation_groups.rst | 2
.../filesystems/xfs-data-structures/rmapbt.rst | 336 ++++++++++++++++++++
2 files changed, 338 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/rmapbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/allocation_groups.rst b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
index 30d169ab5cc5..6c0ffd3a170b 100644
--- a/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
+++ b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
@@ -1379,3 +1379,5 @@ response times that come from metadata operations.
None of the XFS per-AG B+trees are involved with real time files. It is not
possible for real time files to share data blocks.
+
+.. include:: rmapbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/rmapbt.rst b/Documentation/filesystems/xfs-data-structures/rmapbt.rst
new file mode 100644
index 000000000000..eefcee5d4e95
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/rmapbt.rst
@@ -0,0 +1,336 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Reverse-Mapping B+tree
+~~~~~~~~~~~~~~~~~~~~~~
+
+If the feature is enabled, each allocation group has its own reverse
+block-mapping B+tree, which grows in the free space like the free space
+B+trees. As mentioned in the chapter about
+`reconstruction <#metadata-reconstruction>`__, this data structure is another piece of
+the puzzle necessary to reconstruct the data or attribute fork of a file from
+reverse-mapping records; we can also use it to double-check allocations to
+ensure that we are not accidentally cross-linking blocks, which can cause
+severe damage to the filesystem.
+
+This B+tree is only present if the XFS\_SB\_FEAT\_RO\_COMPAT\_RMAPBT feature
+is enabled. The feature requires a version 5 filesystem.
+
+Each record in the reverse-mapping B+tree has the following structure:
+
+.. code:: c
+
+ struct xfs_rmap_rec {
+ __be32 rm_startblock;
+ __be32 rm_blockcount;
+ __be64 rm_owner;
+ __be64 rm_fork:1;
+ __be64 rm_bmbt:1;
+ __be64 rm_unwritten:1;
+ __be64 rm_unused:7;
+ __be64 rm_offset:54;
+ };
+
+**rm\_startblock**
+ AG block number of this record.
+
+**rm\_blockcount**
+ The length of this extent.
+
+**rm\_owner**
+ A 64-bit number describing the owner of this extent. This is typically the
+ absolute inode number, but can also correspond to one of the following:
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+ * - XFS\_RMAP\_OWN\_NULL
+ - No owner. This should never appear on disk.
+
+ * - XFS\_RMAP\_OWN\_UNKNOWN
+ - Unknown owner; for EFI recovery. This should never appear on disk.
+
+ * - XFS\_RMAP\_OWN\_FS
+ - Allocation group headers.
+
+ * - XFS\_RMAP\_OWN\_LOG
+ - XFS log blocks.
+
+ * - XFS\_RMAP\_OWN\_AG
+ - Per-allocation group B+tree blocks. This means free space B+tree blocks,
+ blocks on the freelist, and reverse-mapping B+tree blocks.
+
+ * - XFS\_RMAP\_OWN\_INOBT
+ - Per-allocation group inode B+tree blocks. This includes free inode
+ B+tree blocks.
+
+ * - XFS\_RMAP\_OWN\_INODES
+ - Inode chunks.
+
+ * - XFS\_RMAP\_OWN\_REFC
+ - Per-allocation group refcount B+tree blocks. This will be used for
+ reflink support.
+
+ * - XFS\_RMAP\_OWN\_COW
+ - Blocks that have been reserved for a copy-on-write operation that has
+ not completed.
+
+Table: Special owner values
+
+**rm\_fork**
+ If rm\_owner describes an inode, this can be 1 if this record is for an
+ attribute fork.
+
+**rm\_bmbt**
+ If rm\_owner describes an inode, this can be 1 to signify that this record
+ is for a block map B+tree block. In this case, rm\_offset has no meaning.
+
+**rm\_unwritten**
+ A flag indicating that the extent is unwritten. This corresponds to the
+ flag in the `extent record <#data-extents>`__ format which means
+ XFS\_EXT\_UNWRITTEN.
+
+**rm\_offset**
+ The 54-bit logical file block offset, if rm\_owner describes an inode.
+ Meaningless otherwise.
+
+ **Note**
+
+ The single-bit flag values rm\_unwritten, rm\_fork, and rm\_bmbt are
+ packed into the larger fields in the C structure definition.
+
+The key has the following structure:
+
+.. code:: c
+
+ struct xfs_rmap_key {
+ __be32 rm_startblock;
+ __be64 rm_owner;
+ __be64 rm_fork:1;
+ __be64 rm_bmbt:1;
+ __be64 rm_reserved:1;
+ __be64 rm_unused:7;
+ __be64 rm_offset:54;
+ };
+
+For the reverse-mapping B+tree on a filesystem that supports sharing of file
+data blocks, the key definition is larger than the usual AG block number. On a
+classic XFS filesystem, each block has only one owner, which means that
+rm\_startblock is sufficient to uniquely identify each record. However, shared
+block support (reflink) on XFS breaks that assumption; now filesystem blocks
+can be linked to any logical block offset of any file inode. Therefore, the
+key must include the owner and offset information to preserve the 1 to 1
+relation between key and record.
+
+- As the reference counting is AG relative, all the block numbers are only
+ 32-bits.
+
+- The bb\_magic value is "RMB3" (0x524d4233).
+
+- The xfs\_btree\_sblock\_t header is used for intermediate B+tree node as
+ well as the leaves.
+
+- Each pointer is associated with two keys. The first of these is the "low
+ key", which is the key of the smallest record accessible through the
+ pointer. This low key has the same meaning as the key in all other btrees.
+ The second key is the high key, which is the maximum of the largest key
+ that can be used to access a given record underneath the pointer. Recall
+ that each record in the reverse mapping b+tree describes an interval of
+ physical blocks mapped to an interval of logical file block offsets;
+ therefore, it makes sense that a range of keys can be used to find to a
+ record.
+
+xfs\_db rmapbt Example
+^^^^^^^^^^^^^^^^^^^^^^
+
+This example shows a reverse-mapping B+tree from a freshly populated root
+filesystem:
+
+::
+
+ xfs_db> agf 0
+ xfs_db> addr rmaproot
+ xfs_db> p
+ magic = 0x524d4233
+ level = 1
+ numrecs = 43
+ leftsib = null
+ rightsib = null
+ bno = 56
+ lsn = 0x3000004c8
+ uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
+ owner = 0
+ crc = 0x7cf8be6f (correct)
+ keys[1-43] = [startblock,owner,offset]
+ keys[1-43] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,
+ offset_hi,attrfork_hi,bmbtblock_hi]
+ 1:[0,-3,0,0,0,351,4418,66,0,0]
+ 2:[417,285,0,0,0,827,4419,2,0,0]
+ 3:[829,499,0,0,0,2352,573,55,0,0]
+ 4:[1292,710,0,0,0,32168,262923,47,0,0]
+ 5:[32215,-5,0,0,0,34655,2365,3411,0,0]
+ 6:[34083,1161,0,0,0,34895,265220,1,0,1]
+ 7:[34896,256191,0,0,0,36522,-9,0,0,0]
+ ...
+ 41:[50998,326734,0,0,0,51430,-5,0,0,0]
+ 42:[51431,327010,0,0,0,51600,325722,11,0,0]
+ 43:[51611,327112,0,0,0,94063,23522,28375272,0,0]
+ ptrs[1-43] = 1:5 2:6 3:8 4:9 5:10 6:11 7:418 ... 41:46377 42:48784 43:49522
+
+We arbitrarily pick pointer 17 to traverse downwards:
+
+::
+
+ xfs_db> addr ptrs[17]
+ xfs_db> p
+ magic = 0x524d4233
+ level = 0
+ numrecs = 168
+ leftsib = 36284
+ rightsib = 37617
+ bno = 294760
+ lsn = 0x200002761
+ uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
+ owner = 0
+ crc = 0x2dad3fbe (correct)
+ recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+ 1:[40326,1,259615,0,0,0,0] 2:[40327,1,-5,0,0,0,0]
+ 3:[40328,2,259618,0,0,0,0] 4:[40330,1,259619,0,0,0,0]
+ ...
+ 127:[40540,1,324266,0,0,0,0] 128:[40541,1,324266,8388608,0,0,0]
+ 129:[40542,2,324266,1,0,0,0] 130:[40544,32,-7,0,0,0,0]
+
+Several interesting things pop out here. The first record shows that inode
+259,615 has mapped AG block 40,326 at offset 0. We confirm this by looking at
+the block map for that inode:
+
+::
+
+ xfs_db> inode 259615
+ xfs_db> bmap
+ data offset 0 startblock 40326 (0/40326) count 1 flag 0
+
+Next, notice records 127 and 128, which describe neighboring AG blocks that
+are mapped to non-contiguous logical blocks in inode 324,266. Given the
+logical offset of 8,388,608 we surmise that this is a leaf directory, but let
+us confirm:
+
+::
+
+ xfs_db> inode 324266
+ xfs_db> p core.mode
+ core.mode = 040755
+ xfs_db> bmap
+ data offset 0 startblock 40540 (0/40540) count 1 flag 0
+ data offset 1 startblock 40542 (0/40542) count 2 flag 0
+ data offset 3 startblock 40576 (0/40576) count 1 flag 0
+ data offset 8388608 startblock 40541 (0/40541) count 1 flag 0
+ xfs_db> p core.mode
+ core.mode = 0100644
+ xfs_db> dblock 0
+ xfs_db> p dhdr.hdr.magic
+ dhdr.hdr.magic = 0x58444433
+ xfs_db> dblock 8388608
+ xfs_db> p lhdr.info.hdr.magic
+ lhdr.info.hdr.magic = 0x3df1
+
+Indeed, this inode 324,266 appears to be a leaf directory, as it has regular
+directory data blocks at low offsets, and a single leaf block.
+
+Notice further the two reverse-mapping records with negative owners. An owner
+of -7 corresponds to XFS\_RMAP\_OWN\_INODES, which is an inode chunk, and an
+owner code of -5 corresponds to XFS\_RMAP\_OWN\_AG, which covers free space
+B+trees and free space. Let’s see if block 40,544 is part of an inode chunk:
+
+::
+
+ xfs_db> blockget
+ xfs_db> fsblock 40544
+ xfs_db> blockuse
+ block 40544 (0/40544) type inode
+ xfs_db> stack
+ 1:
+ byte offset 166068224, length 4096
+ buffer block 324352 (fsbno 40544), 8 bbs
+ inode 324266, dir inode 324266, type data
+ xfs_db> type inode
+ xfs_db> p
+ core.magic = 0x494e
+
+Our suspicions are confirmed. Let’s also see if 40,327 is part of a free space
+tree:
+
+::
+
+ xfs_db> fsblock 40327
+ xfs_db> blockuse
+ block 40327 (0/40327) type btrmap
+ xfs_db> type rmapbt
+ xfs_db> p
+ magic = 0x524d4233
+
+As you can see, the reverse block-mapping B+tree is an important secondary
+metadata structure, which can be used to reconstruct damaged primary metadata.
+Now let’s look at an extend rmap btree:
+
+::
+
+ xfs_db> agf 0
+ xfs_db> addr rmaproot
+ xfs_db> p
+ magic = 0x34524d42
+ level = 1
+ numrecs = 5
+ leftsib = null
+ rightsib = null
+ bno = 6368
+ lsn = 0x100000d1b
+ uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
+ owner = 0
+ crc = 0x8d4ace05 (correct)
+ keys[1-5] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,offset_hi,attrfork_hi,bmbtblock_hi]
+ 1:[0,-3,0,0,0,705,132,681,0,0]
+ 2:[24,5761,0,0,0,548,5761,524,0,0]
+ 3:[24,5929,0,0,0,380,5929,356,0,0]
+ 4:[24,6097,0,0,0,212,6097,188,0,0]
+ 5:[24,6277,0,0,0,807,-7,0,0,0]
+ ptrs[1-5] = 1:5 2:771 3:9 4:10 5:11
+
+The second pointer stores both the low key [24,5761,0,0,0] and the high key
+[548,5761,524,0,0], which means that we can expect block 771 to contain
+records starting at physical block 24, inode 5761, offset zero; and that one
+of the records can be used to find a reverse mapping for physical block 548,
+inode 5761, and offset 524:
+
+::
+
+ xfs_db> addr ptrs[2]
+ xfs_db> p
+ magic = 0x34524d42
+ level = 0
+ numrecs = 168
+ leftsib = 5
+ rightsib = 9
+ bno = 6168
+ lsn = 0x100000d1b
+ uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
+ owner = 0
+ crc = 0xd58eff0e (correct)
+ recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+ 1:[24,525,5761,0,0,0,0]
+ 2:[24,524,5762,0,0,0,0]
+ 3:[24,523,5763,0,0,0,0]
+ ...
+ 166:[24,360,5926,0,0,0,0]
+ 167:[24,359,5927,0,0,0,0]
+ 168:[24,358,5928,0,0,0,0]
+
+Observe that the first record in the block starts at physical block 24, inode
+5761, offset zero, just as we expected. Note that this first record is also
+indexed by the highest key as provided in the node block; physical block 548,
+inode 5761, offset 524 is the very last block mapped by this record.
+Furthermore, note that record 168, despite being the last record in this
+block, has a lower maximum key (physical block 382, inode 5928, offset 23)
+than the first record.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 13/22] docs: add XFS refcount btree structure to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (10 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 12/22] docs: add XFS reverse mapping structures " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 14/22] docs: add XFS log to the " Darrick J. Wong
` (9 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/allocation_groups.rst | 1
.../filesystems/xfs-data-structures/refcountbt.rst | 154 ++++++++++++++++++++
2 files changed, 155 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/refcountbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/allocation_groups.rst b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
index 6c0ffd3a170b..76c6ddcd02ac 100644
--- a/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
+++ b/Documentation/filesystems/xfs-data-structures/allocation_groups.rst
@@ -1381,3 +1381,4 @@ None of the XFS per-AG B+trees are involved with real time files. It is not
possible for real time files to share data blocks.
.. include:: rmapbt.rst
+.. include:: refcountbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/refcountbt.rst b/Documentation/filesystems/xfs-data-structures/refcountbt.rst
new file mode 100644
index 000000000000..0f2b818959df
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/refcountbt.rst
@@ -0,0 +1,154 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Reference Count B+tree
+~~~~~~~~~~~~~~~~~~~~~~
+
+To support the sharing of file data blocks (reflink), each allocation group
+has its own reference count B+tree, which grows in the allocated space like
+the inode B+trees. This data could be gleaned by performing an interval query
+of the reverse-mapping B+tree, but doing so would come at a huge performance
+penalty. Therefore, this data structure is a cache of computable information.
+
+This B+tree is only present if the XFS\_SB\_FEAT\_RO\_COMPAT\_REFLINK feature
+is enabled. The feature requires a version 5 filesystem.
+
+Each record in the reference count B+tree has the following structure:
+
+.. code:: c
+
+ struct xfs_refcount_rec {
+ __be32 rc_startblock;
+ __be32 rc_blockcount;
+ __be32 rc_refcount;
+ };
+
+**rc\_startblock**
+ AG block number of this record. The high bit is set for all records
+ referring to an extent that is being used to stage a copy on write
+ operation. This reduces recovery time during mount operations. The
+ reference count of these staging events must only be 1.
+
+**rc\_blockcount**
+ The length of this extent.
+
+**rc\_refcount**
+ Number of mappings of this filesystem extent.
+
+Node pointers are an AG relative block pointer:
+
+.. code:: c
+
+ struct xfs_refcount_key {
+ __be32 rc_startblock;
+ };
+
+- As the reference counting is AG relative, all the block numbers are only
+ 32-bits.
+
+- The bb\_magic value is "R3FC" (0x52334643).
+
+- The xfs\_btree\_sblock\_t header is used for intermediate B+tree node as
+ well as the leaves.
+
+xfs\_db refcntbt Example
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+For this example, an XFS filesystem was populated with a root filesystem and a
+deduplication program was run to create shared blocks:
+
+::
+
+ xfs_db> agf 0
+ xfs_db> addr refcntroot
+ xfs_db> p
+ magic = 0x52334643
+ level = 1
+ numrecs = 6
+ leftsib = null
+ rightsib = null
+ bno = 36892
+ lsn = 0x200004ec2
+ uuid = f1f89746-e00b-49c9-96b3-ecef0f2f14ae
+ owner = 0
+ crc = 0x75f35128 (correct)
+ keys[1-6] = [startblock] 1:[14] 2:[65633] 3:[65780] 4:[94571] 5:[117201] 6:[152442]
+ ptrs[1-6] = 1:7 2:25836 3:25835 4:18447 5:18445 6:18449
+ xfs_db> addr ptrs[3]
+ xfs_db> p
+ magic = 0x52334643
+ level = 0
+ numrecs = 80
+ leftsib = 25836
+ rightsib = 18447
+ bno = 51670
+ lsn = 0x200004ec2
+ uuid = f1f89746-e00b-49c9-96b3-ecef0f2f14ae
+ owner = 0
+ crc = 0xc3962813 (correct)
+ recs[1-80] = [startblock,blockcount,refcount,cowflag]
+ 1:[65780,1,2,0] 2:[65781,1,3,0] 3:[65785,2,2,0] 4:[66640,1,2,0]
+ 5:[69602,4,2,0] 6:[72256,16,2,0] 7:[72871,4,2,0] 8:[72879,20,2,0]
+ 9:[73395,4,2,0] 10:[75063,4,2,0] 11:[79093,4,2,0] 12:[86344,16,2,0]
+ ...
+ 80:[35235,10,1,1]
+
+Notice record 80. The copy on write flag is set and the reference count is 1,
+which indicates that the extent 35,235 - 35,244 are being used to stage a copy
+on write activity. The "cowflag" field is the high bit of rc\_startblock.
+
+Record 6 in the reference count B+tree for AG 0 indicates that the AG extent
+starting at block 72,256 and running for 16 blocks has a reference count of 2.
+This means that there are two files sharing the block:
+
+::
+
+ xfs_db> blockget -n
+ xfs_db> fsblock 72256
+ xfs_db> blockuse
+ block 72256 (0/72256) type rldata inode 25169197
+
+The blockuse type changes to "rldata" to indicate that the block is shared
+data. Unfortunately, blockuse only tells us about one block owner. If we
+happen to have enabled the reverse-mapping B+tree, we can use it to find all
+inodes that own this block:
+
+::
+
+ xfs_db> agf 0
+ xfs_db> addr rmaproot
+ ...
+ xfs_db> addr ptrs[3]
+ ...
+ xfs_db> addr ptrs[7]
+ xfs_db> p
+ magic = 0x524d4233
+ level = 0
+ numrecs = 22
+ leftsib = 65057
+ rightsib = 65058
+ bno = 291478
+ lsn = 0x200004ec2
+ uuid = f1f89746-e00b-49c9-96b3-ecef0f2f14ae
+ owner = 0
+ crc = 0xed7da3f7 (correct)
+ recs[1-22] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+ 1:[68957,8,3201,0,0,0,0] 2:[68965,4,25260953,0,0,0,0]
+ ...
+ 18:[72232,58,3227,0,0,0,0] 19:[72256,16,25169197,24,0,0,0]
+ 20:[72290,75,3228,0,0,0,0] 21:[72365,46,3229,0,0,0,0]
+
+Records 18 and 19 intersect the block 72,256; they tell us that inodes 3,227
+and 25,169,197 both claim ownership. Let us confirm this:
+
+::
+
+ xfs_db> inode 25169197
+ xfs_db> bmap
+ data offset 0 startblock 12632259 (3/49347) count 24 flag 0
+ data offset 24 startblock 72256 (0/72256) count 16 flag 0
+ data offset 40 startblock 12632299 (3/49387) count 18 flag 0
+ xfs_db> inode 3227
+ xfs_db> bmap
+ data offset 0 startblock 72232 (0/72232) count 58 flag 0
+
+Inodes 25,169,197 and 3,227 both contain mappings to block 0/72,256.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 14/22] docs: add XFS log to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (11 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 13/22] docs: add XFS refcount btree structure to " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:19 ` [PATCH 15/22] docs: add XFS internal inodes " Darrick J. Wong
` (8 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/globals.rst | 1
.../xfs-data-structures/journaling_log.rst | 1442 ++++++++++++++++++++
2 files changed, 1443 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/journaling_log.rst
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index c91b1d24d6e7..8ce83deafae5 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -6,3 +6,4 @@ Global Structures
.. include:: btrees.rst
.. include:: dabtrees.rst
.. include:: allocation_groups.rst
+.. include:: journaling_log.rst
diff --git a/Documentation/filesystems/xfs-data-structures/journaling_log.rst b/Documentation/filesystems/xfs-data-structures/journaling_log.rst
new file mode 100644
index 000000000000..78d8fa1933ae
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/journaling_log.rst
@@ -0,0 +1,1442 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Journaling Log
+--------------
+
+ **Note**
+
+ Only v2 log format is covered here.
+
+The XFS journal exists on disk as a reserved extent of blocks within the
+filesystem, or as a separate journal device. The journal itself can be thought
+of as a series of log records; each log record contains a part of or a whole
+transaction. A transaction consists of a series of log operation headers
+("log items"), formatting structures, and raw data. The first operation in
+a transaction establishes the transaction ID and the last operation is a
+commit record. The operations recorded between the start and commit operations
+represent the metadata changes made by the transaction. If the commit
+operation is missing, the transaction is incomplete and cannot be recovered.
+
+Log Records
+~~~~~~~~~~~
+
+The XFS log is split into a series of log records. Log records seem to
+correspond to an in-core log buffer, which can be up to 256KiB in size. Each
+record has a log sequence number, which is the same LSN recorded in the v5
+metadata integrity fields.
+
+Log sequence numbers are a 64-bit quantity consisting of two 32-bit
+quantities. The upper 32 bits are the
+"cycle number", which increments every time XFS
+cycles through the log. The lower 32 bits are the "block number", which
+is assigned when a transaction is committed, and should correspond to the
+block offset within the log.
+
+A log record begins with the following header, which occupies 512 bytes on
+disk:
+
+.. code:: c
+
+ typedef struct xlog_rec_header {
+ __be32 h_magicno;
+ __be32 h_cycle;
+ __be32 h_version;
+ __be32 h_len;
+ __be64 h_lsn;
+ __be64 h_tail_lsn;
+ __le32 h_crc;
+ __be32 h_prev_block;
+ __be32 h_num_logops;
+ __be32 h_cycle_data[XLOG_HEADER_CYCLE_SIZE / BBSIZE];
+ /* new fields */
+ __be32 h_fmt;
+ uuid_t h_fs_uuid;
+ __be32 h_size;
+ } xlog_rec_header_t;
+
+**h\_magicno**
+ The magic number of log records, 0xfeedbabe.
+
+**h\_cycle**
+ Cycle number of this log record.
+
+**h\_version**
+ Log record version, currently 2.
+
+**h\_len**
+ Length of the log record, in bytes. Must be aligned to a 64-bit boundary.
+
+**h\_lsn**
+ Log sequence number of this record.
+
+**h\_tail\_lsn**
+ Log sequence number of the first log record with uncommitted buffers.
+
+**h\_crc**
+ Checksum of the log record header, the cycle data, and the log records
+ themselves.
+
+**h\_prev\_block**
+ Block number of the previous log record.
+
+**h\_num\_logops**
+ The number of log operations in this record.
+
+**h\_cycle\_data**
+ The first u32 of each log sector must contain the cycle number. Since log
+ item buffers are formatted without regard to this requirement, the
+ original contents of the first four bytes of each sector in the log are
+ copied into the corresponding element of this array. After that, the first
+ four bytes of those sectors are stamped with the cycle number. This
+ process is reversed at recovery time. If there are more sectors in this
+ log record than there are slots in this array, the cycle data continues
+ for as many sectors are needed; each sector is formatted as type
+ xlog\_rec\_ext\_header.
+
+**h\_fmt**
+ Format of the log record. This is one of the following values:
+
+.. list-table::
+ :widths: 24 56
+ :header-rows: 1
+
+ * - Format value
+ - Log format
+
+ * - XLOG\_FMT\_UNKNOWN
+ - Unknown. Perhaps this log is corrupt.
+
+ * - XLOG\_FMT\_LINUX\_LE
+ - Little-endian Linux.
+
+ * - XLOG\_FMT\_LINUX\_BE
+ - Big-endian Linux.
+
+ * - XLOG\_FMT\_IRIX\_BE
+ - Big-endian Irix.
+
+Table: Log record formats
+
+**h\_fs\_uuid**
+ Filesystem UUID.
+
+**h\_size**
+ In-core log record size. This is somewhere between 16 and 256KiB, with
+ 32KiB being the default.
+
+As mentioned earlier, if this log record is longer than 256 sectors, the cycle
+data overflows into the next sector(s) in the log. Each of those sectors is
+formatted as follows:
+
+.. code:: c
+
+ typedef struct xlog_rec_ext_header {
+ __be32 xh_cycle;
+ __be32 xh_cycle_data[XLOG_HEADER_CYCLE_SIZE / BBSIZE];
+ } xlog_rec_ext_header_t;
+
+**xh\_cycle**
+ Cycle number of this log record. Should match h\_cycle.
+
+**xh\_cycle\_data**
+ Overflow cycle data.
+
+Log Operations
+~~~~~~~~~~~~~~
+
+Within a log record, log operations are recorded as a series consisting of an
+operation header immediately followed by a data region. The operation header
+has the following format:
+
+.. code:: c
+
+ typedef struct xlog_op_header {
+ __be32 oh_tid;
+ __be32 oh_len;
+ __u8 oh_clientid;
+ __u8 oh_flags;
+ __u16 oh_res2;
+ } xlog_op_header_t;
+
+**oh\_tid**
+ Transaction ID of this operation.
+
+**oh\_len**
+ Number of bytes in the data region.
+
+**oh\_clientid**
+ The originator of this operation. This can be one of the following:
+
+.. list-table::
+ :widths: 24 56
+ :header-rows: 1
+
+ * - Client ID
+ - Originator
+
+ * - XFS\_TRANSACTION
+ - Operation came from a transaction.
+
+ * - XFS\_VOLUME
+ - ???
+
+ * - XFS\_LOG
+ - ???
+
+Table: Log Operation Client ID
+
+**oh\_flags**
+ Specifies flags associated with this operation. This can be a combination
+ of the following values (though most likely only one will be set at a
+ time):
+
+.. list-table::
+ :widths: 24 56
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XLOG\_START\_TRANS
+ - Start a new transaction. The next operation header should describe a
+ transaction header.
+
+ * - XLOG\_COMMIT\_TRANS
+ - Commit this transaction.
+
+ * - XLOG\_CONTINUE\_TRANS
+ - Continue this trans into new log record.
+
+ * - XLOG\_WAS\_CONT\_TRANS
+ - This transaction started in a previous log record.
+
+ * - XLOG\_END\_TRANS
+ - End of a continued transaction.
+
+ * - XLOG\_UNMOUNT\_TRANS
+ - Transaction to unmount a filesystem.
+
+Table: Log Operation Flags
+
+**oh\_res2**
+ Padding.
+
+The data region follows immediately after the operation header and is exactly
+oh\_len bytes long. These payloads are in host-endian order, which means that
+one cannot replay the log from an unclean XFS filesystem on a system with a
+different byte order.
+
+Log Items
+~~~~~~~~~
+
+Following are the types of log item payloads that can follow an
+xlog\_op\_header. Except for buffer data and inode cores, all log items have a
+magic number to distinguish themselves. Buffer data items only appear after
+xfs\_buf\_log\_format items; and inode core items only appear after
+xfs\_inode\_log\_format items.
+
+.. list-table::
+ :widths: 24 12 44
+ :header-rows: 1
+
+ * - Magic
+ - Hexadecimal
+ - Operation Type
+
+ * - XFS\_TRANS\_HEADER\_MAGIC
+ - 0x5452414e
+ - Log Transaction Header
+
+ * - XFS\_LI\_EFI
+ - 0x1236
+ - Extent Freeing Intent
+
+ * - XFS\_LI\_EFD
+ - 0x1237
+ - Extent Freeing Done
+
+ * - XFS\_LI\_IUNLINK
+ - 0x1238
+ - Unknown?
+
+ * - XFS\_LI\_INODE
+ - 0x123b
+ - Inode Updates
+
+ * - XFS\_LI\_BUF
+ - 0x123c
+ - Buffer Writes
+
+ * - XFS\_LI\_DQUOT
+ - 0x123d
+ - Update Quota
+
+ * - XFS\_LI\_QUOTAOFF
+ - 0x123e
+ - Quota Off
+
+ * - XFS\_LI\_ICREATE
+ - 0x123f
+ - Inode Creation
+
+ * - XFS\_LI\_RUI
+ - 0x1240
+ - Reverse Mapping Update Intent
+
+ * - XFS\_LI\_RUD
+ - 0x1241
+ - Reverse Mapping Update Done
+
+ * - XFS\_LI\_CUI
+ - 0x1242
+ - Reference Count Update Intent
+
+ * - XFS\_LI\_CUD
+ - 0x1243
+ - Reference Count Update Done
+
+ * - XFS\_LI\_BUI
+ - 0x1244
+ - File Block Mapping Update Intent
+
+ * - XFS\_LI\_BUD
+ - 0x1245
+ - File Block Mapping Update Done
+
+Table: Log Operation Magic Numbers
+
+Note that all log items (except for transaction headers) MUST start with the
+following header structure. The type and size fields are baked into each log
+item header, but there is not a separately defined header.
+
+.. code:: c
+
+ struct xfs_log_item {
+ __uint16_t magic;
+ __uint16_t size;
+ };
+
+Transaction Headers
+^^^^^^^^^^^^^^^^^^^
+
+A transaction header is an operation payload that starts a transaction.
+
+.. code:: c
+
+ typedef struct xfs_trans_header {
+ uint th_magic;
+ uint th_type;
+ __int32_t th_tid;
+ uint th_num_items;
+ } xfs_trans_header_t;
+
+**th\_magic**
+ The signature of a transaction header, "TRAN" (0x5452414e). Note that
+ this value is in host-endian order, not big-endian like the rest of XFS.
+
+**th\_type**
+ Transaction type. This is one of the following values:
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Type
+ - Description
+
+ * - XFS\_TRANS\_SETATTR\_NOT\_SIZE
+ - Set an inode attribute that isn’t the inode’s size.
+
+ * - XFS\_TRANS\_SETATTR\_SIZE
+ - Setting the size attribute of an inode.
+
+ * - XFS\_TRANS\_INACTIVE
+ - Freeing blocks from an unlinked inode.
+
+ * - XFS\_TRANS\_CREATE
+ - Create a file.
+
+ * - XFS\_TRANS\_CREATE\_TRUNC
+ - Unused?
+
+ * - XFS\_TRANS\_TRUNCATE\_FILE
+ - Truncate a quota file.
+
+ * - XFS\_TRANS\_REMOVE
+ - Remove a file.
+
+ * - XFS\_TRANS\_LINK
+ - Link an inode into a directory.
+
+ * - XFS\_TRANS\_RENAME
+ - Rename a path.
+
+ * - XFS\_TRANS\_MKDIR
+ - Create a directory.
+
+ * - XFS\_TRANS\_RMDIR
+ - Remove a directory.
+
+ * - XFS\_TRANS\_SYMLINK
+ - Create a symbolic link.
+
+ * - XFS\_TRANS\_SET\_DMATTRS
+ - Set the DMAPI attributes of an inode.
+
+ * - XFS\_TRANS\_GROWFS
+ - Expand the filesystem.
+
+ * - XFS\_TRANS\_STRAT\_WRITE
+ - Convert an unwritten extent or delayed-allocate some blocks to
+ handle a write.
+
+ * - XFS\_TRANS\_DIOSTRAT
+ - Allocate some blocks to handle a direct I/O write.
+
+ * - XFS\_TRANS\_WRITEID
+ - Update an inode’s preallocation flag.
+
+ * - XFS\_TRANS\_ADDAFORK
+ - Add an attribute fork to an inode.
+
+ * - XFS\_TRANS\_ATTRINVAL
+ - Erase the attribute fork of an inode.
+
+ * - XFS\_TRANS\_ATRUNCATE
+ - Unused?
+
+ * - XFS\_TRANS\_ATTR\_SET
+ - Set an extended attribute.
+
+ * - XFS\_TRANS\_ATTR\_RM
+ - Remove an extended attribute.
+
+ * - XFS\_TRANS\_ATTR\_FLAG
+ - Unused?
+
+ * - XFS\_TRANS\_CLEAR\_AGI\_BUCKET
+ - Clear a bad inode pointer in the AGI unlinked inode hash bucket.
+
+ * - XFS\_TRANS\_SB\_CHANGE
+ - Write the superblock to disk.
+
+ * - XFS\_TRANS\_QM\_QUOTAOFF
+ - Start disabling quotas.
+
+ * - XFS\_TRANS\_QM\_DQALLOC
+ - Allocate a disk quota structure.
+
+ * - XFS\_TRANS\_QM\_SETQLIM
+ - Adjust quota limits.
+
+ * - XFS\_TRANS\_QM\_DQCLUSTER
+ - Unused?
+
+ * - XFS\_TRANS\_QM\_QINOCREATE
+ - Create a (quota) inode with reference taken.
+
+ * - XFS\_TRANS\_QM\_QUOTAOFF\_END
+ - Finish disabling quotas.
+
+ * - XFS\_TRANS\_FSYNC\_TS
+ - Update only inode timestamps.
+
+ * - XFS\_TRANS\_GROWFSRT\_ALLOC
+ - Grow the realtime bitmap and summary data for growfs.
+
+ * - XFS\_TRANS\_GROWFSRT\_ZERO
+ - Zero space in the realtime bitmap and summary data.
+
+ * - XFS\_TRANS\_GROWFSRT\_FREE
+ - Free space in the realtime bitmap and summary data.
+
+ * - XFS\_TRANS\_SWAPEXT
+ - Swap data fork of two inodes.
+
+ * - XFS\_TRANS\_CHECKPOINT
+ - Checkpoint the log.
+
+ * - XFS\_TRANS\_ICREATE
+ - Unknown?
+
+ * - XFS\_TRANS\_CREATE\_TMPFILE
+ - Create a temporary file.
+
+**th\_tid**
+ Transaction ID.
+
+**th\_num\_items**
+ The number of operations appearing after this operation, not including the
+ commit operation. In effect, this tracks the number of metadata change
+ operations in this transaction.
+
+Intent to Free an Extent
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The next two operation types work together to handle the freeing of filesystem
+blocks. Naturally, the ranges of blocks to be freed can be expressed in terms
+of extents:
+
+.. code:: c
+
+ typedef struct xfs_extent_32 {
+ __uint64_t ext_start;
+ __uint32_t ext_len;
+ } __attribute__((packed)) xfs_extent_32_t;
+
+ typedef struct xfs_extent_64 {
+ __uint64_t ext_start;
+ __uint32_t ext_len;
+ __uint32_t ext_pad;
+ } xfs_extent_64_t;
+
+**ext\_start**
+ Start block of this extent.
+
+**ext\_len**
+ Length of this extent.
+
+The "extent freeing intent" operation comes first; it tells the log that XFS
+wants to free some extents. This record is crucial for correct log recovery
+because it prevents the log from replaying blocks that are subsequently freed.
+If the log lacks a corresponding "extent freeing done" operation, the
+recovery process will free the extents.
+
+.. code:: c
+
+ typedef struct xfs_efi_log_format {
+ __uint16_t efi_type;
+ __uint16_t efi_size;
+ __uint32_t efi_nextents;
+ __uint64_t efi_id;
+ xfs_extent_t efi_extents[1];
+ } xfs_efi_log_format_t;
+
+**efi\_type**
+ The signature of an EFI operation, 0x1236. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**efi\_size**
+ Size of this log item. Should be 1.
+
+**efi\_nextents**
+ Number of extents to free.
+
+**efi\_id**
+ A 64-bit number that binds the corresponding EFD log item to this EFI log
+ item.
+
+**efi\_extents**
+ Variable-length array of extents to be freed. The array length is given by
+ efi\_nextents. The record type will be either xfs\_extent\_64\_t or
+ xfs\_extent\_32\_t; this can be determined from the log item size
+ (oh\_len) and the number of extents (efi\_nextents).
+
+Completion of Intent to Free an Extent
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The "extent freeing done" operation complements the "extent freeing
+intent" operation. This second operation indicates that the block freeing
+actually happened, so that log recovery needn’t try to free the blocks.
+Typically, the operations to update the free space B+trees follow immediately
+after the EFD.
+
+.. code:: c
+
+ typedef struct xfs_efd_log_format {
+ __uint16_t efd_type;
+ __uint16_t efd_size;
+ __uint32_t efd_nextents;
+ __uint64_t efd_efi_id;
+ xfs_extent_t efd_extents[1];
+ } xfs_efd_log_format_t;
+
+**efd\_type**
+ The signature of an EFD operation, 0x1237. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**efd\_size**
+ Size of this log item. Should be 1.
+
+**efd\_nextents**
+ Number of extents to free.
+
+**efd\_id**
+ A 64-bit number that binds the corresponding EFI log item to this EFD log
+ item.
+
+**efd\_extents**
+ Variable-length array of extents to be freed. The array length is given by
+ efd\_nextents. The record type will be either xfs\_extent\_64\_t or
+ xfs\_extent\_32\_t; this can be determined from the log item size
+ (oh\_len) and the number of extents (efd\_nextents).
+
+Reverse Mapping Updates Intent
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The next two operation types work together to handle deferred reverse mapping
+updates. Naturally, the mappings to be updated can be expressed in terms of
+mapping extents:
+
+.. code:: c
+
+ struct xfs_map_extent {
+ __uint64_t me_owner;
+ __uint64_t me_startblock;
+ __uint64_t me_startoff;
+ __uint32_t me_len;
+ __uint32_t me_flags;
+ };
+
+**me\_owner**
+ Owner of this reverse mapping. See the values in the section about
+ `reverse mapping <#reverse-mapping-b-tree>`__ for more information.
+
+**me\_startblock**
+ Filesystem block of this mapping.
+
+**me\_startoff**
+ Logical block offset of this mapping.
+
+**me\_len**
+ The length of this mapping.
+
+**me\_flags**
+ The lower byte of this field is a type code indicating what sort of
+ reverse mapping operation we want. The upper three bytes are flag bits.
+
+.. list-table::
+ :widths: 36 44
+ :header-rows: 1
+
+ * - Value
+ - Description
+
+ * - XFS\_RMAP\_EXTENT\_MAP
+ - Add a reverse mapping for file data.
+
+ * - XFS\_RMAP\_EXTENT\_MAP\_SHARED
+ - Add a reverse mapping for file data for a file with shared blocks.
+
+ * - XFS\_RMAP\_EXTENT\_UNMAP
+ - Remove a reverse mapping for file data.
+
+ * - XFS\_RMAP\_EXTENT\_UNMAP\_SHARED
+ - Remove a reverse mapping for file data for a file with shared blocks.
+
+ * - XFS\_RMAP\_EXTENT\_CONVERT
+ - Convert a reverse mapping for file data between unwritten and normal.
+
+ * - XFS\_RMAP\_EXTENT\_CONVERT\_SHARED
+ - Convert a reverse mapping for file data between unwritten and normal for
+ a file with shared blocks.
+
+ * - XFS\_RMAP\_EXTENT\_ALLOC
+ - Add a reverse mapping for non-file data.
+
+ * - XFS\_RMAP\_EXTENT\_FREE
+ - Remove a reverse mapping for non-file data.
+
+Table: Reverse mapping update log intent types
+
+.. list-table::
+ :widths: 36 44
+ :header-rows: 1
+
+ * - Value
+ - Description
+
+ * - XFS\_RMAP\_EXTENT\_ATTR\_FORK
+ - Extent is for the attribute fork.
+
+ * - XFS\_RMAP\_EXTENT\_BMBT\_BLOCK
+ - Extent is for a block mapping btree block.
+
+ * - XFS\_RMAP\_EXTENT\_UNWRITTEN
+ - Extent is unwritten.
+
+Table: Reverse mapping update log intent flags
+
+The "rmap update intent" operation comes first; it tells the log that XFS
+wants to update some reverse mappings. This record is crucial for correct log
+recovery because it enables us to spread a complex metadata update across
+multiple transactions while ensuring that a crash midway through the complex
+update will be replayed fully during log recovery.
+
+.. code:: c
+
+ struct xfs_rui_log_format {
+ __uint16_t rui_type;
+ __uint16_t rui_size;
+ __uint32_t rui_nextents;
+ __uint64_t rui_id;
+ struct xfs_map_extent rui_extents[1];
+ };
+
+**rui\_type**
+ The signature of an RUI operation, 0x1240. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**rui\_size**
+ Size of this log item. Should be 1.
+
+**rui\_nextents**
+ Number of reverse mappings.
+
+**rui\_id**
+ A 64-bit number that binds the corresponding RUD log item to this RUI log
+ item.
+
+**rui\_extents**
+ Variable-length array of reverse mappings to update.
+
+Completion of Reverse Mapping Updates
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The "reverse mapping update done" operation complements the "reverse
+mapping update intent" operation. This second operation indicates that the
+update actually happened, so that log recovery needn’t replay the update. The
+RUD and the actual updates are typically found in a new transaction following
+the transaction in which the RUI was logged.
+
+.. code:: c
+
+ struct xfs_rud_log_format {
+ __uint16_t rud_type;
+ __uint16_t rud_size;
+ __uint32_t __pad;
+ __uint64_t rud_rui_id;
+ };
+
+**rud\_type**
+ The signature of an RUD operation, 0x1241. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**rud\_size**
+ Size of this log item. Should be 1.
+
+**rud\_rui\_id**
+ A 64-bit number that binds the corresponding RUI log item to this RUD log
+ item.
+
+Reference Count Updates Intent
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The next two operation types work together to handle reference count updates.
+Naturally, the ranges of extents having reference count updates can be
+expressed in terms of physical extents:
+
+.. code:: c
+
+ struct xfs_phys_extent {
+ __uint64_t pe_startblock;
+ __uint32_t pe_len;
+ __uint32_t pe_flags;
+ };
+
+**pe\_startblock**
+ Filesystem block of this extent.
+
+**pe\_len**
+ The length of this extent.
+
+**pe\_flags**
+ The lower byte of this field is a type code indicating what sort of
+ reverse mapping operation we want. The upper three bytes are flag bits.
+
+.. list-table::
+ :widths: 34 46
+ :header-rows: 1
+
+ * - Value
+ - Description
+
+ * - XFS\_REFCOUNT\_EXTENT\_INCREASE
+ - Increase the reference count for this extent.
+
+ * - XFS\_REFCOUNT\_EXTENT\_DECREASE
+ - Decrease the reference count for this extent.
+
+ * - XFS\_REFCOUNT\_EXTENT\_ALLOC\_COW
+ - Reserve an extent for staging copy on write.
+
+ * - XFS\_REFCOUNT\_EXTENT\_FREE\_COW
+ - Unreserve an extent for staging copy on write.
+
+Table: Reference count update log intent types
+
+The "reference count update intent" operation comes first; it tells the
+log that XFS wants to update some reference counts. This record is crucial for
+correct log recovery because it enables us to spread a complex metadata update
+across multiple transactions while ensuring that a crash midway through the
+complex update will be replayed fully during log recovery.
+
+.. code:: c
+
+ struct xfs_cui_log_format {
+ __uint16_t cui_type;
+ __uint16_t cui_size;
+ __uint32_t cui_nextents;
+ __uint64_t cui_id;
+ struct xfs_map_extent cui_extents[1];
+ };
+
+**cui\_type**
+ The signature of an CUI operation, 0x1242. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**cui\_size**
+ Size of this log item. Should be 1.
+
+**cui\_nextents**
+ Number of reference count updates.
+
+**cui\_id**
+ A 64-bit number that binds the corresponding RUD log item to this RUI log
+ item.
+
+**cui\_extents**
+ Variable-length array of reference count update information.
+
+Completion of Reference Count Updates
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The "reference count update done" operation complements the "reference
+count update intent" operation. This second operation indicates that the
+update actually happened, so that log recovery needn’t replay the update. The
+CUD and the actual updates are typically found in a new transaction following
+the transaction in which the CUI was logged.
+
+.. code:: c
+
+ struct xfs_cud_log_format {
+ __uint16_t cud_type;
+ __uint16_t cud_size;
+ __uint32_t __pad;
+ __uint64_t cud_cui_id;
+ };
+
+**cud\_type**
+ The signature of an RUD operation, 0x1243. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**cud\_size**
+ Size of this log item. Should be 1.
+
+**cud\_cui\_id**
+ A 64-bit number that binds the corresponding CUI log item to this CUD log
+ item.
+
+File Block Mapping Intent
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The next two operation types work together to handle deferred file block
+mapping updates. The extents to be mapped are expressed via the
+xfs\_map\_extent structure discussed in the section about `reverse mapping
+intents <#reverse-mapping-updates-intent>`__.
+
+The lower byte of the me\_flags field is a type code indicating what sort of
+file block mapping operation we want. The upper three bytes are flag bits.
+
+.. list-table::
+ :widths: 32 48
+ :header-rows: 1
+
+ * - Value
+ - Description
+
+ * - XFS\_BMAP\_EXTENT\_MAP
+ - Add a mapping for file data.
+
+ * - XFS\_BMAP\_EXTENT\_UNMAP
+ - Remove a mapping for file data.
+
+Table: File block mapping update log intent types
+
+.. list-table::
+ :widths: 32 48
+ :header-rows: 1
+
+ * - Value
+ - Description
+
+ * - XFS\_BMAP\_EXTENT\_ATTR\_FORK
+ - Extent is for the attribute fork.
+
+ * - XFS\_BMAP\_EXTENT\_UNWRITTEN
+ - Extent is unwritten.
+
+Table: File block mapping update log intent flags
+
+The "file block mapping update intent" operation comes first; it tells the
+log that XFS wants to map or unmap some extents in a file. This record is
+crucial for correct log recovery because it enables us to spread a complex
+metadata update across multiple transactions while ensuring that a crash
+midway through the complex update will be replayed fully during log recovery.
+
+.. code:: c
+
+ struct xfs_bui_log_format {
+ __uint16_t bui_type;
+ __uint16_t bui_size;
+ __uint32_t bui_nextents;
+ __uint64_t bui_id;
+ struct xfs_map_extent bui_extents[1];
+ };
+
+**bui\_type**
+ The signature of an BUI operation, 0x1244. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**bui\_size**
+ Size of this log item. Should be 1.
+
+**bui\_nextents**
+ Number of file mappings. Should be 1.
+
+**bui\_id**
+ A 64-bit number that binds the corresponding BUD log item to this BUI log
+ item.
+
+**bui\_extents**
+ Variable-length array of file block mappings to update. There should only
+ be one mapping present.
+
+Completion of File Block Mapping Updates
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The "file block mapping update done" operation complements the "file
+block mapping update intent" operation. This second operation indicates that
+the update actually happened, so that log recovery needn’t replay the update.
+The BUD and the actual updates are typically found in a new transaction
+following the transaction in which the BUI was logged.
+
+.. code:: c
+
+ struct xfs_bud_log_format {
+ __uint16_t bud_type;
+ __uint16_t bud_size;
+ __uint32_t __pad;
+ __uint64_t bud_bui_id;
+ };
+
+**bud\_type**
+ The signature of an BUD operation, 0x1245. This value is in host-endian
+ order, not big-endian like the rest of XFS.
+
+**bud\_size**
+ Size of this log item. Should be 1.
+
+**bud\_bui\_id**
+ A 64-bit number that binds the corresponding BUI log item to this BUD log
+ item.
+
+Inode Updates
+^^^^^^^^^^^^^
+
+This operation records changes to an inode record. There are several types of
+inode updates, each corresponding to different parts of the inode record.
+Allowing updates to proceed at a sub-inode granularity reduces contention for
+the inode, since different parts of the inode can be updated simultaneously.
+
+The actual buffer data are stored in subsequent log items.
+
+The inode log format header is as follows:
+
+.. code:: c
+
+ typedef struct xfs_inode_log_format_64 {
+ __uint16_t ilf_type;
+ __uint16_t ilf_size;
+ __uint32_t ilf_fields;
+ __uint16_t ilf_asize;
+ __uint16_t ilf_dsize;
+ __uint32_t ilf_pad;
+ __uint64_t ilf_ino;
+ union {
+ __uint32_t ilfu_rdev;
+ uuid_t ilfu_uuid;
+ } ilf_u;
+ __int64_t ilf_blkno;
+ __int32_t ilf_len;
+ __int32_t ilf_boffset;
+ } xfs_inode_log_format_64_t;
+
+**ilf\_type**
+ The signature of an inode update operation, 0x123b. This value is in
+ host-endian order, not big-endian like the rest of XFS.
+
+**ilf\_size**
+ Number of operations involved in this update, including this format
+ operation.
+
+**ilf\_fields**
+ Specifies which parts of the inode are being updated. This can be certain
+ combinations of the following:
+
+.. list-table::
+ :widths: 24 56
+ :header-rows: 1
+
+ * - Flag
+ - Inode changes to log include:
+
+ * - XFS\_ILOG\_CORE
+ - The standard inode fields.
+
+ * - XFS\_ILOG\_DDATA
+ - Data fork’s local data.
+
+ * - XFS\_ILOG\_DEXT
+ - Data fork’s extent list.
+
+ * - XFS\_ILOG\_DBROOT
+ - Data fork’s B+tree root.
+
+ * - XFS\_ILOG\_DEV
+ - Data fork’s device number.
+
+ * - XFS\_ILOG\_UUID
+ - Data fork’s UUID contents.
+
+ * - XFS\_ILOG\_ADATA
+ - Attribute fork’s local data.
+
+ * - XFS\_ILOG\_AEXT
+ - Attribute fork’s extent list.
+
+ * - XFS\_ILOG\_ABROOT
+ - Attribute fork’s B+tree root.
+
+ * - XFS\_ILOG\_DOWNER
+ - Change the data fork owner on replay.
+
+ * - XFS\_ILOG\_AOWNER
+ - Change the attr fork owner on replay.
+
+ * - XFS\_ILOG\_TIMESTAMP
+ - Timestamps are dirty, but not necessarily anything else. Should never
+ appear on disk.
+
+ * - XFS\_ILOG\_NONCORE
+ - ( XFS_ILOG_DDATA \| XFS_ILOG_DEXT \| XFS_ILOG_DBROOT \|
+ XFS_ILOG_DEV \| XFS_ILOG_UUID \| XFS_ILOG_ADATA \| XFS_ILOG_AEXT
+ \| XFS_ILOG_ABROOT \| XFS_ILOG_DOWNER \| XFS_ILOG_AOWNER )
+
+ * - XFS\_ILOG\_DFORK
+ - ( XFS_ILOG_DDATA \| XFS_ILOG_DEXT \| XFS_ILOG_DBROOT
+
+ * - XFS\_ILOG\_AFORK
+ - ( XFS_ILOG_ADATA \| XFS_ILOG_AEXT \| XFS_ILOG_ABROOT )
+
+
+ * - XFS\_ILOG\_ALL
+ - ( XFS_ILOG_CORE \| XFS_ILOG_DDATA \| XFS_ILOG_DEXT \|
+ XFS_ILOG_DBROOT \| XFS_ILOG_DEV \| XFS_ILOG_UUID \|
+ XFS_ILOG_ADATA \| XFS_ILOG_AEXT \| XFS_ILOG_ABROOT \|
+ XFS_ILOG_TIMESTAMP \| XFS_ILOG_DOWNER \| XFS_ILOG_AOWNER )
+
+**ilf\_asize**
+ Size of the attribute fork, in bytes.
+
+**ilf\_dsize**
+ Size of the data fork, in bytes.
+
+**ilf\_ino**
+ Absolute node number.
+
+**ilfu\_rdev**
+ Device number information, for a device file update.
+
+**ilfu\_uuid**
+ UUID, for a UUID update?
+
+**ilf\_blkno**
+ Block number of the inode buffer, in sectors.
+
+**ilf\_len**
+ Length of inode buffer, in sectors.
+
+**ilf\_boffset**
+ Byte offset of the inode in the buffer.
+
+Be aware that there is a nearly identical xfs\_inode\_log\_format\_32 which
+may appear on disk. It is the same as xfs\_inode\_log\_format\_64, except that
+it is missing the ilf\_pad field and is 52 bytes long as opposed to 56 bytes.
+
+Inode Data Log Item
+^^^^^^^^^^^^^^^^^^^
+
+This region contains the new contents of a part of an inode, as described in
+the `previous section <#inode-updates>`__. There are no magic numbers.
+
+If XFS\_ILOG\_CORE is set in ilf\_fields, the correpsonding data buffer must
+be in the format struct xfs\_icdinode, which has the same format as the first
+96 bytes of an `inode <#on-disk-inode>`__, but is recorded in host byte order.
+
+Buffer Log Item
+^^^^^^^^^^^^^^^
+
+This operation writes parts of a buffer to disk. The regions to write are
+tracked in the data map; the actual buffer data are stored in subsequent log
+items.
+
+.. code:: c
+
+ typedef struct xfs_buf_log_format {
+ unsigned short blf_type;
+ unsigned short blf_size;
+ ushort blf_flags;
+ ushort blf_len;
+ __int64_t blf_blkno;
+ unsigned int blf_map_size;
+ unsigned int blf_data_map[XFS_BLF_DATAMAP_SIZE];
+ } xfs_buf_log_format_t;
+
+**blf\_type**
+ Magic number to specify a buffer log item, 0x123c.
+
+**blf\_size**
+ Number of buffer data items following this item.
+
+**blf\_flags**
+ Specifies flags associated with the buffer item. This can be any of the
+ following:
+
+.. list-table::
+ :widths: 24 56
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS\_BLF\_INODE\_BUF
+ - Inode buffer. These must be recovered before replaying items that change
+ this buffer.
+
+ * - XFS\_BLF\_CANCEL
+ - Don’t recover this buffer, blocks are being freed.
+
+ * - XFS\_BLF\_UDQUOT\_BUF
+ - User quota buffer, don’t recover if there’s a subsequent quotaoff.
+
+ * - XFS\_BLF\_PDQUOT\_BUF
+ - Project quota buffer, don’t recover if there’s a subsequent quotaoff.
+
+ * - XFS\_BLF\_GDQUOT\_BUF
+ - Group quota buffer, don’t recover if there’s a subsequent quotaoff.
+
+**blf\_len**
+ Number of sectors affected by this buffer.
+
+**blf\_blkno**
+ Block number to write, in sectors.
+
+**blf\_map\_size**
+ The size of blf\_data\_map, in 32-bit words.
+
+**blf\_data\_map**
+ This variable-sized array acts as a dirty bitmap for the logged buffer.
+ Each 1 bit represents a dirty region in the buffer, and each run of 1 bits
+ corresponds to a subsequent log item containing the new contents of the
+ buffer area. Each bit represents (blf\_len \* 512) / (blf\_map\_size \*
+ NBBY) bytes.
+
+Buffer Data Log Item
+^^^^^^^^^^^^^^^^^^^^
+
+This region contains the new contents of a part of a buffer, as described in
+the `previous section <#buffer-log-item>`__. There are no magic numbers.
+
+Update Quota File
+^^^^^^^^^^^^^^^^^
+
+This updates a block in a quota file. The buffer data must be in the next log
+item.
+
+.. code:: c
+
+ typedef struct xfs_dq_logformat {
+ __uint16_t qlf_type;
+ __uint16_t qlf_size;
+ xfs_dqid_t qlf_id;
+ __int64_t qlf_blkno;
+ __int32_t qlf_len;
+ __uint32_t qlf_boffset;
+ } xfs_dq_logformat_t;
+
+**qlf\_type**
+ The signature of an inode create operation, 0x123e. This value is in
+ host-endian order, not big-endian like the rest of XFS.
+
+**qlf\_size**
+ Size of this log item. Should be 2.
+
+**qlf\_id**
+ The user/group/project ID to alter.
+
+**qlf\_blkno**
+ Block number of the quota buffer, in sectors.
+
+**qlf\_len**
+ Length of the quota buffer, in sectors.
+
+**qlf\_boffset**
+ Buffer offset of the quota data to update, in bytes.
+
+Quota Update Data Log Item
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This region contains the new contents of a part of a buffer, as described in
+the `previous section <#quota-update-data-log-item>`__. There are no magic numbers.
+
+Disable Quota Log Item
+^^^^^^^^^^^^^^^^^^^^^^
+
+A request to disable quota controls has the following format:
+
+.. code:: c
+
+ typedef struct xfs_qoff_logformat {
+ unsigned short qf_type;
+ unsigned short qf_size;
+ unsigned int qf_flags;
+ char qf_pad[12];
+ } xfs_qoff_logformat_t;
+
+**qf\_type**
+ The signature of an inode create operation, 0x123d. This value is in
+ host-endian order, not big-endian like the rest of XFS.
+
+**qf\_size**
+ Size of this log item. Should be 1.
+
+**qf\_flags**
+ Specifies which quotas are being turned off. Can be a combination of the
+ following:
+
+.. list-table::
+ :widths: 20 60
+ :header-rows: 1
+
+ * - Flag
+ - Quota type to disable
+
+ * - XFS\_UQUOTA\_ACCT
+ - User quotas.
+
+ * - XFS\_PQUOTA\_ACCT
+ - Project quotas.
+
+ * - XFS\_GQUOTA\_ACCT
+ - Group quotas.
+
+Inode Creation Log Item
+^^^^^^^^^^^^^^^^^^^^^^^
+
+This log item is created when inodes are allocated in-core. When replaying
+this item, the specified inode records will be zeroed and some of the inode
+fields populated with default values.
+
+.. code:: c
+
+ struct xfs_icreate_log {
+ __uint16_t icl_type;
+ __uint16_t icl_size;
+ __be32 icl_ag;
+ __be32 icl_agbno;
+ __be32 icl_count;
+ __be32 icl_isize;
+ __be32 icl_length;
+ __be32 icl_gen;
+ };
+
+**icl\_type**
+ The signature of an inode create operation, 0x123f. This value is in
+ host-endian order, not big-endian like the rest of XFS.
+
+**icl\_size**
+ Size of this log item. Should be 1.
+
+**icl\_ag**
+ AG number of the inode chunk to create.
+
+**icl\_agbno**
+ AG block number of the inode chunk.
+
+**icl\_count**
+ Number of inodes to initialize.
+
+**icl\_isize**
+ Size of each inode, in bytes.
+
+**icl\_length**
+ Length of the extent being initialized, in blocks.
+
+**icl\_gen**
+ Inode generation number to write into the new inodes.
+
+xfs\_logprint Example
+~~~~~~~~~~~~~~~~~~~~~
+
+Here’s an example of dumping the XFS log contents with xfs\_logprint:
+
+::
+
+ # xfs_logprint /dev/sda
+ xfs_logprint: /dev/sda contains a mounted and writable filesystem
+ xfs_logprint:
+ data device: 0xfc03
+ log device: 0xfc03 daddr: 900931640 length: 879816
+
+ cycle: 48 version: 2 lsn: 48,0 tail_lsn: 47,879760
+ length of Log Record: 19968 prev offset: 879808 num ops: 53
+ uuid: 24afeec2-f418-46a2-a573-10091f5e200e format: little endian linux
+ h_size: 32768
+
+This is the log record header.
+
+::
+
+ Oper (0): tid: 30483aec len: 0 clientid: TRANS flags: START
+
+This operation indicates that we’re starting a transaction, so the next
+operation should record the transaction header.
+
+::
+
+ Oper (1): tid: 30483aec len: 16 clientid: TRANS flags: none
+ TRAN: type: CHECKPOINT tid: 30483aec num_items: 50
+
+This operation records a transaction header. There should be fifty operations
+in this transaction and the transaction ID is 0x30483aec.
+
+::
+
+ Oper (2): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 2 start blkno: 145400496 (0x8aaa2b0) len: 8 bmap size: 1 flags: 0x2000
+ Oper (3): tid: 30483aec len: 3712 clientid: TRANS flags: none
+ BUF DATA
+ ...
+ Oper (4): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 3 start blkno: 59116912 (0x3860d70) len: 8 bmap size: 1 flags: 0x2000
+ Oper (5): tid: 30483aec len: 128 clientid: TRANS flags: none
+ BUF DATA
+ 0 43544241 49010000 fa347000 2c357000 3a40b200 13000000 2343c200 13000000
+ 8 3296d700 13000000 375deb00 13000000 8a551501 13000000 56be1601 13000000
+ 10 af081901 13000000 ec741c01 13000000 9e911c01 13000000 69073501 13000000
+ 18 4e539501 13000000 6549501 13000000 5d0e7f00 14000000 c6908200 14000000
+
+ Oper (6): tid: 30483aec len: 640 clientid: TRANS flags: none
+ BUF DATA
+ 0 7f47c800 21000000 23c0e400 21000000 2d0dfe00 21000000 e7060c01 21000000
+ 8 34b91801 21000000 9cca9100 22000000 26e69800 22000000 4c969900 22000000
+ ...
+ 90 1cf69900 27000000 42f79c00 27000000 6a99e00 27000000 6a99e00 27000000
+ 98 6a99e00 27000000 6a99e00 27000000 6a99e00 27000000 6a99e00 27000000
+
+Operations 4-6 describe two updates to a single dirty buffer at disk address
+59,116,912. The first chunk of dirty data is 128 bytes long. Notice how the
+first four bytes of the first chunk is 0x43544241? Remembering that log items
+are in host byte order, reverse that to 0x41425443, which is the magic number
+for the free space B+tree ordered by size.
+
+The second chunk is 640 bytes. There are more buffer changes, so we’ll skip
+ahead a few operations:
+
+::
+
+ Oper (19): tid: 30483aec len: 56 clientid: TRANS flags: none
+ INODE: #regs: 2 ino: 0x63a73b4e flags: 0x1 dsize: 40
+ blkno: 1412688704 len: 16 boff: 7168
+ Oper (20): tid: 30483aec len: 96 clientid: TRANS flags: none
+ INODE CORE
+ magic 0x494e mode 0100600 version 2 format 3
+ nlink 1 uid 1000 gid 1000
+ atime 0x5633d58d mtime 0x563a391b ctime 0x563a391b
+ size 0x109dc8 nblocks 0x111 extsize 0x0 nextents 0x1b
+ naextents 0x0 forkoff 0 dmevmask 0x0 dmstate 0x0
+ flags 0x0 gen 0x389071be
+
+This is an update to the core of inode 0x63a73b4e. There were similar inode
+core updates after this, so we’ll skip ahead a bit:
+
+::
+
+ Oper (32): tid: 30483aec len: 56 clientid: TRANS flags: none
+ INODE: #regs: 3 ino: 0x4bde428 flags: 0x5 dsize: 16
+ blkno: 79553568 len: 16 boff: 4096
+ Oper (33): tid: 30483aec len: 96 clientid: TRANS flags: none
+ INODE CORE
+ magic 0x494e mode 0100644 version 2 format 2
+ nlink 1 uid 1000 gid 1000
+ atime 0x563a3924 mtime 0x563a3931 ctime 0x563a3931
+ size 0x1210 nblocks 0x2 extsize 0x0 nextents 0x1
+ naextents 0x0 forkoff 0 dmevmask 0x0 dmstate 0x0
+ flags 0x0 gen 0x2829c6f9
+ Oper (34): tid: 30483aec len: 16 clientid: TRANS flags: none
+ EXTENTS inode data
+
+This inode update changes both the core and also the data fork. Since we’re
+changing the block map, it’s unsurprising that one of the subsequent
+operations is an EFI:
+
+::
+
+ Oper (37): tid: 30483aec len: 32 clientid: TRANS flags: none
+ EFI: #regs: 1 num_extents: 1 id: 0xffff8801147b5c20
+ (s: 0x720daf, l: 1)
+ \----------------------------------------------------------------------------
+ Oper (38): tid: 30483aec len: 32 clientid: TRANS flags: none
+ EFD: #regs: 1 num_extents: 1 id: 0xffff8801147b5c20
+ \----------------------------------------------------------------------------
+ Oper (39): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 2 start blkno: 8 (0x8) len: 8 bmap size: 1 flags: 0x2800
+ Oper (40): tid: 30483aec len: 128 clientid: TRANS flags: none
+ AGF Buffer: XAGF
+ ver: 1 seq#: 0 len: 56308224
+ root BNO: 18174905 CNT: 18175030
+ level BNO: 2 CNT: 2
+ 1st: 41 last: 46 cnt: 6 freeblks: 35790503 longest: 19343245
+ \----------------------------------------------------------------------------
+ Oper (41): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 3 start blkno: 145398760 (0x8aa9be8) len: 8 bmap size: 1 flags: 0x2000
+ Oper (42): tid: 30483aec len: 128 clientid: TRANS flags: none
+ BUF DATA
+ Oper (43): tid: 30483aec len: 128 clientid: TRANS flags: none
+ BUF DATA
+ \----------------------------------------------------------------------------
+ Oper (44): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 3 start blkno: 145400224 (0x8aaa1a0) len: 8 bmap size: 1 flags: 0x2000
+ Oper (45): tid: 30483aec len: 128 clientid: TRANS flags: none
+ BUF DATA
+ Oper (46): tid: 30483aec len: 3584 clientid: TRANS flags: none
+ BUF DATA
+ \----------------------------------------------------------------------------
+ Oper (47): tid: 30483aec len: 24 clientid: TRANS flags: none
+ BUF: #regs: 3 start blkno: 59066216 (0x3854768) len: 8 bmap size: 1 flags: 0x2000
+ Oper (48): tid: 30483aec len: 128 clientid: TRANS flags: none
+ BUF DATA
+ Oper (49): tid: 30483aec len: 768 clientid: TRANS flags: none
+ BUF DATA
+
+Here we see an EFI, followed by an EFD, followed by updates to the AGF and the
+free space B+trees. Most probably, we just unmapped a few blocks from a file.
+
+::
+
+ Oper (50): tid: 30483aec len: 56 clientid: TRANS flags: none
+ INODE: #regs: 2 ino: 0x3906f20 flags: 0x1 dsize: 16
+ blkno: 59797280 len: 16 boff: 0
+ Oper (51): tid: 30483aec len: 96 clientid: TRANS flags: none
+ INODE CORE
+ magic 0x494e mode 0100644 version 2 format 2
+ nlink 1 uid 1000 gid 1000
+ atime 0x563a3938 mtime 0x563a3938 ctime 0x563a3938
+ size 0x0 nblocks 0x0 extsize 0x0 nextents 0x0
+ naextents 0x0 forkoff 0 dmevmask 0x0 dmstate 0x0
+ flags 0x0 gen 0x35ed661
+ \----------------------------------------------------------------------------
+ Oper (52): tid: 30483aec len: 0 clientid: TRANS flags: COMMIT
+
+One more inode core update and this transaction commits.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 15/22] docs: add XFS internal inodes to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (12 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 14/22] docs: add XFS log to the " Darrick J. Wong
@ 2018-10-04 4:19 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 16/22] docs: add preliminary XFS realtime rmapbt structures " Darrick J. Wong
` (7 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:19 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/globals.rst | 1
.../xfs-data-structures/internal_inodes.rst | 208 ++++++++++++++++++++
2 files changed, 209 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/internal_inodes.rst
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index 8ce83deafae5..1662540e40ef 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -7,3 +7,4 @@ Global Structures
.. include:: dabtrees.rst
.. include:: allocation_groups.rst
.. include:: journaling_log.rst
+.. include:: internal_inodes.rst
diff --git a/Documentation/filesystems/xfs-data-structures/internal_inodes.rst b/Documentation/filesystems/xfs-data-structures/internal_inodes.rst
new file mode 100644
index 000000000000..4c3a1bf1f822
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/internal_inodes.rst
@@ -0,0 +1,208 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Internal Inodes
+---------------
+
+XFS allocates several inodes when a filesystem is created. These are internal
+and not accessible from the standard directory structure. These inodes are
+only accessible from the superblock.
+
+Quota Inodes
+~~~~~~~~~~~~
+
+Prior to version 5 filesystems, two inodes can be allocated for quota
+management. The first inode will be used for user quotas. The second inode
+will be used for group quotas or project quotas, depending on mount options.
+Group and project quotas are mutually exclusive features in these
+environments.
+
+In version 5 or later filesystems, each quota type is allocated its own inode,
+making it possible to use group and project quota management simultaneously.
+
+- Project quota’s primary purpose is to track and monitor disk usage for
+ directories. For this to occur, the directory inode must have the
+ XFS\_DIFLAG\_PROJINHERIT flag set so all inodes created underneath the
+ directory inherit the project ID.
+
+- Inodes and blocks owned by ID zero do not have enforced quotas, but only
+ quota accounting.
+
+- Extended attributes do not contribute towards the ID’s quota.
+
+- To access each ID’s quota information in the file, seek to the ID offset
+ multiplied by the size of xfs\_dqblk\_t (136 bytes).
+
+.. figure:: images/76.png
+ :alt: Quota inode layout
+
+ Quota inode layout
+
+Quota information is stored in the data extents of the reserved quota inodes
+as an array of the xfs\_dqblk structures, where there is one array element for
+each ID in the system:
+
+.. code:: c
+
+ struct xfs_disk_dquot {
+ __be16 d_magic;
+ __u8 d_version;
+ __u8 d_flags;
+ __be32 d_id;
+ __be64 d_blk_hardlimit;
+ __be64 d_blk_softlimit;
+ __be64 d_ino_hardlimit;
+ __be64 d_ino_softlimit;
+ __be64 d_bcount;
+ __be64 d_icount;
+ __be32 d_itimer;
+ __be32 d_btimer;
+ __be16 d_iwarns;
+ __be16 d_bwarns;
+ __be32 d_pad0;
+ __be64 d_rtb_hardlimit;
+ __be64 d_rtb_softlimit;
+ __be64 d_rtbcount;
+ __be32 d_rtbtimer;
+ __be16 d_rtbwarns;
+ __be16 d_pad;
+ };
+ struct xfs_dqblk {
+ struct xfs_disk_dquot dd_diskdq;
+ char dd_fill[4];
+
+ /* version 5 filesystem fields begin here */
+ __be32 dd_crc;
+ __be64 dd_lsn;
+ uuid_t dd_uuid;
+ };
+
+**d\_magic**
+ Specifies the signature where these two bytes are 0x4451
+ (XFS\_DQUOT\_MAGIC), or \`\`DQ'' in ASCII.
+
+**d\_version**
+ The structure version, currently this is 1 (XFS\_DQUOT\_VERSION).
+
+**d\_flags**
+ Specifies which type of ID the structure applies to:
+
+.. code:: c
+
+ #define XFS_DQ_USER 0x0001
+ #define XFS_DQ_PROJ 0x0002
+ #define XFS_DQ_GROUP 0x0004
+
+**d\_id**
+ The ID for the quota structure. This will be a uid, gid or projid based on
+ the value of d\_flags.
+
+**d\_blk\_hardlimit**
+ The hard limit for the number of filesystem blocks the ID can own. The ID
+ will not be able to use more space than this limit. If it is attempted,
+ ENOSPC will be returned.
+
+**d\_blk\_softlimit**
+ The soft limit for the number of filesystem blocks the ID can own. The ID
+ can temporarily use more space than by d\_blk\_softlimit up to
+ d\_blk\_hardlimit. If the space is not freed by the time limit specified
+ by ID zero’s d\_btimer value, the ID will be denied more space until the
+ total blocks owned goes below d\_blk\_softlimit.
+
+**d\_ino\_hardlimit**
+ The hard limit for the number of inodes the ID can own. The ID will not be
+ able to create or own any more inodes if d\_icount reaches this value.
+
+**d\_ino\_softlimit**
+ The soft limit for the number of inodes the ID can own. The ID can
+ temporarily create or own more inodes than specified by d\_ino\_softlimit
+ up to d\_ino\_hardlimit. If the inode count is not reduced by the time
+ limit specified by ID zero’s d\_itimer value, the ID will be denied from
+ creating or owning more inodes until the count goes below
+ d\_ino\_softlimit.
+
+**d\_bcount**
+ How many filesystem blocks are actually owned by the ID.
+
+**d\_icount**
+ How many inodes are actually owned by the ID.
+
+**d\_itimer**
+ Specifies the time when the ID’s d\_icount exceeded d\_ino\_softlimit. The
+ soft limit will turn into a hard limit after the elapsed time exceeds ID
+ zero’s d\_itimer value. When d\_icount goes back below d\_ino\_softlimit,
+ d\_itimer is reset back to zero.
+
+**d\_btimer**
+ Specifies the time when the ID’s d\_bcount exceeded d\_blk\_softlimit. The
+ soft limit will turn into a hard limit after the elapsed time exceeds ID
+ zero’s d\_btimer value. When d\_bcount goes back below d\_blk\_softlimit,
+ d\_btimer is reset back to zero.
+
+**d\_iwarns**; \ **d\_bwarns**; \ **d\_rtbwarns**
+ Specifies how many times a warning has been issued. Currently not used.
+
+**d\_rtb\_hardlimit**
+ The hard limit for the number of real-time blocks the ID can own. The ID
+ cannot own more space on the real-time subvolume beyond this limit.
+
+**d\_rtb\_softlimit**
+ The soft limit for the number of real-time blocks the ID can own. The ID
+ can temporarily own more space than specified by d\_rtb\_softlimit up to
+ d\_rtb\_hardlimit. If d\_rtbcount is not reduced by the time limit
+ specified by ID zero’s d\_rtbtimer value, the ID will be denied from
+ owning more space until the count goes below d\_rtb\_softlimit.
+
+**d\_rtbcount**
+ How many real-time blocks are currently owned by the ID.
+
+**d\_rtbtimer**
+ Specifies the time when the ID’s d\_rtbcount exceeded d\_rtb\_softlimit.
+ The soft limit will turn into a hard limit after the elapsed time exceeds
+ ID zero’s d\_rtbtimer value. When d\_rtbcount goes back below
+ d\_rtb\_softlimit, d\_rtbtimer is reset back to zero.
+
+**dd\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**dd\_lsn**
+ Log sequence number of the last DQ block write.
+
+**dd\_crc**
+ Checksum of the DQ block.
+
+Real-time Inodes
+~~~~~~~~~~~~~~~~
+
+There are two inodes allocated to managing the real-time device’s space, the
+Bitmap Inode and the Summary Inode.
+
+Real-Time Bitmap Inode
+^^^^^^^^^^^^^^^^^^^^^^
+
+The real time bitmap inode, sb\_rbmino, tracks the used/free space in the
+real-time device using an old-style bitmap. One bit is allocated per real-time
+extent. The size of an extent is specified by the superblock’s sb\_rextsize
+value.
+
+The number of blocks used by the bitmap inode is equal to the number of
+real-time extents (sb\_rextents) divided by the block size (sb\_blocksize) and
+bits per byte. This value is stored in sb\_rbmblocks. The nblocks and extent
+array for the inode should match this. Each real time block gets its own bit
+in the bitmap.
+
+Real-Time Summary Inode
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The real time summary inode, sb\_rsumino, tracks the used and free space
+accounting information for the real-time device. This file indexes the
+approximate location of each free extent on the real-time device first by
+log2(extent size) and then by the real-time bitmap block number. The size of
+the summary inode file is equal to sb\_rbmblocks × log2(realtime device size)
+× sizeof(xfs\_suminfo\_t). The entry for a given log2(extent size) and
+rtbitmap block number is 0 if there is no free extents of that size at that
+rtbitmap location, and positive if there are any.
+
+This data structure is not particularly space efficient, however it is a very
+fast way to provide the same data as the two free space B+trees for regular
+files since the space is preallocated and metadata maintenance is minimal.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 16/22] docs: add preliminary XFS realtime rmapbt structures to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (13 preceding siblings ...)
2018-10-04 4:19 ` [PATCH 15/22] docs: add XFS internal inodes " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 17/22] docs: add XFS inode format " Darrick J. Wong
` (6 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/internal_inodes.rst | 2
.../filesystems/xfs-data-structures/rtrmapbt.rst | 230 ++++++++++++++++++++
2 files changed, 232 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/rtrmapbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/internal_inodes.rst b/Documentation/filesystems/xfs-data-structures/internal_inodes.rst
index 4c3a1bf1f822..0faf58caf8f6 100644
--- a/Documentation/filesystems/xfs-data-structures/internal_inodes.rst
+++ b/Documentation/filesystems/xfs-data-structures/internal_inodes.rst
@@ -206,3 +206,5 @@ rtbitmap location, and positive if there are any.
This data structure is not particularly space efficient, however it is a very
fast way to provide the same data as the two free space B+trees for regular
files since the space is preallocated and metadata maintenance is minimal.
+
+.. include:: rtrmapbt.rst
diff --git a/Documentation/filesystems/xfs-data-structures/rtrmapbt.rst b/Documentation/filesystems/xfs-data-structures/rtrmapbt.rst
new file mode 100644
index 000000000000..1573ec4f09ec
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/rtrmapbt.rst
@@ -0,0 +1,230 @@
+Real-Time Reverse-Mapping B+tree
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ **Note**
+
+ This data structure is under construction! Details may change.
+
+If the reverse-mapping B+tree and real-time storage device features are
+enabled, the real-time device has its own reverse block-mapping B+tree.
+
+As mentioned in the chapter about `reconstruction <#metadata-reconstruction>`__, this
+data structure is another piece of the puzzle necessary to reconstruct the
+data or attribute fork of a file from reverse-mapping records; we can also use
+it to double-check allocations to ensure that we are not accidentally
+cross-linking blocks, which can cause severe damage to the filesystem.
+
+This B+tree is only present if the XFS\_SB\_FEAT\_RO\_COMPAT\_RMAPBT feature
+is enabled and a real time device is present. The feature requires a version 5
+filesystem.
+
+The real-time reverse mapping B+tree is rooted in an inode’s data fork; the
+inode number is given by the sb\_rrmapino field in the superblock. The B+tree
+blocks themselves are stored in the regular filesystem. The structures used
+for an inode’s B+tree root are:
+
+.. code:: c
+
+ struct xfs_rtrmap_root {
+ __be16 bb_level;
+ __be16 bb_numrecs;
+ };
+
+- On disk, the B+tree node starts with the xfs\_rtrmap\_root header followed
+ by an array of xfs\_rtrmap\_key values and then an array of
+ xfs\_rtrmap\_ptr\_t values. The size of both arrays is specified by the
+ header’s bb\_numrecs value.
+
+- The root node in the inode can only contain up to 10 key/pointer pairs for
+ a standard 512 byte inode before a new level of nodes is added between the
+ root and the leaves. di\_forkoff should always be zero, because there are
+ no extended attributes.
+
+Each record in the real-time reverse-mapping B+tree has the following
+structure:
+
+.. code:: c
+
+ struct xfs_rtrmap_rec {
+ __be64 rm_startblock;
+ __be64 rm_blockcount;
+ __be64 rm_owner;
+ __be64 rm_fork:1;
+ __be64 rm_bmbt:1;
+ __be64 rm_unwritten:1;
+ __be64 rm_unused:7;
+ __be64 rm_offset:54;
+ };
+
+**rm\_startblock**
+ Real-time device block number of this record.
+
+**rm\_blockcount**
+ The length of this extent, in real-time blocks.
+
+**rm\_owner**
+ A 64-bit number describing the owner of this extent. This must be an inode
+ number, because the real-time device is for file data only.
+
+**rm\_fork**
+ If rm\_owner describes an inode, this can be 1 if this record is for an
+ attribute fork. This value will always be zero for real-time extents.
+
+**rm\_bmbt**
+ If rm\_owner describes an inode, this can be 1 to signify that this record
+ is for a block map B+tree block. In this case, rm\_offset has no meaning.
+ This value will always be zero for real-time extents.
+
+**rm\_unwritten**
+ A flag indicating that the extent is unwritten. This corresponds to the
+ flag in the `extent record <#data-extents>`__ format which means
+ XFS\_EXT\_UNWRITTEN.
+
+**rm\_offset**
+ The 54-bit logical file block offset, if rm\_owner describes an inode.
+
+ **Note**
+
+ The single-bit flag values rm\_unwritten, rm\_fork, and rm\_bmbt are
+ packed into the larger fields in the C structure definition.
+
+The key has the following structure:
+
+.. code:: c
+
+ struct xfs_rtrmap_key {
+ __be64 rm_startblock;
+ __be64 rm_owner;
+ __be64 rm_fork:1;
+ __be64 rm_bmbt:1;
+ __be64 rm_reserved:1;
+ __be64 rm_unused:7;
+ __be64 rm_offset:54;
+ };
+
+- All block numbers are 64-bit real-time device block numbers.
+
+- The bb\_magic value is "MAPR" (0x4d415052).
+
+- The xfs\_btree\_lblock\_t header is used for intermediate B+tree node as
+ well as the leaves.
+
+- Each pointer is associated with two keys. The first of these is the "low
+ key", which is the key of the smallest record accessible through the
+ pointer. This low key has the same meaning as the key in all other btrees.
+ The second key is the high key, which is the maximum of the largest key
+ that can be used to access a given record underneath the pointer. Recall
+ that each record in the real-time reverse mapping b+tree describes an
+ interval of physical blocks mapped to an interval of logical file block
+ offsets; therefore, it makes sense that a range of keys can be used to find
+ to a record.
+
+xfs\_db rtrmapbt Example
+""""""""""""""""""""""""
+
+This example shows a real-time reverse-mapping B+tree from a freshly populated
+root filesystem:
+
+::
+
+ xfs_db> sb 0
+ xfs_db> addr rrmapino
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0100000
+ core.version = 3
+ core.format = 5 (rtrmapbt)
+ ...
+ u3.rtrmapbt.level = 3
+ u3.rtrmapbt.numrecs = 1
+ u3.rtrmapbt.keys[1] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,
+ owner_hi,offset_hi,attrfork_hi,bmbtblock_hi]
+ 1:[1,132,1,0,0,1705337,133,54431,0,0]
+ u3.rtrmapbt.ptrs[1] = 1:671
+ xfs_db> addr u3.rtrmapbt.ptrs[1]
+ xfs_db> p
+ magic = 0x4d415052
+ level = 2
+ numrecs = 8
+ leftsib = null
+ rightsib = null
+ bno = 5368
+ lsn = 0x400000000
+ uuid = 98bbde42-67e7-46a5-a73e-d64a76b1b5ce
+ owner = 131
+ crc = 0x2560d199 (correct)
+ keys[1-8] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,
+ offset_hi,attrfork_hi,bmbtblock_hi]
+ 1:[1,132,1,0,0,17749,132,17749,0,0]
+ 2:[17751,132,17751,0,0,35499,132,35499,0,0]
+ 3:[35501,132,35501,0,0,53249,132,53249,0,0]
+ 4:[53251,132,53251,0,0,1658473,133,7567,0,0]
+ 5:[1658475,133,7569,0,0,1667473,133,16567,0,0]
+ 6:[1667475,133,16569,0,0,1685223,133,34317,0,0]
+ 7:[1685225,133,34319,0,0,1694223,133,43317,0,0]
+ 8:[1694225,133,43319,0,0,1705337,133,54431,0,0]
+ ptrs[1-8] = 1:134 2:238 3:345 4:453 5:795 6:563 7:670 8:780
+
+We arbitrarily pick pointer 7 (twice) to traverse downwards:
+
+::
+
+ xfs_db> addr ptrs[7]
+ xfs_db> p
+ magic = 0x4d415052
+ level = 1
+ numrecs = 36
+ leftsib = 563
+ rightsib = 780
+ bno = 5360
+ lsn = 0
+ uuid = 98bbde42-67e7-46a5-a73e-d64a76b1b5ce
+ owner = 131
+ crc = 0x6807761d (correct)
+ keys[1-36] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,
+ offset_hi,attrfork_hi,bmbtblock_hi]
+ 1:[1685225,133,34319,0,0,1685473,133,34567,0,0]
+ 2:[1685475,133,34569,0,0,1685723,133,34817,0,0]
+ 3:[1685725,133,34819,0,0,1685973,133,35067,0,0]
+ ...
+ 34:[1693475,133,42569,0,0,1693723,133,42817,0,0]
+ 35:[1693725,133,42819,0,0,1693973,133,43067,0,0]
+ 36:[1693975,133,43069,0,0,1694223,133,43317,0,0]
+ ptrs[1-36] = 1:669 2:672 3:674...34:722 35:723 36:725
+ xfs_db> addr ptrs[7]
+ xfs_db> p
+ magic = 0x4d415052
+ level = 0
+ numrecs = 125
+ leftsib = 678
+ rightsib = 681
+ bno = 5440
+ lsn = 0
+ uuid = 98bbde42-67e7-46a5-a73e-d64a76b1b5ce
+ owner = 131
+ crc = 0xefce34d4 (correct)
+ recs[1-125] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+ 1:[1686725,1,133,35819,0,0,0]
+ 2:[1686727,1,133,35821,0,0,0]
+ 3:[1686729,1,133,35823,0,0,0]
+ ...
+ 123:[1686969,1,133,36063,0,0,0]
+ 124:[1686971,1,133,36065,0,0,0]
+ 125:[1686973,1,133,36067,0,0,0]
+
+Several interesting things pop out here. The first record shows that inode 133
+has mapped real-time block 1,686,725 at offset 35,819. We confirm this by
+looking at the block map for that inode:
+
+::
+
+ xfs_db> inode 133
+ xfs_db> p core.realtime
+ core.realtime = 1
+ xfs_db> bmap
+ data offset 35817 startblock 1686723 (1/638147) count 1 flag 0
+ data offset 35819 startblock 1686725 (1/638149) count 1 flag 0
+ data offset 35821 startblock 1686727 (1/638151) count 1 flag 0
+
+Notice that inode 133 has the real-time flag set, which means that its data
+blocks are all allocated from the real-time device.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 17/22] docs: add XFS inode format to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (14 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 16/22] docs: add preliminary XFS realtime rmapbt structures " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 18/22] docs: add XFS data extent map doc " Darrick J. Wong
` (5 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/dynamic.rst | 2
.../xfs-data-structures/ondisk_inode.rst | 558 ++++++++++++++++++++
2 files changed, 560 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/ondisk_inode.rst
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 895c94e95889..945b07be2034 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -2,3 +2,5 @@
Dynamic Allocated Structures
============================
+
+.. include:: ondisk_inode.rst
diff --git a/Documentation/filesystems/xfs-data-structures/ondisk_inode.rst b/Documentation/filesystems/xfs-data-structures/ondisk_inode.rst
new file mode 100644
index 000000000000..77ecd2917489
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/ondisk_inode.rst
@@ -0,0 +1,558 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+On-Disk Inode
+-------------
+
+All files, directories, and links are stored on disk with inodes and descend
+from the root inode with its number defined in the
+`superblock <#superblocks>`__. The previous section on `AG Inode
+Management <#ag-inode-management>`__ describes the allocation and management
+of inodes on disk. This section describes the contents of inodes themselves.
+
+An inode is divided into 3 parts:
+
+.. figure:: images/23.png
+ :alt: On-disk inode sections
+
+ On-disk inode sections
+
+- The core contains what the inode represents, stat data, and information
+ describing the data and attribute forks.
+
+- The di\_u "data fork" contains normal data related to the inode. Its
+ contents depends on the file type specified by di\_core.di\_mode (eg.
+ regular file, directory, link, etc) and how much information is contained
+ in the file which determined by di\_core.di\_format. The following union to
+ represent this data is declared as follows:
+
+.. code:: c
+
+ union {
+ xfs_bmdr_block_t di_bmbt;
+ xfs_bmbt_rec_t di_bmx[1];
+ xfs_dir2_sf_t di_dir2sf;
+ char di_c[1];
+ xfs_dev_t di_dev;
+ uuid_t di_muuid;
+ char di_symlink[1];
+ } di_u;
+
+- The di\_a "attribute fork" contains extended attributes. Its layout is
+ determined by the di\_core.di\_aformat value. Its representation is
+ declared as follows:
+
+.. code:: c
+
+ union {
+ xfs_bmdr_block_t di_abmbt;
+ xfs_bmbt_rec_t di_abmx[1];
+ xfs_attr_shortform_t di_attrsf;
+ } di_a;
+
+- The above two unions are rarely used in the XFS code, but the structures
+ within the union are directly cast depending on the di\_mode/di\_format
+ and di\_aformat values. They are referenced in this document to make it
+ easier to explain the various structures in use within the inode.
+
+The remaining space in the inode after di\_next\_unlinked where the two forks
+are located is called the inode’s "literal area". This starts at offset
+100 (0x64) in a version 1 or 2 inode, and offset 176 (0xb0) in a version 3
+inode.
+
+The space for each of the two forks in the literal area is determined by the
+inode size, and di\_core.di\_forkoff. The data fork is located between the
+start of the literal area and di\_forkoff. The attribute fork is located
+between di\_forkoff and the end of the inode.
+
+Inode Core
+~~~~~~~~~~
+
+The inode’s core is 96 bytes on a V4 filesystem and 176 bytes on a V5
+filesystem. It contains information about the file itself including most stat
+data information about data and attribute forks after the core within the
+inode. It uses the following structure:
+
+.. code:: c
+
+ struct xfs_dinode_core {
+ __uint16_t di_magic;
+ __uint16_t di_mode;
+ __int8_t di_version;
+ __int8_t di_format;
+ __uint16_t di_onlink;
+ __uint32_t di_uid;
+ __uint32_t di_gid;
+ __uint32_t di_nlink;
+ __uint16_t di_projid;
+ __uint16_t di_projid_hi;
+ __uint8_t di_pad[6];
+ __uint16_t di_flushiter;
+ xfs_timestamp_t di_atime;
+ xfs_timestamp_t di_mtime;
+ xfs_timestamp_t di_ctime;
+ xfs_fsize_t di_size;
+ xfs_rfsblock_t di_nblocks;
+ xfs_extlen_t di_extsize;
+ xfs_extnum_t di_nextents;
+ xfs_aextnum_t di_anextents;
+ __uint8_t di_forkoff;
+ __int8_t di_aformat;
+ __uint32_t di_dmevmask;
+ __uint16_t di_dmstate;
+ __uint16_t di_flags;
+ __uint32_t di_gen;
+
+ /* di_next_unlinked is the only non-core field in the old dinode */
+ __be32 di_next_unlinked;
+
+ /* version 5 filesystem (inode version 3) fields start here */
+ __le32 di_crc;
+ __be64 di_changecount;
+ __be64 di_lsn;
+ __be64 di_flags2;
+ __be32 di_cowextsize;
+ __u8 di_pad2[12];
+ xfs_timestamp_t di_crtime;
+ __be64 di_ino;
+ uuid_t di_uuid;
+
+ };
+
+**di\_magic**
+ The inode signature; these two bytes are "IN" (0x494e).
+
+**di\_mode**
+ Specifies the mode access bits and type of file using the standard S\_Ixxx
+ values defined in stat.h.
+
+**di\_version**
+ Specifies the inode version which currently can only be 1, 2, or 3. The
+ inode version specifies the usage of the di\_onlink, di\_nlink and
+ di\_projid values in the inode core. Initially, inodes are created as v1
+ but can be converted on the fly to v2 when required. v3 inodes are created
+ only for v5 filesystems.
+
+**di\_format**
+ Specifies the format of the data fork in conjunction with the di\_mode
+ type. This can be one of several values. For directories and links, it can
+ be "local"
+ where all metadata associated with the file is within the inode; "extents"
+ where the inode contains an array of extents to other filesystem blocks
+ which contain the associated metadata or data; or
+ "btree" where the inode contains a B+tree
+ root node which points to filesystem blocks containing the metadata or data.
+ Migration between the formats depends on the amount of metadata associated with
+ the inode. "dev" is used for character and block devices while
+ "uuid" is
+ currently not used. "rmap" indicates that a reverse-mapping B+tree is
+ rooted in the fork.
+
+.. code:: c
+
+ typedef enum xfs_dinode_fmt {
+ XFS_DINODE_FMT_DEV,
+ XFS_DINODE_FMT_LOCAL,
+ XFS_DINODE_FMT_EXTENTS,
+ XFS_DINODE_FMT_BTREE,
+ XFS_DINODE_FMT_UUID,
+ XFS_DINODE_FMT_RMAP,
+ } xfs_dinode_fmt_t;
+
+**di\_onlink**
+ In v1 inodes, this specifies the number of links to the inode from
+ directories. When the number exceeds 65535, the inode is converted to v2
+ and the link count is stored in di\_nlink.
+
+**di\_uid**
+ Specifies the owner’s UID of the inode.
+
+**di\_gid**
+ Specifies the owner’s GID of the inode.
+
+**di\_nlink**
+ Specifies the number of links to the inode from directories. This is
+ maintained for both inode versions for current versions of XFS. Prior to
+ v2 inodes, this field was part of di\_pad.
+
+**di\_projid**
+ Specifies the owner’s project ID in v2 inodes. An inode is converted to v2
+ if the project ID is set. This value must be zero for v1 inodes.
+
+**di\_projid\_hi**
+ Specifies the high 16 bits of the owner’s project ID in v2 inodes, if the
+ XFS\_SB\_VERSION2\_PROJID32BIT feature is set; and zero otherwise.
+
+**di\_pad[6]**
+ Reserved, must be zero.
+
+**di\_flushiter**
+ Incremented on flush.
+
+**di\_atime**
+ Specifies the last access time of the files using UNIX time conventions
+ the following structure. This value may be undefined if the filesystem is
+ mounted with the "noatime" option. XFS supports timestamps with
+ nanosecond resolution:
+
+.. code:: c
+
+ struct xfs_timestamp {
+ __int32_t t_sec;
+ __int32_t t_nsec;
+ };
+
+**di\_mtime**
+ Specifies the last time the file was modified.
+
+**di\_ctime**
+ Specifies when the inode’s status was last changed.
+
+**di\_size**
+ Specifies the EOF of the inode in bytes. This can be larger or smaller
+ than the extent space (therefore actual disk space) used for the inode.
+ For regular files, this is the filesize in bytes, directories, the space
+ taken by directory entries and for links, the length of the symlink.
+
+**di\_nblocks**
+ Specifies the number of filesystem blocks used to store the inode’s data
+ including relevant metadata like B+trees. This does not include blocks
+ used for extended attributes.
+
+**di\_extsize**
+ Specifies the extent size for filesystems with real-time devices or an
+ extent size hint for standard filesystems. For normal filesystems, and
+ with directories, the XFS\_DIFLAG\_EXTSZINHERIT flag must be set in
+ di\_flags if this field is used. Inodes created in these directories will
+ inherit the di\_extsize value and have XFS\_DIFLAG\_EXTSIZE set in their
+ di\_flags. When a file is written to beyond allocated space, XFS will
+ attempt to allocate additional disk space based on this value.
+
+**di\_nextents**
+ Specifies the number of data extents associated with this inode.
+
+**di\_anextents**
+ Specifies the number of extended attribute extents associated with this
+ inode.
+
+**di\_forkoff**
+ Specifies the offset into the inode’s literal area where the extended
+ attribute fork starts. This is an 8-bit value that is multiplied by 8 to
+ determine the actual offset in bytes (ie. attribute data is 64-bit
+ aligned). This also limits the maximum size of the inode to 2048 bytes.
+ This value is initially zero until an extended attribute is created. When
+ in attribute is added, the nature of di\_forkoff depends on the
+ XFS\_SB\_VERSION2\_ATTR2BIT flag in the superblock. Refer to `Extended
+ Attribute Versions <#extended-attribute-versions>`__ for more details.
+
+**di\_aformat**
+ Specifies the format of the attribute fork. This uses the same values as
+ di\_format, but restricted to "local", "extents" and "btree"
+ formats for extended attribute data.
+
+**di\_dmevmask**
+ DMAPI event mask.
+
+**di\_dmstate**
+ DMAPI state.
+
+**di\_flags**
+ Specifies flags associated with the inode. This can be a combination of
+ the following values:
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_DIFLAG_REALTIME
+ - The inode's data is located on the real-time device.
+
+ * - XFS_DIFLAG_PREALLOC
+ - The inode's extents have been preallocated.
+
+ * - XFS_DIFLAG_NEWRTBM
+ - Specifies the +sb_rbmino+ uses the new real-time bitmap format
+
+ * - XFS_DIFLAG_IMMUTABLE
+ - Specifies the inode cannot be modified.
+
+ * - XFS_DIFLAG_APPEND
+ - The inode is in append only mode.
+
+ * - XFS_DIFLAG_SYNC
+ - The inode is written synchronously.
+
+ * - XFS_DIFLAG_NOATIME
+ - The inode's +di_atime+ is not updated.
+
+ * - XFS_DIFLAG_NODUMP
+ - Specifies the inode is to be ignored by xfsdump.
+
+ * - XFS_DIFLAG_RTINHERIT
+ - For directory inodes, new inodes inherit the XFS_DIFLAG_REALTIME bit.
+
+ * - XFS_DIFLAG_PROJINHERIT
+ - For directory inodes, new inodes inherit the ``di_projid`` value.
+
+ * - XFS_DIFLAG_NOSYMLINKS
+ - For directory inodes, symlinks cannot be created.
+
+ * - XFS_DIFLAG_EXTSIZE
+ - Specifies the extent size for real-time files or an extent size hint for
+ regular files.
+
+ * - XFS_DIFLAG_EXTSZINHERIT
+ - For directory inodes, new inodes inherit the +di_extsize+ value.
+
+ * - XFS_DIFLAG_NODEFRAG
+ - Specifies the inode is to be ignored when defragmenting the filesystem.
+
+ * - XFS_DIFLAG_FILESTREAMS
+ - Use the filestream allocator. The filestreams allocator allows a
+ directory to reserve an entire allocation group for exclusive use by
+ files created in that directory. Files in other directories cannot use
+ AGs reserved by other directories.
+
+Table: Version 2 Inode flags
+
+**di\_gen**
+ A generation number used for inode identification. This is used by tools
+ that do inode scanning such as backup tools and xfsdump. An inode’s
+ generation number can change by unlinking and creating a new file that
+ reuses the inode.
+
+**di\_next\_unlinked**
+ See the section on `unlinked inode pointers <#unlinked-pointer>`__ for
+ more information.
+
+**di\_crc**
+ Checksum of the inode.
+
+**di\_changecount**
+ Counts the number of changes made to the attributes in this inode.
+
+**di\_lsn**
+ Log sequence number of the last inode write.
+
+**di\_flags2**
+ Specifies extended flags associated with a v3 inode.
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS\_DIFLAG2\_DAX
+ - For a file, enable DAX to increase performance on persistent-memory
+ storage. If set on a directory, files created in the directory will
+ inherit this flag.
+
+ * - XFS\_DIFLAG2\_REFLINK
+ - This inode shares (or has shared) data blocks with another inode.
+
+ * - XFS\_DIFLAG2\_COWEXTSIZE
+ - For files, this is the extent size hint for copy on write operations;
+ see di\_cowextsize for details. For directories, the value in
+ di\_cowextsize will be copied to all newly created files and
+ directories.
+
+Table: Version 3 Inode flags
+
+**di\_cowextsize**
+ Specifies the extent size hint for copy on write operations. When
+ allocating extents for a copy on write operation, the allocator will be
+ asked to align its allocations to either di\_cowextsize blocks or
+ di\_extsize blocks, whichever is greater. The XFS\_DIFLAG2\_COWEXTSIZE
+ flag must be set if this field is used. If this field and its flag are set
+ on a directory file, the value will be copied into any files or
+ directories created within this directory. During a block sharing
+ operation, this value will be copied from the source file to the
+ destination file if the sharing operation completely overwrites the
+ destination file’s contents and the destination file does not already have
+ di\_cowextsize set.
+
+**di\_pad2**
+ Padding for future expansion of the inode.
+
+**di\_crtime**
+ Specifies the time when this inode was created.
+
+**di\_ino**
+ The full inode number of this inode.
+
+**di\_uuid**
+ The UUID of this inode, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+Unlinked Pointer
+~~~~~~~~~~~~~~~~
+
+The di\_next\_unlinked value in the inode is used to track inodes that have
+been unlinked (deleted) but are still open by a program. When an inode is in
+this state, the inode is added to one of the `AGI’s <#ag-inode-management>`__
+agi\_unlinked hash buckets. The AGI unlinked bucket points to an inode and the
+di\_next\_unlinked value points to the next inode in the chain. The last inode
+in the chain has di\_next\_unlinked set to NULL (-1).
+
+Once the last reference is released, the inode is removed from the unlinked
+hash chain and di\_next\_unlinked is set to NULL. In the case of a system
+crash, XFS recovery will complete the unlink process for any inodes found in
+these lists.
+
+The only time the unlinked fields can be seen to be used on disk is either on
+an active filesystem or a crashed system. A cleanly unmounted or recovered
+filesystem will not have any inodes in these unlink hash chains.
+
+.. figure:: images/28.png
+ :alt: Unlinked inode pointer
+
+ Unlinked inode pointer
+
+Data Fork
+~~~~~~~~~
+
+The structure of the inode’s data fork based is on the inode’s type and
+di\_format. The data fork begins at the start of the inode’s "literal area".
+This area starts at offset 100 (0x64), or offset 176 (0xb0) in a v3 inode. The
+size of the data fork is determined by the type and format. The maximum size is
+determined by the inode size and di_forkoff. In code, use the XFS_DFORK_PTR
+macro specifying XFS_DATA_FORK for the "which" parameter. Alternatively,
+the XFS\_DFORK\_DPTR macro can be used.
+
+Each of the following sub-sections summarises the contents of the data fork
+based on the inode type.
+
+Regular Files (S\_IFREG)
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The data fork specifies the file’s data extents. The extents specify where the
+file’s actual data is located within the filesystem. Extents can have 2
+formats which is defined by the di\_format value:
+
+- XFS\_DINODE\_FMT\_EXTENTS: The extent data is fully contained within the
+ inode which contains an array of extents to the filesystem blocks for the
+ file’s data. To access the extents, cast the return value from
+ XFS\_DFORK\_DPTR to xfs\_bmbt\_rec\_t\*.
+
+- XFS\_DINODE\_FMT\_BTREE: The extent data is contained in the leaves of a
+ B+tree. The inode contains the root node of the tree and is accessed by
+ casting the return value from XFS\_DFORK\_DPTR to xfs\_bmdr\_block\_t\*.
+
+Details for each of these data extent formats are covered in the `Data
+Extents <#data-extents>`__ later on.
+
+Directories (S\_IFDIR)
+^^^^^^^^^^^^^^^^^^^^^^
+
+The data fork contains the directory’s entries and associated data. The format
+of the entries is also determined by the di\_format value and can be one of 3
+formats:
+
+- XFS\_DINODE\_FMT\_LOCAL: The directory entries are fully contained within
+ the inode. This is accessed by casting the value from XFS\_DFORK\_DPTR to
+ xfs\_dir2\_sf\_t\*.
+
+- XFS\_DINODE\_FMT\_EXTENTS: The actual directory entries are located in
+ another filesystem block, the inode contains an array of extents to these
+ filesystem blocks (xfs\_bmbt\_rec\_t\*).
+
+- XFS\_DINODE\_FMT\_BTREE: The directory entries are contained in the leaves
+ of a B+tree. The inode contains the root node (xfs\_bmdr\_block\_t\*).
+
+Details for each of these directory formats are covered in the
+`Directories <#directories>`__ later on.
+
+Symbolic Links (S\_IFLNK)
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The data fork contains the contents of the symbolic link. The format of the
+link is determined by the di\_format value and can be one of 2 formats:
+
+- XFS\_DINODE\_FMT\_LOCAL: The symbolic link is fully contained within the
+ inode. This is accessed by casting the return value from XFS\_DFORK\_DPTR
+ to char\*.
+
+- XFS\_DINODE\_FMT\_EXTENTS: The actual symlink is located in another
+ filesystem block, the inode contains the extents to these filesystem blocks
+ (xfs\_bmbt\_rec\_t\*).
+
+Details for symbolic links is covered in the section about `Symbolic
+Links <#symbolic-links>`__.
+
+Other File Types
+^^^^^^^^^^^^^^^^
+
+For character and block devices (S\_IFCHR and S\_IFBLK), cast the value from
+XFS\_DFORK\_DPTR to xfs\_dev\_t\*.
+
+Attribute Fork
+~~~~~~~~~~~~~~
+
+The attribute fork in the inode always contains the location of the extended
+attributes associated with the inode.
+
+The location of the attribute fork in the inode’s literal area is specified by
+the di\_forkoff value in the inode’s core. If this value is zero, the inode
+does not contain any extended attributes. If non-zero, the attribute fork’s
+byte offset into the literal area can be computed from di\_forkoff × 8.
+Attributes must be allocated on a 64-bit boundary on the disk. To access the
+extended attributes in code, use the XFS\_DFORK\_PTR macro specifying
+XFS\_ATTR\_FORK for the "which" parameter. Alternatively, the
+XFS\_DFORK\_APTR macro can be used.
+
+The structure of the attribute fork depends on the di\_aformat value in the
+inode. It can be one of the following values:
+
+- XFS\_DINODE\_FMT\_LOCAL: The extended attributes are contained entirely
+ within the inode. This is accessed by casting the value from
+ XFS\_DFORK\_APTR to xfs\_attr\_shortform\_t\*.
+
+- XFS\_DINODE\_FMT\_EXTENTS: The attributes are located in another filesystem
+ block, the inode contains an array of pointers to these filesystem blocks.
+ They are accessed by casting the value from XFS\_DFORK\_APTR to
+ xfs\_bmbt\_rec\_t\*.
+
+- XFS\_DINODE\_FMT\_BTREE: The extents for the attributes are contained in
+ the leaves of a B+tree. The inode contains the root node of the tree and is
+ accessed by casting the value from XFS\_DFORK\_APTR to
+ xfs\_bmdr\_block\_t\*.
+
+Detailed information on the layouts of extended attributes are covered in the
+`Extended Attributes <#extended-attributes>`__ in this document.
+
+Extended Attribute Versions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Extended attributes come in two versions: "attr1" or "attr2". The
+attribute version is specified by the XFS\_SB\_VERSION2\_ATTR2BIT flag in the
+sb\_features2 field in the superblock. It determines how the inode’s extra
+space is split between di\_u and di\_a forks which also determines how the
+di\_forkoff value is maintained in the inode’s core.
+
+With "attr1" attributes, the di\_forkoff is set to somewhere in the middle
+of the space between the core and end of the inode and never changes (which
+has the effect of artificially limiting the space for data information). As
+the data fork grows, when it gets to di\_forkoff, it will move the data to the
+next format level (ie. local < extent < btree). If very little space is used
+for either attributes or data, then a good portion of the available inode
+space is wasted with this version.
+
+"attr2" was introduced to maximum the utilisation of the inode’s literal
+area. The di\_forkoff starts at the end of the inode and works its way to the
+data fork as attributes are added. Attr2 is highly recommended if extended
+attributes are used.
+
+The following diagram compares the two versions:
+
+.. figure:: images/30.png
+ :alt: Extended attribute layouts
+
+ Extended attribute layouts
+
+Note that because di\_forkoff is an 8-bit value measuring units of 8 bytes,
+the maximum size of an inode is 2\ :sup:`8` × 2\ :sup:`3` = 2\ :sup:`11` =
+2048 bytes.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 18/22] docs: add XFS data extent map doc to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (15 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 17/22] docs: add XFS inode format " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 19/22] docs: add XFS directory structure " Darrick J. Wong
` (4 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/data_extents.rst | 337 ++++++++++++++++++++
.../filesystems/xfs-data-structures/dynamic.rst | 1
2 files changed, 338 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/data_extents.rst
diff --git a/Documentation/filesystems/xfs-data-structures/data_extents.rst b/Documentation/filesystems/xfs-data-structures/data_extents.rst
new file mode 100644
index 000000000000..a410397e9892
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/data_extents.rst
@@ -0,0 +1,337 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Data Extents
+------------
+
+XFS manages space using extents, which are defined as a starting location and
+length. A fork in an XFS inode maps a logical offset to a space extent. This
+enables a file’s extent map to support sparse files (i.e. "holes" in the
+file). A flag is also used to specify if the extent has been preallocated but
+has not yet been written (unwritten extent).
+
+A file can have more than one extent if one chunk of contiguous disk space is
+not available for the file. As a file grows, the XFS space allocator will
+attempt to keep space contiguous and to merge extents. If more than one file
+is being allocated space in the same AG at the same time, multiple extents for
+the files will occur as the extent allocations interleave. The effect of this
+can vary depending on the extent allocator used in the XFS driver.
+
+An extent is 128 bits in size and uses the following packed layout:
+
+.. figure:: images/31.png
+ :alt: Extent record format
+
+ Extent record format
+
+The extent is represented by the xfs\_bmbt\_rec structure which uses a big
+endian format on-disk. In-core management of extents use the xfs\_bmbt\_irec
+structure which is the unpacked version of xfs\_bmbt\_rec:
+
+.. code:: c
+
+ struct xfs_bmbt_irec {
+ xfs_fileoff_t br_startoff;
+ xfs_fsblock_t br_startblock;
+ xfs_filblks_t br_blockcount;
+ xfs_exntst_t br_state;
+ };
+
+**br\_startoff**
+ Logical block offset of this mapping.
+
+**br\_startblock**
+ Filesystem block of this mapping.
+
+**br\_blockcount**
+ The length of this mapping.
+
+**br\_state**
+ The extent br\_state field uses the following enum declaration:
+
+.. code:: c
+
+ typedef enum {
+ XFS_EXT_NORM,
+ XFS_EXT_UNWRITTEN,
+ XFS_EXT_INVALID
+ } xfs_exntst_t;
+
+Some other points about extents:
+
+- The xfs\_bmbt\_rec\_32\_t and xfs\_bmbt\_rec\_64\_t structures were
+ effectively the same as xfs\_bmbt\_rec\_t, just different representations
+ of the same 128 bits in on-disk big endian format. xfs\_bmbt\_rec\_32\_t
+ was removed and xfs\_bmbt\_rec\_64\_t renamed to xfs\_bmbt\_rec\_t some
+ time ago.
+
+- When a file is created and written to, XFS will endeavour to keep the
+ extents within the same AG as the inode. It may use a different AG if the
+ AG is busy or there is no space left in it.
+
+- If a file is zero bytes long, it will have no extents and di\_nblocks and
+ di\_nexents will be zero. Any file with data will have at least one extent,
+ and each extent can use from 1 to over 2 million blocks (2:sup:`21`) on the
+ filesystem. For a default 4KB block size filesystem, a single extent can be
+ up to 8GB in length.
+
+The following two subsections cover the two methods of storing extent
+information for a file. The first is the fastest and simplest where the inode
+completely contains an extent array to the file’s data. The second is slower
+and more complex B+tree which can handle thousands to millions of extents
+efficiently.
+
+Extent List
+~~~~~~~~~~~
+
+If the entire extent list is short enough to fit within the inode’s fork
+region, we say that the fork is in "extent list" format. This is the most
+optimal in terms of speed and resource consumption. The trade-off is the file
+can only have a few extents before the inode runs out of space.
+
+The data fork of the inode contains an array of extents; the size of the array
+is determined by the inode’s di\_nextents value.
+
+.. figure:: images/32.png
+ :alt: Inode data fork extent layout
+
+ Inode data fork extent layout
+
+The number of extents that can fit in the inode depends on the inode size and
+di\_forkoff. For a default 256 byte inode with no extended attributes, a file
+can have up to 9 extents with this format. On a default v5 filesystem with 512
+byte inodes, a file can have up to 21 extents with this format. Beyond that,
+extents have to use the B+tree format.
+
+xfs\_db Inode Data Fork Extents Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+An 8MB file with one extent:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0100644
+ core.version = 1
+ core.format = 2 (extents)
+ ...
+ core.size = 8294400
+ core.nblocks = 2025
+ core.extsize = 0
+ core.nextents = 1
+ core.naextents = 0
+ core.forkoff = 0
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,25356,2025,0]
+
+A 24MB file with three extents:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.format = 2 (extents)
+ ...
+ core.size = 24883200
+ core.nblocks = 6075
+ core.nextents = 3
+ ...
+ u.bmx[0-2] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,27381,2025,0]
+ 1:[2025,31431,2025,0]
+ 2:[4050,35481,2025,0]
+
+Raw disk version of the inode with the third extent highlighted (di\_u starts
+at offset 0x64):
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e 81 a4 01 02 00 01 00 00 00 00 00 00 00 00 IN..............
+ 10: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 01 ................
+ 20: 44 b6 88 dd 2f 8a ed d0 44 b6 88 f7 10 8c 5b de D.......D.......
+ 30: 44 b6 88 f7 10 8c 5b d0 00 00 00 00 01 7b b0 00 D...............
+ 40: 00 00 00 00 00 00 17 bb 00 00 00 00 00 00 00 03 ................
+ 50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 0d ................
+ 70: 5e a0 07 e9 00 00 00 00 00 0f d2 00 00 00 00 0f ................
+ 80: 58 e0 07 e9 00 00 00 00 00 1f a4 00 00 00 00 11 X...............
+ 90: 53 20 07 e9 00 00 00 00 00 00 00 00 00 00 00 00 S...............
+ a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ be: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ co: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ do: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ fo: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+We can expand the highlighted section into the following bit array from MSB to
+LSB with the file offset and the block count highlighted:
+
+::
+
+ 127-96: 0000 0000 0000 0000 0000 0000 0000 0000
+ 95-64: 0000 0000 0001 1111 1010 0100 0000 0000
+ 63-32: 0000 0000 0000 0000 0000 0000 0000 1111
+ 31-0 : 0101 1000 1110 0000 0000 0111 1110 1001
+
+ Grouping by highlights we get:
+ file offset = 0x0fd2 (4050)
+ start block = 0x7ac7 (31431)
+ block count = 0x07e9 (2025)
+
+A 4MB file with two extents and a hole in the middle, the first extent
+containing 64KB of data, the second about 4MB in containing 32KB (write 64KB,
+lseek 4MB, write 32KB operations):
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.format = 2 (extents)
+ ...
+ core.size = 4063232
+ core.nblocks = 24
+ core.nextents = 2
+ ...
+ u.bmx[0-1] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,37506,16,0]
+ 1:[984,37522,8,0]
+
+B+tree Extent List
+~~~~~~~~~~~~~~~~~~
+
+To manage extent maps that cannot fit in the inode fork area, XFS uses `long
+format B+trees <#long-format-b-trees>`__. The root node of the B+tree is stored
+in the inode’s data fork. All block pointers for extent B+trees are 64-bit
+filesystem block numbers.
+
+For a single level B+tree, the root node points to the B+tree’s leaves. Each
+leaf occupies one filesystem block and contains a header and an array of
+extents sorted by the file’s offset. Each leaf has left and right (or backward
+and forward) block pointers to adjacent leaves. For a standard 4KB filesystem
+block, a leaf can contain up to 254 extents before a B+tree rebalance is
+triggered.
+
+For a multi-level B+tree, the root node points to other B+tree nodes which
+eventually point to the extent leaves. B+tree keys are based on the file’s
+offset and have pointers to the next level down. Nodes at each level in the
+B+tree also have pointers to the adjacent nodes.
+
+The base B+tree node is used for extents, directories and extended attributes.
+The structures used for an inode’s B+tree root are:
+
+.. code:: c
+
+ struct xfs_bmdr_block {
+ __be16 bb_level;
+ __be16 bb_numrecs;
+ };
+ struct xfs_bmbt_key {
+ xfs_fileoff_t br_startoff;
+ };
+ typedef xfs_fsblock_t xfs_bmbt_ptr_t, xfs_bmdr_ptr_t;
+
+- On disk, the B+tree node starts with the xfs\_bmdr\_block\_t header
+ followed by an array of xfs\_bmbt\_key\_t values and then an array of
+ xfs\_bmbt\_ptr\_t values. The size of both arrays is specified by the
+ header’s bb\_numrecs value.
+
+- The root node in the inode can only contain up to 9 key/pointer pairs for a
+ standard 256 byte inode before a new level of nodes is added between the
+ root and the leaves. This will be less if di\_forkoff is not zero (i.e.
+ attributes are in use on the inode).
+
+- The magic number for a BMBT block is "BMAP" (0x424d4150). On a v5
+ filesystem, this is "BMA3" (0x424d4133).
+
+- For intermediate nodes, the data following xfs\_btree\_lblock is the same
+ as the root node: array of xfs\_bmbt\_key value followed by an array of
+ xfs\_bmbt\_ptr\_t values that starts halfway through the block (offset
+ 0x808 for a 4096 byte filesystem block).
+
+- For leaves, an array of xfs\_bmbt\_rec extents follow the
+ xfs\_btree\_lblock header.
+
+- Nodes and leaves use the same value for bb\_magic.
+
+- The bb\_level value determines if the node is an intermediate node or a
+ leaf. Leaves have a bb\_level of zero, nodes are one or greater.
+
+- Intermediate nodes, like leaves, can contain up to 254 pointers to leaf
+ blocks for a standard 4KB filesystem block size as both the keys and
+ pointers are 64 bits in size.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/35.png
+ :alt: Single level extent B+tree
+
+ Single level extent B+tree
+
+ .. figure:: images/36.png
+ :alt: Multiple level extent B+tree
+
+ Multiple level extent B+tree
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/35.png
+ :scale: 45%
+ :alt: Single level extent B+tree
+
+ Single level extent B+tree
+
+ .. figure:: images/36.png
+ :scale: 40%
+ :alt: Multiple level extent B+tree
+
+ Multiple level extent B+tree
+
+xfs\_db bmbt Example
+^^^^^^^^^^^^^^^^^^^^
+
+In this example, we dissect the data fork of a VM image that is sufficiently
+sparse and interleaved to have become a B+tree.
+
+::
+
+ xfs_db> inode 132
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0100600
+ core.version = 3
+ core.format = 3 (btree)
+ ...
+ u3.bmbt.level = 1
+ u3.bmbt.numrecs = 3
+ u3.bmbt.keys[1-3] = [startoff] 1:[0] 2:[9072] 3:[13136]
+ u3.bmbt.ptrs[1-3] = 1:8568 2:8569 3:8570
+
+As you can see, the block map B+tree is rooted in the inode. This tree has two
+levels, so let’s go down a level to look at the records:
+
+::
+
+ xfs_db> addr u3.bmbt.ptrs[1]
+ xfs_db> p
+ magic = 0x424d4133
+ level = 0
+ numrecs = 251
+ leftsib = null
+ rightsib = 8569
+ bno = 68544
+ lsn = 0x100000006
+ uuid = 9579903c-333f-4673-a7d4-3254c05816ea
+ owner = 132
+ crc = 0xc61513dc (correct)
+ recs[1-251] = [startoff,startblock,blockcount,extentflag]
+ 1:[0,8520,48,0] 2:[48,4421,16,0] 3:[80,9136,16,0] 4:[96,8569,16,0]
+ 5:[144,8601,32,0] 6:[192,8637,16,0] 7:[240,8680,16,0] 8:[288,9870,16,0]
+ 9:[320,9920,16,0] 10:[336,9950,16,0] 11:[384,4004,32,0]
+ 12:[432,6771,16,0] 13:[480,2702,16,0] 14:[528,8420,16,0]
+ ...
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 945b07be2034..5ba6f3940808 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -4,3 +4,4 @@ Dynamic Allocated Structures
============================
.. include:: ondisk_inode.rst
+.. include:: data_extents.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 19/22] docs: add XFS directory structure to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (16 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 18/22] docs: add XFS data extent map doc " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 20/22] docs: add XFS extended attributes structures " Darrick J. Wong
` (3 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../xfs-data-structures/directories.rst | 1688 ++++++++++++++++++++
.../filesystems/xfs-data-structures/dynamic.rst | 1
2 files changed, 1689 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/directories.rst
diff --git a/Documentation/filesystems/xfs-data-structures/directories.rst b/Documentation/filesystems/xfs-data-structures/directories.rst
new file mode 100644
index 000000000000..34f34c7aedea
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/directories.rst
@@ -0,0 +1,1688 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Directories
+-----------
+
+ **Note**
+
+ Only v2 directories covered here. v1 directories are obsolete.
+
+ **Note**
+
+ The term "block" in this section will refer to directory blocks, not
+ filesystem blocks unless otherwise specified.
+
+The size of a "directory block" is defined by the
+`superblock’s <#superblocks>`__ sb\_dirblklog value. The size in bytes =
+sb\_blocksize × 2\ :sup:`sb\_dirblklog`. For example, if sb\_blocksize = 4096
+and sb\_dirblklog = 2, the directory block size is 16384 bytes. Directory
+blocks are always allocated in multiples based on sb\_dirblklog. Directory
+blocks cannot be more that 65536 bytes in size.
+
+All directory entries contain the following "data":
+
+- The entry’s name (counted string consisting of a single byte namelen
+ followed by name consisting of an array of 8-bit chars without a NULL
+ terminator).
+
+- The entry’s absolute `inode number <#inode-numbers>`__, which are always 64
+ bits (8 bytes) in size except a special case for shortform directories.
+
+- An offset or tag used for iterative readdir calls.
+
+- If the XFS\_SB\_FEAT\_INCOMPAT\_FTYPE feature flag is set, each directory
+ entry contains an ftype field that caches the inode’s type to avoid having
+ to perform an inode lookup.
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_DIR3_FT_UNKNOWN
+ - Entry points to an unknown inode type. This should never appear on
+ disk.
+
+ * - XFS_DIR3_FT_REG_FILE
+ - Entry points to a file.
+
+ * - XFS_DIR3_FT_DIR
+ - Entry points to another directory.
+
+ * - XFS_DIR3_FT_CHRDEV
+ - Entry points to a character device.
+
+ * - XFS_DIR3_FT_BLKDEV
+ - Entry points to a block device.
+
+ * - XFS_DIR3_FT_FIFO
+ - Entry points to a FIFO.
+
+ * - XFS_DIR3_FT_SOCK
+ - Entry points to a socket.
+
+ * - XFS_DIR3_FT_SYMLINK
+ - Entry points to a symbolic link.
+
+ * - XFS_DIR3_FT_WHT
+ - Entry points to an overlayfs whiteout file. This (as far as the author
+ knows) has never appeared on disk.
+
+Table: ftype values
+
+All non-shortform directories also contain two additional structures:
+"leaves"
+and "freespace indexes".
+
+- Leaves contain the sorted hashed name value (xfs\_da\_hashname() in
+ xfs\_da\_btree.c) and associated "address" which points to the
+ effective offset into the directory’s data structures. Leaves are used to
+ optimise lookup operations.
+
+- Freespace indexes contain free space/empty entry tracking for quickly
+ finding an appropriately sized location for new entries. They maintain the
+ largest free space for each "data" block.
+
+A few common types are used for the directory structures:
+
+.. code:: c
+
+ typedef __uint16_t xfs_dir2_data_off_t;
+ typedef __uint32_t xfs_dir2_dataptr_t;
+
+Short Form Directories
+~~~~~~~~~~~~~~~~~~~~~~
+
+- Directory entries are stored within the inode.
+
+- The only data stored is the name, inode number, and offset. No "leaf" or
+ "freespace index" information is required as an inode can only store a
+ few entries.
+
+- "." is not stored (as it’s in the inode itself), and ".." is a
+ dedicated parent field in the header.
+
+- The number of directories that can be stored in an inode depends on the
+ `inode <#on-disk-inode>`__ size, the number of entries, the length of the
+ entry names, and extended attribute data.
+
+- Once the number of entries exceeds the space available in the inode, the
+ format is converted to a `block directory <#block-directories>`__.
+
+- Shortform directory data is packed as tightly as possible on the disk with
+ the remaining space zeroed:
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf {
+ xfs_dir2_sf_hdr_t hdr;
+ xfs_dir2_sf_entry_t list[1];
+ } xfs_dir2_sf_t;
+
+**hdr**
+ Short form directory header.
+
+**list**
+ An array of variable-length directory entry records.
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf_hdr {
+ __uint8_t count;
+ __uint8_t i8count;
+ xfs_dir2_inou_t parent;
+ } xfs_dir2_sf_hdr_t;
+
+**count**
+ Number of directory entries.
+
+**i8count**
+ Number of directory entries requiring 64-bit entries, if any inode numbers
+ require 64-bits. Zero otherwise.
+
+**parent**
+ The absolute inode number of this directory’s parent.
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf_entry {
+ __uint8_t namelen;
+ xfs_dir2_sf_off_t offset;
+ __uint8_t name[1];
+ __uint8_t ftype;
+ xfs_dir2_inou_t inumber;
+ } xfs_dir2_sf_entry_t;
+
+**namelen**
+ Length of the name, in bytes.
+
+**offset**
+ Offset tag used to assist with directory iteration.
+
+**name**
+ The name of the directory entry. The entry is not NULL-terminated.
+
+**ftype**
+ The type of the inode. This is used to avoid reading the inode while
+ iterating a directory. The XFS\_SB\_VERSION2\_FTYPE feature must be set,
+ or this field will not be present.
+
+**inumber**
+ The inode number that this entry points to. The length is either 32 or 64
+ bits, depending on whether icount or i8count, respectively, are set in the
+ header.
+
+.. figure:: images/39.png
+ :alt: Short form directory layout
+
+ Short form directory layout
+
+- Inode numbers are stored using 4 or 8 bytes depending on whether all the
+ inode numbers for the directory fit in 4 bytes (32 bits) or not. If all
+ inode numbers fit in 4 bytes, the header’s count value specifies the number
+ of entries in the directory and i8count will be zero. If any inode number
+ exceeds 4 bytes, all inode numbers will be 8 bytes in size and the header’s
+ i8count value specifies the number of entries requiring larger inodes.
+ i4count is still the number of entries. The following union covers the
+ shortform inode number structure:
+
+.. code:: c
+
+ typedef struct { __uint8_t i[8]; } xfs_dir2_ino8_t;
+ typedef struct { __uint8_t i[4]; } xfs_dir2_ino4_t;
+ typedef union {
+ xfs_dir2_ino8_t i8;
+ xfs_dir2_ino4_t i4;
+ } xfs_dir2_inou_t;
+
+xfs\_db Short Form Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A directory is created with 4 files, all inode numbers fitting within 4 bytes:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 1 (local)
+ core.nlinkv1 = 2
+ ...
+ core.size = 94
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u.sfdir2.hdr.count = 4
+ u.sfdir2.hdr.i8count = 0
+ u.sfdir2.hdr.parent.i4 = 128 /* parent = root inode */
+ u.sfdir2.list[0].namelen = 15
+ u.sfdir2.list[0].offset = 0x30
+ u.sfdir2.list[0].name = "frame000000.tst"
+ u.sfdir2.list[0].inumber.i4 = 25165953
+ u.sfdir2.list[1].namelen = 15
+ u.sfdir2.list[1].offset = 0x50
+ u.sfdir2.list[1].name = "frame000001.tst"
+ u.sfdir2.list[1].inumber.i4 = 25165954
+ u.sfdir2.list[2].namelen = 15
+ u.sfdir2.list[2].offset = 0x70
+ u.sfdir2.list[2].name = "frame000002.tst"
+ u.sfdir2.list[2].inumber.i4 = 25165955
+ u.sfdir2.list[3].namelen = 15
+ u.sfdir2.list[3].offset = 0x90
+ u.sfdir2.list[3].name = "frame000003.tst"
+ u.sfdir2.list[3].inumber.i4 = 25165956
+
+The raw data on disk with the first entry highlighted. The six byte header
+precedes the first entry:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e 41 ed 01 01 00 02 00 00 00 00 00 00 00 00 INA.............
+ 10: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 02 ................
+ 20: 44 ad 3a 83 1d a9 4a d0 44 ad 3a ab 0b c7 a7 d0 D.....J.D.......
+ 30: 44 ad 3a ab 0b c7 a7 d0 00 00 00 00 00 00 00 5e D...............
+ 40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 04 00 00 00 00 80 0f 00 30 66 72 61 ............0fra
+ 70: 6d 65 30 30 30 30 30 30 2e 74 73 74 01 80 00 81 me000000.tst....
+ 80: 0f 00 50 66 72 61 6d 65 30 30 30 30 30 31 2e 74 ..Pframe000001.t
+ 90: 73 74 01 80 00 82 0f 00 70 66 72 61 6d 65 30 30 st......pframe00
+ a0: 30 30 30 32 2e 74 73 74 01 80 00 83 0f 00 90 66 0002.tst........
+ b0: 72 61 6d 65 30 30 30 30 30 33 2e 74 73 74 01 80 rame000003.tst..
+ cO: 00 84 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+Next, an entry is deleted (frame000001.tst), and any entries after the deleted
+entry are moved or compacted to "cover" the hole:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 1 (local)
+ core.nlinkv1 = 2
+ ...
+ core.size = 72
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u.sfdir2.hdr.count = 3
+ u.sfdir2.hdr.i8count = 0
+ u.sfdir2.hdr.parent.i4 = 128
+ u.sfdir2.list[0].namelen = 15
+ u.sfdir2.list[0].offset = 0x30
+ u.sfdir2.list[0].name = "frame000000.tst"
+ u.sfdir2.list[0].inumber.i4 = 25165953
+ u.sfdir2.list[1].namelen = 15
+ u.sfdir2.list[1].offset = 0x70
+ u.sfdir2.list[1].name = "frame000002.tst"
+ u.sfdir2.list[1].inumber.i4 = 25165955
+ u.sfdir2.list[2].namelen = 15
+ u.sfdir2.list[2].offset = 0x90
+ u.sfdir2.list[2].name = "frame000003.tst"
+ u.sfdir2.list[2].inumber.i4 = 25165956
+
+Raw disk data, the space beyond the shortform entries is invalid and could be
+non-zero:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e 41 ed 01 01 00 02 00 00 00 00 00 00 00 00 INA.............
+ 10: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 03 ................
+ 20: 44 b2 45 a2 09 fd e4 50 44 b2 45 a3 12 ee b5 d0 D.E....PD.E.....
+ 30: 44 b2 45 a3 12 ee b5 d0 00 00 00 00 00 00 00 48 D.E............H
+ 40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 03 00 00 00 00 80 0f 00 30 66 72 61 ............0fra
+ 70: 6d 65 30 30 30 30 30 30 2e 74 73 74 01 80 00 81 me000000.tst....
+ 80: 0f 00 70 66 72 61 6d 65 30 30 30 30 30 32 2e 74 ..pframe000002.t
+ 90: 73 74 01 80 00 83 0f 00 90 66 72 61 6d 65 30 30 st.......frame00
+ a0: 30 30 30 33 2e 74 73 74 01 80 00 84 0f 00 90 66 0003.tst.......f
+ b0: 72 61 6d 65 30 30 30 30 30 33 2e 74 73 74 01 80 rame000003.tst..
+ c0: 00 84 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+This is an example of mixed 4-byte and 8-byte inodes in a directory:
+
+::
+
+ xfs_db> inode 1024
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 3
+ core.format = 1 (local)
+ core.nlinkv2 = 9
+ ...
+ core.size = 125
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u3.sfdir3.hdr.count = 7
+ u3.sfdir3.hdr.i8count = 4
+ u3.sfdir3.hdr.parent.i8 = 1024
+ u3.sfdir3.list[0].namelen = 3
+ u3.sfdir3.list[0].offset = 0x60
+ u3.sfdir3.list[0].name = "git"
+ u3.sfdir3.list[0].inumber.i8 = 1027
+ u3.sfdir3.list[0].filetype = 2
+ u3.sfdir3.list[1].namelen = 4
+ u3.sfdir3.list[1].offset = 0x70
+ u3.sfdir3.list[1].name = "home"
+ u3.sfdir3.list[1].inumber.i8 = 13422826546
+ u3.sfdir3.list[1].filetype = 2
+ u3.sfdir3.list[2].namelen = 10
+ u3.sfdir3.list[2].offset = 0x80
+ u3.sfdir3.list[2].name = "mike"
+ u3.sfdir3.list[2].inumber.i8 = 4299308032
+ u3.sfdir3.list[2].filetype = 2
+ u3.sfdir3.list[3].namelen = 3
+ u3.sfdir3.list[3].offset = 0x98
+ u3.sfdir3.list[3].name = "mtr"
+ u3.sfdir3.list[3].inumber.i8 = 13433252916
+ u3.sfdir3.list[3].filetype = 2
+ u3.sfdir3.list[4].namelen = 3
+ u3.sfdir3.list[4].offset = 0xa8
+ u3.sfdir3.list[4].name = "vms"
+ u3.sfdir3.list[4].inumber.i8 = 16647516355
+ u3.sfdir3.list[4].filetype = 2
+ u3.sfdir3.list[5].namelen = 5
+ u3.sfdir3.list[5].offset = 0xb8
+ u3.sfdir3.list[5].name = "rsync"
+ u3.sfdir3.list[5].inumber.i8 = 3494912
+ u3.sfdir3.list[5].filetype = 2
+ u3.sfdir3.list[6].namelen = 3
+ u3.sfdir3.list[6].offset = 0xd0
+ u3.sfdir3.list[6].name = "tmp"
+ u3.sfdir3.list[6].inumber.i8 = 1593379
+ u3.sfdir3.list[6].filetype = 2
+
+Block Directories
+~~~~~~~~~~~~~~~~~
+
+When the shortform directory space exceeds the space in an inode, the
+directory data is moved into a new single directory block outside the inode.
+The inode’s format is changed from "local" to "extent" Following is a
+list of points about block directories.
+
+- All directory data is stored within the one directory block, including
+ "." and
+ ".." entries which are mandatory.
+
+- The block also contains "leaf" and "freespace index" information.
+
+- The location of the block is defined by the inode’s in-core `extent
+ list <#extent-list>`__: the di\_u.u\_bmx[0] value. The file offset in the
+ extent must always be zero and the length = (directory block size /
+ filesystem block size). The block number points to the filesystem block
+ containing the directory data.
+
+- Block directory data is stored in the following structures:
+
+.. code:: c
+
+ #define XFS_DIR2_DATA_FD_COUNT 3
+ typedef struct xfs_dir2_block {
+ xfs_dir2_data_hdr_t hdr;
+ xfs_dir2_data_union_t u[1];
+ xfs_dir2_leaf_entry_t leaf[1];
+ xfs_dir2_block_tail_t tail;
+ } xfs_dir2_block_t;
+
+**hdr**
+ Directory block header. On a v5 filesystem this is
+ xfs\_dir3\_data\_hdr\_t.
+
+**u**
+ Union of directory and unused entries.
+
+**leaf**
+ Hash values of the entries in this block.
+
+**tail**
+ Bookkeeping for the leaf entries.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_hdr {
+ __uint32_t magic;
+ xfs_dir2_data_free_t bestfree[XFS_DIR2_DATA_FD_COUNT];
+ } xfs_dir2_data_hdr_t;
+
+**magic**
+ Magic number for this directory block.
+
+**bestfree**
+ An array pointing to free regions in the directory block.
+
+On a v5 filesystem, directory and attribute blocks are formatted with v3
+headers, which contain extra data:
+
+.. code:: c
+
+ struct xfs_dir3_blk_hdr {
+ __be32 magic;
+ __be32 crc;
+ __be64 blkno;
+ __be64 lsn;
+ uuid_t uuid;
+ __be64 owner;
+ };
+
+**magic**
+ Magic number for this directory block.
+
+**crc**
+ Checksum of the directory block.
+
+**blkno**
+ Block number of this directory block.
+
+**lsn**
+ Log sequence number of the last write to this block.
+
+**uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**owner**
+ The inode number that this directory block belongs to.
+
+.. code:: c
+
+ struct xfs_dir3_data_hdr {
+ struct xfs_dir3_blk_hdr hdr;
+ xfs_dir2_data_free_t best_free[XFS_DIR2_DATA_FD_COUNT];
+ __be32 pad;
+ };
+
+**hdr**
+ The v5 directory/attribute block header.
+
+**best\_free**
+ An array pointing to free regions in the directory block.
+
+**pad**
+ Padding to maintain a 64-bit alignment.
+
+Within the block, data structures are as follows:
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_free {
+ xfs_dir2_data_off_t offset;
+ xfs_dir2_data_off_t length;
+ } xfs_dir2_data_free_t;
+
+**offset**
+ Block offset of a free block, in bytes.
+
+**length**
+ Length of the free block, in bytes.
+
+Space inside the directory block can be used for directory entries or unused
+entries. This is signified via a union of the two types:
+
+.. code:: c
+
+ typedef union {
+ xfs_dir2_data_entry_t entry;
+ xfs_dir2_data_unused_t unused;
+ } xfs_dir2_data_union_t;
+
+**entry**
+ A directory entry.
+
+**unused**
+ An unused entry.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_entry {
+ xfs_ino_t inumber;
+ __uint8_t namelen;
+ __uint8_t name[1];
+ __uint8_t ftype;
+ xfs_dir2_data_off_t tag;
+ } xfs_dir2_data_entry_t;
+
+**inumber**
+ The inode number that this entry points to.
+
+**namelen**
+ Length of the name, in bytes.
+
+**name**
+ The name associated with this entry.
+
+**ftype**
+ The type of the inode. This is used to avoid reading the inode while
+ iterating a directory. The XFS\_SB\_VERSION2\_FTYPE feature must be set,
+ or this field will not be present.
+
+**tag**
+ Starting offset of the entry, in bytes. This is used for directory
+ iteration.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_unused {
+ __uint16_t freetag; /* 0xffff */
+ xfs_dir2_data_off_t length;
+ xfs_dir2_data_off_t tag;
+ } xfs_dir2_data_unused_t;
+
+**freetag**
+ Magic number signifying that this is an unused entry. Must be 0xFFFF.
+
+**length**
+ Length of this unused entry, in bytes.
+
+**tag**
+ Starting offset of the entry, in bytes.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_entry {
+ xfs_dahash_t hashval;
+ xfs_dir2_dataptr_t address;
+ } xfs_dir2_leaf_entry_t;
+
+**hashval**
+ Hash value of the name of the directory entry. This is used to speed up
+ entry lookups.
+
+**address**
+ Block offset of the entry, in eight byte units.
+
+.. code:: c
+
+ typedef struct xfs_dir2_block_tail {
+ __uint32_t count;
+ __uint32_t stale;
+ } xfs_dir2_block_tail_t;
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of free leaf entries.
+
+Following is a diagram of how these pieces fit together for a block directory.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/43.png
+ :alt: Block directory layout
+
+ Block directory layout
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/43.png
+ :scale: 45%
+ :alt: Block directory layout
+
+ Block directory layout
+
+- The magic number in the header is "XD2B" (0x58443242), or "XDB3"
+ (0x58444233) on a v5 filesystem.
+
+- The tag in the xfs\_dir2\_data\_entry\_t structure stores its offset from
+ the start of the block.
+
+- The start of a free space region is marked with the
+ xfs\_dir2\_data\_unused\_t structure where the freetag is 0xffff. The
+ freetag and length overwrites the inumber for an entry. The tag is located
+ at length - sizeof(tag) from the start of the unused entry on-disk.
+
+- The bestfree array in the header points to as many as three of the largest
+ spaces of free space within the block for storing new entries sorted by
+ largest to third largest. If there are less than 3 empty regions, the
+ remaining bestfree elements are zeroed. The offset specifies the offset
+ from the start of the block in bytes, and the length specifies the size of
+ the free space in bytes. The location each points to must contain the above
+ xfs\_dir2\_data\_unused\_t structure. As a block cannot exceed 64KB in
+ size, each is a 16-bit value. bestfree is used to optimise the time
+ required to locate space to create an entry. It saves scanning through the
+ block to find a location suitable for every entry created.
+
+- The tail structure specifies the number of elements in the leaf array and
+ the number of stale entries in the array. The tail is always located at the
+ end of the block. The leaf data immediately precedes the tail structure.
+
+- The leaf array, which grows from the end of the block just before the tail
+ structure, contains an array of hash/address pairs for quickly looking up a
+ name by a hash value. Hash values are covered by the introduction to
+ directories. The address on-disk is the offset into the block divided by 8
+ (XFS\_DIR2\_DATA\_ALIGN). Hash/address pairs are stored on disk to optimise
+ lookup speed for large directories. If they were not stored, the hashes
+ would have to be calculated for all entries each time a lookup occurs in a
+ directory.
+
+xfs\_db Block Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A directory is created with 8 entries, directory block size = filesystem block
+size:
+
+::
+
+ xfs_db> sb 0
+ xfs_db> p
+ magicnum = 0x58465342
+ blocksize = 4096
+ ...
+ dirblklog = 0
+ ...
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ core.nlinkv1 = 2
+ ...
+ core.size = 4096
+ core.nblocks = 1
+ core.extsize = 0
+ core.nextents = 1
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,2097164,1,0]
+
+Go to the "startblock" and show the raw disk data:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type text
+ xfs_db> p
+ 000: 58 44 32 42 01 30 0e 78 00 00 00 00 00 00 00 00 XD2B.0.x........
+ 010: 00 00 00 00 02 00 00 80 01 2e 00 00 00 00 00 10 ................
+ 020: 00 00 00 00 00 00 00 80 02 2e 2e 00 00 00 00 20 ................
+ 030: 00 00 00 00 02 00 00 81 0f 66 72 61 6d 65 30 30 .........frame00
+ 040: 30 30 30 30 2e 74 73 74 80 8e 59 00 00 00 00 30 0000.tst..Y....0
+ 050: 00 00 00 00 02 00 00 82 0f 66 72 61 6d 65 30 30 .........frame00
+ 060: 30 30 30 31 2e 74 73 74 d0 ca 5c 00 00 00 00 50 0001.tst.......P
+ 070: 00 00 00 00 02 00 00 83 0f 66 72 61 6d 65 30 30 .........frame00
+ 080: 30 30 30 32 2e 74 73 74 00 00 00 00 00 00 00 70 0002.tst.......p
+ 090: 00 00 00 00 02 00 00 84 0f 66 72 61 6d 65 30 30 .........frame00
+ 0a0: 30 30 30 33 2e 74 73 74 00 00 00 00 00 00 00 90 0003.tst........
+ 0b0: 00 00 00 00 02 00 00 85 0f 66 72 61 6d 65 30 30 .........frame00
+ 0c0: 30 30 30 34 2e 74 73 74 00 00 00 00 00 00 00 b0 0004.tst........
+ 0d0: 00 00 00 00 02 00 00 86 0f 66 72 61 6d 65 30 30 .........frame00
+ 0e0: 30 30 30 35 2e 74 73 74 00 00 00 00 00 00 00 d0 0005.tst........
+ 0f0: 00 00 00 00 02 00 00 87 0f 66 72 61 6d 65 30 30 .........frame00
+ 100: 30 30 30 36 2e 74 73 74 00 00 00 00 00 00 00 f0 0006.tst........
+ 110: 00 00 00 00 02 00 00 88 0f 66 72 61 6d 65 30 30 .........frame00
+ 120: 30 30 30 37 2e 74 73 74 00 00 00 00 00 00 01 10 0007.tst........
+ 130: ff ff 0e 78 00 00 00 00 00 00 00 00 00 00 00 00 ...x............
+
+The "leaf" and "tail" structures are stored at the end of the block, so
+as the directory grows, the middle is filled in:
+
+::
+
+ fa0: 00 00 00 00 00 00 01 30 00 00 00 2e 00 00 00 02 .......0........
+ fb0: 00 00 17 2e 00 00 00 04 83 a0 40 b4 00 00 00 0e ................
+ fc0: 93 a0 40 b4 00 00 00 12 a3 a0 40 b4 00 00 00 06 ................
+ fd0: b3 a0 40 b4 00 00 00 0a c3 a0 40 b4 00 00 00 1e ................
+ fe0: d3 a0 40 b4 00 00 00 22 e3 a0 40 b4 00 00 00 16 ................
+ ff0: f3 a0 40 b4 00 00 00 1a 00 00 00 0a 00 00 00 00 ................
+
+In a readable format:
+
+::
+
+ xfs_db> type dir2
+ xfs_db> p
+ bhdr.magic = 0x58443242
+ bhdr.bestfree[0].offset = 0x130
+ bhdr.bestfree[0].length = 0xe78
+ bhdr.bestfree[1].offset = 0
+ bhdr.bestfree[1].length = 0
+ bhdr.bestfree[2].offset = 0
+ bhdr.bestfree[2].length = 0
+ bu[0].inumber = 33554560
+ bu[0].namelen = 1
+ bu[0].name = "."
+ bu[0].tag = 0x10
+ bu[1].inumber = 128
+ bu[1].namelen = 2
+ bu[1].name = ".."
+ bu[1].tag = 0x20
+ bu[2].inumber = 33554561
+ bu[2].namelen = 15
+ bu[2].name = "frame000000.tst"
+ bu[2].tag = 0x30
+ bu[3].inumber = 33554562
+ bu[3].namelen = 15
+ bu[3].name = "frame000001.tst"
+ bu[3].tag = 0x50
+ ...
+ bu[8].inumber = 33554567
+ bu[8].namelen = 15
+ bu[8].name = "frame000006.tst"
+ bu[8].tag = 0xf0
+ bu[9].inumber = 33554568
+ bu[9].namelen = 15
+ bu[9].name = "frame000007.tst"
+ bu[9].tag = 0x110
+ bu[10].freetag = 0xffff
+ bu[10].length = 0xe78
+ bu[10].tag = 0x130
+ bleaf[0].hashval = 0x2e
+ bleaf[0].address = 0x2
+ bleaf[1].hashval = 0x172e
+ bleaf[1].address = 0x4
+ bleaf[2].hashval = 0x83a040b4
+ bleaf[2].address = 0xe
+ ...
+ bleaf[8].hashval = 0xe3a040b4
+ bleaf[8].address = 0x16
+ bleaf[9].hashval = 0xf3a040b4
+ bleaf[9].address = 0x1a
+ btail.count = 10
+ btail.stale = 0
+
+ **Note**
+
+ For block directories, all xfs\_db fields are preceded with "b".
+
+For a simple lookup example, the hash of frame000000.tst is 0xb3a040b4.
+Looking up that value, we get an address of 0x6. Multiply that by 8, it
+becomes offset 0x30 and the inode at that point is 33554561.
+
+When we remove an entry from the middle (frame000004.tst), we can see how the
+freespace details are adjusted:
+
+::
+
+ bhdr.magic = 0x58443242
+ bhdr.bestfree[0].offset = 0x130
+ bhdr.bestfree[0].length = 0xe78
+ bhdr.bestfree[1].offset = 0xb0
+ bhdr.bestfree[1].length = 0x20
+ bhdr.bestfree[2].offset = 0
+ bhdr.bestfree[2].length = 0
+ ...
+ bu[5].inumber = 33554564
+ bu[5].namelen = 15
+ bu[5].name = "frame000003.tst"
+ bu[5].tag = 0x90
+ bu[6].freetag = 0xffff
+ bu[6].length = 0x20
+ bu[6].tag = 0xb0
+ bu[7].inumber = 33554566
+ bu[7].namelen = 15
+ bu[7].name = "frame000005.tst"
+ bu[7].tag = 0xd0
+ ...
+ bleaf[7].hashval = 0xd3a040b4
+ bleaf[7].address = 0x22
+ bleaf[8].hashval = 0xe3a040b4
+ bleaf[8].address = 0
+ bleaf[9].hashval = 0xf3a040b4
+ bleaf[9].address = 0x1a
+ btail.count = 10
+ btail.stale = 1
+
+A new "bestfree" value is added for the entry, the start of the entry is
+marked as unused with 0xffff (which overwrites the inode number for an actual
+entry), and the length of the space. The tag remains intact at the
+offset+length - sizeof(tag). The address for the hash is also cleared. The
+affected areas are highlighted below:
+
+::
+
+ 090: 00 00 00 00 02 00 00 84 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0a0: 30 30 30 33 2e 74 73 74 00 00 00 00 00 00 00 90 0003.tst.........
+ 0b0: ff ff 00 20 02 00 00 85 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0c0: 30 30 30 34 2e 74 73 74 00 00 00 00 00 00 00 b0 0004.tst.........
+ 0d0: 00 00 00 00 02 00 00 86 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0e0: 30 30 30 35 2e 74 73 74 00 00 00 00 00 00 00 0d 0005.tst.........
+ ...
+ fb0: 00 00 17 2e 00 00 00 04 83 a0 40 b4 00 00 00 0e .................
+ fc0: 93 a0 40 b4 00 00 00 12 a3 a0 40 b4 00 00 00 06 .................
+ fd0: b3 a0 40 b4 00 00 00 0a c3 a0 40 b4 00 00 00 1e .................
+ fe0: d3 a0 40 b4 00 00 00 22 e3 a0 40 b4 00 00 00 00 .................
+ ff0: f3 a0 40 b4 00 00 00 1a 00 00 00 0a 00 00 00 01 .................
+
+Leaf Directories
+~~~~~~~~~~~~~~~~
+
+Once a Block Directory has filled the block, the directory data is changed
+into a new format. It still uses `extents <#data-extents>`__ and the same
+basic structures, but the "data" and "leaf" are split up into their own
+extents. The "leaf" information only occupies one extent. As "leaf"
+information is more compact than
+"data" information, more than one "data" extent is common.
+
+- Block to Leaf conversions retain the existing block for the data entries
+ and allocate a new block for the leaf and freespace index information.
+
+- As with all directories, data blocks must start at logical offset zero.
+
+- The "leaf" block has a special offset defined by
+ XFS\_DIR2\_LEAF\_OFFSET. Currently, this is 32GB and in the extent view, a
+ block offset of 32GB / sb\_blocksize. On a 4KB block filesystem, this is
+ 0x800000 (8388608 decimal).
+
+- Blocks with directory entries
+ ("data" extents) have the magic number "X2D2" (0x58443244), or
+ "XDD3" (0x58444433) on a v5 filesystem.
+
+- The "data" extents have a new header (no "leaf" data):
+
+.. code:: c
+
+ typedef struct xfs_dir2_data {
+ xfs_dir2_data_hdr_t hdr;
+ xfs_dir2_data_union_t u[1];
+ } xfs_dir2_data_t;
+
+**hdr**
+ Data block header. On a v5 filesystem, this field is struct
+ xfs\_dir3\_data\_hdr.
+
+**u**
+ Union of directory and unused entries, exactly the same as in a block
+ directory.
+
+- The "leaf" extent uses the following structures:
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf {
+ xfs_dir2_leaf_hdr_t hdr;
+ xfs_dir2_leaf_entry_t ents[1];
+ xfs_dir2_data_off_t bests[1];
+ xfs_dir2_leaf_tail_t tail;
+ } xfs_dir2_leaf_t;
+
+**hdr**
+ Directory leaf header. On a v5 filesystem this is struct
+ xfs\_dir3\_leaf\_hdr\_t.
+
+**ents**
+ Hash values of the entries in this block.
+
+**bests**
+ An array pointing to free regions in the directory block.
+
+**tail**
+ Bookkeeping for the leaf entries.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_hdr {
+ xfs_da_blkinfo_t info;
+ __uint16_t count;
+ __uint16_t stale;
+ } xfs_dir2_leaf_hdr_t;
+
+**info**
+ Leaf btree block header.
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of stale/zeroed leaf entries.
+
+.. code:: c
+
+ struct xfs_dir3_leaf_hdr {
+ struct xfs_da3_blkinfo info;
+ __uint16_t count;
+ __uint16_t stale;
+ __be32 pad;
+ };
+
+**info**
+ Leaf B+tree block header.
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of stale/zeroed leaf entries.
+
+**pad**
+ Padding to maintain alignment rules.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_tail {
+ __uint32_t bestcount;
+ } xfs_dir2_leaf_tail_t;
+
+**bestcount**
+ Number of best free entries.
+
+- The magic number of the leaf block is XFS\_DIR2\_LEAF1\_MAGIC (0xd2f1); on
+ a v5 filesystem it is XFS\_DIR3\_LEAF1\_MAGIC (0x3df1).
+
+- The size of the ents array is specified by hdr.count.
+
+- The size of the bests array is specified by the tail.bestcount, which is
+ also the number of "data" blocks for the directory. The bests array
+ maintains each data block’s bestfree[0].length value.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/48.png
+ :alt: Leaf directory free entry detail
+
+ Leaf directory free entry detail
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/48.png
+ :scale: 40%
+ :alt: Leaf directory free entry detail
+
+ Leaf directory free entry detail
+
+xfs\_db Leaf Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For this example, a directory was created with 256 entries (frame000000.tst to
+frame000255.tst). Some files were deleted (frame00005\*, frame00018\* and
+frame000240.tst) to show free list characteristics.
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ core.nlinkv1 = 2
+ ...
+ core.size = 12288
+ core.nblocks = 4
+ core.extsize = 0
+ core.nextents = 3
+ ...
+ u.bmx[0-2] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,4718604,1,0]
+ 1:[1,4718610,2,0]
+ 2:[8388608,4718605,1,0]
+
+As can be seen in this example, three blocks are used for
+"data" in two extents,
+and the "leaf" extent has a logical offset of 8388608 blocks (32GB).
+
+Examining the first block:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x670
+ dhdr.bestfree[0].length = 0x140
+ dhdr.bestfree[1].offset = 0xff0
+ dhdr.bestfree[1].length = 0x10
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 75497600
+ du[0].namelen = 1
+ du[0].name = "."
+ du[0].tag = 0x10
+ du[1].inumber = 128
+ du[1].namelen = 2
+ du[1].name = ".."
+ du[1].tag = 0x20
+ du[2].inumber = 75497601
+ du[2].namelen = 15
+ du[2].name = "frame000000.tst"
+ du[2].tag = 0x30
+ du[3].inumber = 75497602
+ du[3].namelen = 15
+ du[3].name = "frame000001.tst"
+ du[3].tag = 0x50
+ ...
+ du[51].inumber = 75497650
+ du[51].namelen = 15
+ du[51].name = "frame000049.tst"
+ du[51].tag = 0x650
+ du[52].freetag = 0xffff
+ du[52].length = 0x140
+ du[52].tag = 0x670
+ du[53].inumber = 75497661
+ du[53].namelen = 15
+ du[53].name = "frame000060.tst"
+ du[53].tag = 0x7b0
+ ...
+ du[118].inumber = 75497758
+ du[118].namelen = 15
+ du[118].name = "frame000125.tst"
+ du[118].tag = 0xfd0
+ du[119].freetag = 0xffff
+ du[119].length = 0x10
+ du[119].tag = 0xff0
+
+ **Note**
+
+ The xfs\_db field output is preceded by a "d" for "data".
+
+The next "data" block:
+
+::
+
+ xfs_db> dblock 1
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x6d0
+ dhdr.bestfree[0].length = 0x140
+ dhdr.bestfree[1].offset = 0xe50
+ dhdr.bestfree[1].length = 0x20
+ dhdr.bestfree[2].offset = 0xff0
+ dhdr.bestfree[2].length = 0x10
+ du[0].inumber = 75497759
+ du[0].namelen = 15
+ du[0].name = "frame000126.tst"
+ du[0].tag = 0x10
+ ...
+ du[53].inumber = 75497844
+ du[53].namelen = 15
+ du[53].name = "frame000179.tst"
+ du[53].tag = 0x6b0
+ du[54].freetag = 0xffff
+ du[54].length = 0x140
+ du[54].tag = 0x6d0
+ du[55].inumber = 75497855
+ du[55].namelen = 15
+ du[55].name = "frame000190.tst"
+ du[55].tag = 0x810
+ ...
+ du[104].inumber = 75497904
+ du[104].namelen = 15
+ du[104].name = "frame000239.tst"
+ du[104].tag = 0xe30
+ du[105].freetag = 0xffff
+ du[105].length = 0x20
+ du[105].tag = 0xe50
+ du[106].inumber = 75497906
+ du[106].namelen = 15
+ du[106].name = "frame000241.tst"
+ du[106].tag = 0xe70
+ ...
+ du[117].inumber = 75497917
+ du[117].namelen = 15
+ du[117].name = "frame000252.tst"
+ du[117].tag = 0xfd0
+ du[118].freetag = 0xffff
+ du[118].length = 0x10
+ du[118].tag = 0xff0
+
+And the last data block:
+
+::
+
+ xfs_db> dblock 2
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x70
+ dhdr.bestfree[0].length = 0xf90
+ dhdr.bestfree[1].offset = 0
+ dhdr.bestfree[1].length = 0
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 75497918
+ du[0].namelen = 15
+ du[0].name = "frame000253.tst"
+ du[0].tag = 0x10
+ du[1].inumber = 75497919
+ du[1].namelen = 15
+ du[1].name = "frame000254.tst"
+ du[1].tag = 0x30
+ du[2].inumber = 75497920
+ du[2].namelen = 15
+ du[2].name = "frame000255.tst"
+ du[2].tag = 0x50
+ du[3].freetag = 0xffff
+ du[3].length = 0xf90
+ du[3].tag = 0x70
+
+Examining the "leaf" block (with the fields preceded by an "l" for
+"leaf"):
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2f1
+ lhdr.count = 258
+ lhdr.stale = 0
+ lbests[0-2] = 0:0x10 1:0x10 2:0xf90
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+ lents[257].hashval = 0xf3a048bc
+ lents[257].address = 0x366
+ ltail.bestcount = 3
+
+Note how the lbests array correspond with the bestfree[0].length values in the
+"data" blocks:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xff0
+ dhdr.bestfree[0].length = 0x10
+ ...
+ xfs_db> dblock 1
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xff0
+ dhdr.bestfree[0].length = 0x10
+ ...
+ xfs_db> dblock 2
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x70
+ dhdr.bestfree[0].length = 0xf90
+
+Now after the entries have been deleted:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2f1
+ lhdr.count = 258
+ lhdr.stale = 21
+ lbests[0-2] = 0:0x140 1:0x140 2:0xf90
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+
+As can be seen, the lbests values have been update to contain each
+hdr.bestfree[0].length values. The leaf’s hdr.stale value has also been
+updated to specify the number of stale entries in the array. The stale entries
+have an address of zero.
+
+TODO: Need an example for where new entries get inserted with several large
+free spaces.
+
+Node Directories
+~~~~~~~~~~~~~~~~
+
+When the "leaf" information fills a block, the extents undergo another
+separation. All "freeindex" information moves into its own extent. Like
+Leaf Directories, the
+"leaf" block maintained the best free space information for
+each "data" block. This is not possible with more than one leaf.
+
+- The "data" blocks stay the same as leaf directories.
+
+- After the "freeindex" data moves to its own block, it is possible for
+ the leaf data to fit within a single leaf block. This single leaf block has
+ a magic number of XFS\_DIR2\_LEAFN\_MAGIC (0xd2ff) or on a v5 filesystem,
+ XFS\_DIR3\_LEAFN\_MAGIC (0x3dff).
+
+- The "leaf" blocks eventually change into a B+tree with the generic B+tree
+ header pointing to directory "leaves" as described in `Leaf
+ Directories <#leaf-directories>`__. Blocks with leaf data still have the
+ LEAFN\_MAGIC magic number as outlined above. The top-level tree blocks are
+ called "nodes" and have a magic number of XFS\_DA\_NODE\_MAGIC
+ (0xfebe), or on a v5 filesystem, XFS\_DA3\_NODE\_MAGIC (0x3ebe).
+
+- Distinguishing between a combined leaf/freeindex block (LEAF1\_MAGIC), a
+ leaf-only block (LEAFN\_MAGIC), and a btree node block (NODE\_MAGIC) can
+ only be done by examining the magic number.
+
+- The new "freeindex" block(s) only contains the bests for each data
+ block.
+
+- The freeindex block uses the following structures:
+
+.. code:: c
+
+ typedef struct xfs_dir2_free_hdr {
+ __uint32_t magic;
+ __int32_t firstdb;
+ __int32_t nvalid;
+ __int32_t nused;
+ } xfs_dir2_free_hdr_t;
+
+**magic**
+ The magic number of the free block, "XD2F" (0x0x58443246).
+
+**firstdb**
+ The starting directory block number for the bests array.
+
+**nvalid**
+ Number of valid elements in the bests array. This number must correspond
+ with the number of directory blocks can fit under the inode di\_size.
+
+**nused**
+ Number of used elements in the bests array. This number must correspond
+ with the number of directory blocks actually mapped under the inode
+ di\_size.
+
+.. code:: c
+
+ typedef struct xfs_dir2_free {
+ xfs_dir2_free_hdr_t hdr;
+ xfs_dir2_data_off_t bests[1];
+ } xfs_dir2_free_t;
+
+**hdr**
+ Free block header.
+
+**bests**
+ An array specifying the best free counts in each directory data block.
+
+- On a v5 filesystem, the freeindex block uses the following structures:
+
+.. code:: c
+
+ struct xfs_dir3_free_hdr {
+ struct xfs_dir3_blk_hdr hdr;
+ __int32_t firstdb;
+ __int32_t nvalid;
+ __int32_t nused;
+ __int32_t pad;
+ };
+
+**hdr**
+ v3 directory block header. The magic number is "XDF3" (0x0x58444633).
+
+**firstdb**
+ The starting directory block number for the bests array.
+
+**nvalid**
+ Number of valid elements in the bests array. This number must correspond
+ with the number of directory blocks can fit under the inode di\_size.
+
+**nused**
+ Number of used elements in the bests array. This number must correspond
+ with the number of directory blocks actually mapped under the inode
+ di\_size.
+
+**pad**
+ Padding to maintain alignment.
+
+.. code:: c
+
+ struct xfs_dir3_free {
+ xfs_dir3_free_hdr_t hdr;
+ __be16 bests[1];
+ };
+
+**hdr**
+ Free block header.
+
+**bests**
+ An array specifying the best free counts in each directory data block.
+
+- The location of the leaf blocks can be in any order, the only way to
+ determine the appropriate is by the node block hash/before values. Given a
+ hash to look up, you read the node’s btree array and first hashval in the
+ array that exceeds the given hash and it can then be found in the block
+ pointed to by the before value.
+
+- The freeindex’s bests array starts from the end of the block and grows to
+ the start of the block.
+
+- When an data block becomes unused (ie. all entries in it have been
+ deleted), the block is freed, the data extents contain a hole, and the
+ freeindex’s hdr.nused value is decremented and the associated bests[] entry
+ is set to 0xffff.
+
+- As the first data block always contains "." and "..", it’s invalid for
+ the directory to have a hole at the start.
+
+- The freeindex’s hdr.nused should always be the same as the number of
+ allocated data directory blocks containing name/inode data and will always
+ be less than or equal to hdr.nvalid. The value of hdr.nvalid should be the
+ same as the index of the last data directory block plus one (i.e. when the
+ last data block is freed, nused and nvalid are decremented).
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/54.png
+ :alt: Node directory layout
+
+ Node directory layout
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/54.png
+ :scale: 40%
+ :alt: Node directory layout
+
+ Node directory layout
+
+xfs\_db Node Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+With the node directory examples, we are using a filesystems with 4KB block
+size, and a 16KB directory size. The directory has over 2000 entries:
+
+::
+
+ xfs_db> sb 0
+ xfs_db> p
+ magicnum = 0x58465342
+ blocksize = 4096
+ ...
+ dirblklog = 2
+ ...
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ ...
+ core.size = 81920
+ core.nblocks = 36
+ core.extsize = 0
+ core.nextents = 8
+ ...
+ u.bmx[0-7] = [startoff,startblock,blockcount,extentflag] 0:[0,7368,4,0]
+ 1:[4,7408,4,0] 2:[8,7444,4,0] 3:[12,7480,4,0] 4:[16,7520,4,0]
+ 5:[8388608,7396,4,0] 6:[8388612,7524,8,0] 7:[16777216,7516,4,0]
+
+As can already be observed, all extents are allocated is multiples of 4
+blocks.
+
+Blocks 0 to 19 (16+4-1) are used for directory data blocks. Looking at blocks
+16-19, we can seen that it’s the same as the single-leaf format, except the
+length values are a lot larger to accommodate the increased directory block
+size:
+
+::
+
+ xfs_db> dblock 16
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xb0
+ dhdr.bestfree[0].length = 0x3f50
+ dhdr.bestfree[1].offset = 0
+ dhdr.bestfree[1].length = 0
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 120224
+ du[0].namelen = 15
+ du[0].name = "frame002043.tst"
+ du[0].tag = 0x10
+ du[1].inumber = 120225
+ du[1].namelen = 15
+ du[1].name = "frame002044.tst"
+ du[1].tag = 0x30
+ du[2].inumber = 120226
+ du[2].namelen = 15
+ du[2].name = "frame002045.tst"
+ du[2].tag = 0x50
+ du[3].inumber = 120227
+ du[3].namelen = 15
+ du[3].name = "frame002046.tst"
+ du[3].tag = 0x70
+ du[4].inumber = 120228
+ du[4].namelen = 15
+ du[4].name = "frame002047.tst"
+ du[4].tag = 0x90
+ du[5].freetag = 0xffff
+ du[5].length = 0x3f50
+ du[5].tag = 0
+
+Next, the "node" block, the fields are preceded with 'n' for node blocks:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 2
+ nhdr.level = 1
+ nbtree[0-1] = [hashval,before] 0:[0xa3a440ac,8388616] 1:[0xf3a440bc,8388612]
+
+The two following leaf blocks were allocated as part of the directory’s
+conversion to node format. All hashes less than 0xa3a440ac are located at
+directory offset 8,388,616, and hashes less than 0xf3a440bc are located at
+directory offset 8,388,612. Hashes greater or equal to 0xf3a440bc don’t exist
+in this directory.
+
+::
+
+ xfs_db> dblock 8388616
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388612
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2ff
+ lhdr.count = 1023
+ lhdr.stale = 0
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+ lents[1021].hashval = 0xa3a440a4
+ lents[1021].address = 0x1fa2
+ lents[1022].hashval = 0xa3a440ac
+ lents[1022].address = 0x1fca
+ xfs_db> dblock 8388612
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 8388616
+ lhdr.info.magic = 0xd2ff
+ lhdr.count = 1027
+ lhdr.stale = 0
+ lents[0].hashval = 0xa3a440b4
+ lents[0].address = 0x1f52
+ lents[1].hashval = 0xa3a440bc
+ lents[1].address = 0x1f7a
+ ...
+ lents[1025].hashval = 0xf3a440b4
+ lents[1025].address = 0x1f66
+ lents[1026].hashval = 0xf3a440bc
+ lents[1026].address = 0x1f8e
+
+An example lookup using xfs\_db:
+
+::
+
+ xfs_db> hash frame001845.tst
+ 0xf3a26094
+
+Doing a binary search through the array, we get address 0x1ce6, which is
+offset 0xe730. Each fsblock is 4KB in size (0x1000), so it will be offset
+0x730 into directory offset 14. From the extent map, this will be fsblock
+7482:
+
+::
+
+ xfs_db> fsblock 7482
+ xfs_db> type text
+ xfs_db> p
+ ...
+ 730: 00 00 00 00 00 01 d4 da 0f 66 72 61 6d 65 30 30 .........frame00
+ 740: 31 38 34 35 2e 74 73 74 00 00 00 00 00 00 27 30 1845.tst.......0
+
+Looking at the freeindex information (fields with an 'f' tag):
+
+::
+
+ xfs_db> fsblock 7516
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 0
+ fhdr.nvalid = 5
+ fhdr.nused = 5
+ fbests[0-4] = 0:0x10 1:0x10 2:0x10 3:0x10 4:0x3f50
+
+Like the Leaf Directory, each of the fbests values correspond to each data
+block’s bestfree[0].length value.
+
+The fbests array is highlighted in a raw block dump:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 000: 58 44 32 46 00 00 00 00 00 00 00 05 00 00 00 05 XD2F............
+ 010: 00 10 00 10 00 10 00 10 3f 50 00 00 1f 01 ff ff .........P......
+
+TODO: Example with a hole in the middle
+
+B+tree Directories
+~~~~~~~~~~~~~~~~~~
+
+When the extent map in an inode grows beyond the inode’s space, the inode
+format is changed to a
+"btree". The inode contains a filesystem block point to the
+B+tree extent map for the directory’s blocks. The B+tree extents contain the
+extent map for the "data", "node", "leaf", and "freeindex"
+information as described in Node Directories.
+
+Refer to the previous section on B+tree `Data Extents <#b-tree-extent-list>`__
+for more information on XFS B+tree extents.
+
+The following properties apply to both node and B+tree directories:
+
+- The node/leaf trees can be more than one level deep.
+
+- More than one freeindex block may exist, but this will be quite rare. It
+ would required hundreds of thousand files with quite long file names (or
+ millions with shorter names) to get a second freeindex block.
+
+xfs\_db B+tree Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A directory has been created with 200,000 entries with each entry being 100
+characters long. The filesystem block size and directory block size are 4KB:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 3 (btree)
+ ...
+ core.size = 22757376
+ core.nblocks = 6145
+ core.extsize = 0
+ core.nextents = 234
+ core.naextents = 0
+ core.forkoff = 0
+ ...
+ u.bmbt.level = 1
+ u.bmbt.numrecs = 1
+ u.bmbt.keys[1] = [startoff] 1:[0]
+ u.bmbt.ptrs[1] = 1:89
+ xfs_db> fsblock 89
+ xfs_db> type bmapbtd
+ xfs_db> p
+ magic = 0x424d4150
+ level = 0
+ numrecs = 234
+ leftsib = null
+ rightsib = null
+ recs[1-234] = [startoff,startblock,blockcount,extentflag]
+ 1:[0,53,1,0] 2:[1,55,13,0] 3:[14,69,1,0] 4:[15,72,13,0]
+ 5:[28,86,2,0] 6:[30,90,21,0] 7:[51,112,1,0] 8:[52,114,11,0]
+ ...
+ 125:[5177,902,15,0] 126:[5192,918,6,0] 127:[5198,524786,358,0]
+ 128:[8388608,54,1,0] 129:[8388609,70,2,0] 130:[8388611,85,1,0]
+ ...
+ 229:[8389164,917,1,0] 230:[8389165,924,19,0] 231:[8389184,944,9,0]
+ 232:[16777216,68,1,0] 233:[16777217,7340114,1,0] 234:[16777218,5767362,1,0]
+
+We have 128 extents and a total of 5555 blocks being used to store name/inode
+pairs. With only about 2000 values that can be stored in the freeindex block,
+3 blocks have been allocated for this information. The firstdb field specifies
+the starting directory block number for each array:
+
+::
+
+ xfs_db> dblock 16777216
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 0
+ fhdr.nvalid = 2040
+ fhdr.nused = 2040
+ fbests[0-2039] = ...
+ xfs_db> dblock 16777217
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 2040
+ fhdr.nvalid = 2040
+ fhdr.nused = 2040
+ fbests[0-2039] = ...
+ xfs_db> dblock 16777218
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 4080
+ fhdr.nvalid = 1476
+ fhdr.nused = 1476
+ fbests[0-1475] = ...
+
+Looking at the root node in the node block, it’s a pretty deep tree:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 2
+ nhdr.level = 2
+ nbtree[0-1] = [hashval,before] 0:[0x6bbf6f39,8389121] 1:[0xfbbf7f79,8389120]
+ xfs_db> dblock 8389121
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 8389120
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 263
+ nhdr.level = 1
+ nbtree[0-262] = ... 262:[0x6bbf6f39,8388928]
+ xfs_db> dblock 8389120
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 8389121
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 319
+ nhdr.level = 1
+ nbtree[0-318] = [hashval,before] 0:[0x70b14711,8388919] ...
+
+The leaves at each the end of a node always point to the end leaves in
+adjacent nodes. Directory block 8388928 has a forward pointer to block 8388919
+and block 8388919 has a previous pointer to block 8388928, as highlighted in
+the following example:
+
+::
+
+ xfs_db> dblock 8388928
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388919
+ lhdr.info.back = 8388937
+ lhdr.info.magic = 0xd2ff
+ ...
+
+ xfs_db> dblock 8388919
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388706
+ lhdr.info.back = 8388928
+ lhdr.info.magic = 0xd2ff
+ ...
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 5ba6f3940808..2c12fca905fd 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -5,3 +5,4 @@ Dynamic Allocated Structures
.. include:: ondisk_inode.rst
.. include:: data_extents.rst
+.. include:: directories.rst
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 20/22] docs: add XFS extended attributes structures to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (17 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 19/22] docs: add XFS directory structure " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 21/22] docs: add XFS symlink " Darrick J. Wong
` (2 subsequent siblings)
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/dynamic.rst | 1
.../xfs-data-structures/extended_attributes.rst | 933 ++++++++++++++++++++
2 files changed, 934 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/extended_attributes.rst
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 2c12fca905fd..16755381d0f8 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -6,3 +6,4 @@ Dynamic Allocated Structures
.. include:: ondisk_inode.rst
.. include:: data_extents.rst
.. include:: directories.rst
+.. include:: extended_attributes.rst
diff --git a/Documentation/filesystems/xfs-data-structures/extended_attributes.rst b/Documentation/filesystems/xfs-data-structures/extended_attributes.rst
new file mode 100644
index 000000000000..db6de15227cd
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/extended_attributes.rst
@@ -0,0 +1,933 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Extended Attributes
+-------------------
+
+Extended attributes enable users and administrators to attach (name: value)
+pairs to inodes within the XFS filesystem. They could be used to store
+meta-information about the file.
+
+Attribute names can be up to 256 bytes in length, terminated by the first 0
+byte. The intent is that they be printable ASCII (or other character set)
+names for the attribute. The values can contain up to 64KB of arbitrary binary
+data. Some XFS internal attributes (eg. parent pointers) use non-printable
+names for the attribute.
+
+Access Control Lists (ACLs) and Data Migration Facility (DMF) use extended
+attributes to store their associated metadata with an inode.
+
+XFS uses two disjoint attribute name spaces associated with every inode. These
+are the root and user address spaces. The root address space is accessible
+only to the superuser, and then only by specifying a flag argument to the
+function call. Other users will not see or be able to modify attributes in the
+root address space. The user address space is protected by the normal file
+permissions mechanism, so the owner of the file can decide who is able to see
+and/or modify the value of attributes on any particular file.
+
+To view extended attributes from the command line, use the getfattr command.
+To set or delete extended attributes, use the setfattr command. ACLs control
+should use the getfacl and setfacl commands.
+
+XFS attributes supports three namespaces: "user", "trusted" (or "root" using
+IRIX terminology), and "secure".
+
+See the section about `extended attributes <#extended-attribute-versions>`__
+in the inode for instructions on how to calculate the location of the
+attributes.
+
+The following four sections describe each of the on-disk formats.
+
+Short Form Attributes
+~~~~~~~~~~~~~~~~~~~~~
+
+When the all extended attributes can fit within the inode’s attribute fork,
+the inode’s di\_aformat is set to "local" and the attributes are stored in
+the inode’s literal area starting at offset di\_forkoff × 8.
+
+Shortform attributes use the following structures:
+
+.. code:: c
+
+ typedef struct xfs_attr_shortform {
+ struct xfs_attr_sf_hdr {
+ __be16 totsize;
+ __u8 count;
+ } hdr;
+ struct xfs_attr_sf_entry {
+ __uint8_t namelen;
+ __uint8_t valuelen;
+ __uint8_t flags;
+ __uint8_t nameval[1];
+ } list[1];
+ } xfs_attr_shortform_t;
+ typedef struct xfs_attr_sf_hdr xfs_attr_sf_hdr_t;
+ typedef struct xfs_attr_sf_entry xfs_attr_sf_entry_t;
+
+**totsize**
+ Total size of the attribute structure in bytes.
+
+**count**
+ The number of entries that can be found in this structure.
+
+**namelen** and **valuelen**
+ These values specify the size of the two byte arrays containing the name
+ and value pairs. valuelen is zero for extended attributes with no value.
+
+**nameval[]**
+ A single array whose size is the sum of namelen and valuelen. The names
+ and values are not null terminated on-disk. The value immediately follows
+ the name in the array.
+
+.. _attribute-flags:
+
+**flags**
+ A combination of the following:
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - 0
+ - The attribute's namespace is "user".
+
+ * - XFS_ATTR_ROOT
+ - The attribute's namespace is "trusted".
+
+ * - XFS_ATTR_SECURE
+ - The attribute's namespace is "secure".
+
+ * - XFS_ATTR_INCOMPLETE
+ - This attribute is being modified.
+
+ * - XFS_ATTR_LOCAL
+ - The attribute value is contained within this block.
+
+Table: Attribute Namespaces
+
+.. figure:: images/64.png
+ :alt: Short form attribute layout
+
+ Short form attribute layout
+
+xfs\_db Short Form Attribute Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A file is created and two attributes are set:
+
+::
+
+ # setfattr -n user.empty few_attr
+ # setfattr -n trusted.trust -v val1 few_attr
+
+Using xfs\_db, we dump the inode:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0100644
+ ...
+ core.naextents = 0
+ core.forkoff = 15
+ core.aformat = 1 (local)
+ ...
+ a.sfattr.hdr.totsize = 24
+ a.sfattr.hdr.count = 2
+ a.sfattr.list[0].namelen = 5
+ a.sfattr.list[0].valuelen = 0
+ a.sfattr.list[0].root = 0
+ a.sfattr.list[0].secure = 0
+ a.sfattr.list[0].name = "empty"
+ a.sfattr.list[1].namelen = 5
+ a.sfattr.list[1].valuelen = 4
+ a.sfattr.list[1].root = 1
+ a.sfattr.list[1].secure = 0
+ a.sfattr.list[1].name = "trust"
+ a.sfattr.list[1].value = "val1"
+
+We can determine the actual inode offset to be 220 (15 x 8 + 100) or 0xdc.
+Examining the raw dump, the second attribute is highlighted:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 09: 49 4e 81 a4 01 02 00 01 00 00 00 00 00 00 00 00 IN..............
+ 10: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 02 ................
+ 20: 44 be 19 be 38 d1 26 98 44 be 1a be 38 d1 26 98 D...8...D...8...
+ 30: 44 be 1a e1 3a 9a ea 18 00 00 00 00 00 00 00 04 D...............
+ 40: 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 01 ................
+ 50: 00 00 0f 01 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 12 ................
+ 70: 53 a0 00 01 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 18 02 00 ................
+ ^^ hdr.totsize = 0x18
+ e0: 05 00 00 65 6d 70 74 79 05 04 02 74 72 75 73 74 ...empty...trust
+ f0: 76 61 6c 31 00 00 00 00 00 00 00 00 00 00 00 00 val1............
+
+Adding another attribute with attr1, the format is converted to extents and
+di\_forkoff remains unchanged (and all those zeros in the dump above remain
+unused):
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.naextents = 1
+ core.forkoff = 15
+ core.aformat = 2 (extents)
+ ...
+ a.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,37534,1,0]
+
+Performing the same steps with attr2, adding one attribute at a time, you can
+see di\_forkoff change as attributes are added:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.naextents = 0
+ core.forkoff = 15
+ core.aformat = 1 (local)
+ ...
+ a.sfattr.hdr.totsize = 17
+ a.sfattr.hdr.count = 1
+ a.sfattr.list[0].namelen = 10
+ a.sfattr.list[0].valuelen = 0
+ a.sfattr.list[0].root = 0
+ a.sfattr.list[0].secure = 0
+ a.sfattr.list[0].name = "empty_attr"
+
+Attribute added:
+
+::
+
+ xfs_db> p
+ ...
+ core.naextents = 0
+ core.forkoff = 15
+ core.aformat = 1 (local)
+ ...
+ a.sfattr.hdr.totsize = 31
+ a.sfattr.hdr.count = 2
+ a.sfattr.list[0].namelen = 10
+ a.sfattr.list[0].valuelen = 0
+ a.sfattr.list[0].root = 0
+ a.sfattr.list[0].secure = 0
+ a.sfattr.list[0].name = "empty_attr"
+ a.sfattr.list[1].namelen = 7
+ a.sfattr.list[1].valuelen = 4
+ a.sfattr.list[1].root = 1
+ a.sfattr.list[1].secure = 0
+ a.sfattr.list[1].name = "trust_a"
+ a.sfattr.list[1].value = "val1"
+
+Another attribute is added:
+
+::
+
+ xfs_db> p
+ ...
+ core.naextents = 0
+ core.forkoff = 13
+ core.aformat = 1 (local)
+ ...
+ a.sfattr.hdr.totsize = 52
+ a.sfattr.hdr.count = 3
+ a.sfattr.list[0].namelen = 10
+ a.sfattr.list[0].valuelen = 0
+ a.sfattr.list[0].root = 0
+ a.sfattr.list[0].secure = 0
+ a.sfattr.list[0].name = "empty_attr"
+ a.sfattr.list[1].namelen = 7
+ a.sfattr.list[1].valuelen = 4
+ a.sfattr.list[1].root = 1
+ a.sfattr.list[1].secure = 0
+ a.sfattr.list[1].name = "trust_a"
+ a.sfattr.list[1].value = "val1"
+ a.sfattr.list[2].namelen = 6
+ a.sfattr.list[2].valuelen = 12
+ a.sfattr.list[2].root = 0
+ a.sfattr.list[2].secure = 0
+ a.sfattr.list[2].name = "second"
+ a.sfattr.list[2].value = "second_value"
+
+One more is added:
+
+::
+
+ xfs_db> p
+ core.naextents = 0
+ core.forkoff = 10
+ core.aformat = 1 (local)
+ ...
+ a.sfattr.hdr.totsize = 69
+ a.sfattr.hdr.count = 4
+ a.sfattr.list[0].namelen = 10
+ a.sfattr.list[0].valuelen = 0
+ a.sfattr.list[0].root = 0
+ a.sfattr.list[0].secure = 0
+ a.sfattr.list[0].name = "empty_attr"
+ a.sfattr.list[1].namelen = 7
+ a.sfattr.list[1].valuelen = 4
+ a.sfattr.list[1].root = 1
+ a.sfattr.list[1].secure = 0
+ a.sfattr.list[1].name = "trust_a"
+ a.sfattr.list[1].value = "val1"
+ a.sfattr.list[2].namelen = 6
+ a.sfattr.list[2].valuelen = 12
+ a.sfattr.list[2].root = 0
+ a.sfattr.list[2].secure = 0
+ a.sfattr.list[2].name = "second"
+ a.sfattr.list[2].value = "second_value"
+ a.sfattr.list[3].namelen = 6
+ a.sfattr.list[3].valuelen = 8
+ a.sfattr.list[3].root = 0
+ a.sfattr.list[3].secure = 1
+ a.sfattr.list[3].name = "policy"
+ a.sfattr.list[3].value = "contents"
+
+A raw dump is shown to compare with the attr1 dump on a prior page, the header
+is highlighted:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e 81 a4 01 02 00 01 00 00 00 00 00 00 00 00 IN..............
+ 10: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 05 ................
+ 20: 44 be 24 cd 0f b0 96 18 44 be 24 cd 0f b0 96 18 D.......D.......
+ 30: 44 be 2d f5 01 62 7a 18 00 00 00 00 00 00 00 04 D....bz.........
+ 40: 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 01 ................
+ 50: 00 00 0a 01 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 01 ................
+ 70: 41 c0 00 01 00 00 00 00 00 00 00 00 00 00 00 00 A...............
+ 80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ b0: 00 00 00 00 00 45 04 00 0a 00 00 65 6d 70 74 79 .....E.....empty
+ c0: 5f 61 74 74 72 07 04 02 74 72 75 73 74 5f 61 76 .attr...trust.av
+ d0: 61 6c 31 06 0c 00 73 65 63 6f 6e 64 73 65 63 6f all...secondseco
+ e0: 6e 64 5f 76 61 6c 75 65 06 08 04 70 6f 6c 69 63 nd.value...polic
+ f0: 79 63 6f 6e 74 65 6e 74 73 64 5f 76 61 6c 75 65 ycontentsd.value
+
+It can be clearly seen that attr2 allows many more attributes to be stored in
+an inode before they are moved to another filesystem block.
+
+Leaf Attributes
+~~~~~~~~~~~~~~~
+
+When an inode’s attribute fork space is used up with shortform attributes and
+more are added, the attribute format is migrated to "extents".
+
+Extent based attributes use hash/index pairs to speed up an attribute lookup.
+The first part of the "leaf" contains an array of fixed size hash/index
+pairs with the flags stored as well. The remaining part of the leaf block
+contains the array name/value pairs, where each element varies in length.
+
+Each leaf is based on the xfs\_da\_blkinfo\_t block header declared in the
+section about `directories <#directory-attribute-block-header>`__. On a v5
+filesystem, the block header is xfs\_da3\_blkinfo\_t. The structure
+encapsulating all other structures in the attribute block is
+xfs\_attr\_leafblock\_t.
+
+The structures involved are:
+
+.. code:: c
+
+ typedef struct xfs_attr_leaf_map {
+ __be16 base;
+ __be16 size;
+ } xfs_attr_leaf_map_t;
+
+**base**
+ Block offset of the free area, in bytes.
+
+**size**
+ Size of the free area, in bytes.
+
+.. code:: c
+
+ typedef struct xfs_attr_leaf_hdr {
+ xfs_da_blkinfo_t info;
+ __be16 count;
+ __be16 usedbytes;
+ __be16 firstused;
+ __u8 holes;
+ __u8 pad1;
+ xfs_attr_leaf_map_t freemap[3];
+ } xfs_attr_leaf_hdr_t;
+
+**info**
+ Directory/attribute block header.
+
+**count**
+ Number of entries.
+
+**usedbytes**
+ Number of bytes used in the leaf block.
+
+**firstused**
+ Block offset of the first entry in use, in bytes.
+
+**holes**
+ Set to 1 if block compaction is necessary.
+
+**pad1**
+ Padding to maintain alignment to 64-bit boundaries.
+
+.. code:: c
+
+ typedef struct xfs_attr_leaf_entry {
+ __be32 hashval;
+ __be16 nameidx;
+ __u8 flags;
+ __u8 pad2;
+ } xfs_attr_leaf_entry_t;
+ ----
+
+**hashval**
+ Hash value of the attribute name.
+
+**nameidx**
+ Block offset of the name entry, in bytes.
+
+**flags**
+ Attribute flags, as specified `above <#attribute-flags>`__.
+
+**pad2**
+ Pads the structure to 64-bit boundaries.
+
+.. code:: c
+
+ typedef struct xfs_attr_leaf_name_local {
+ __be16 valuelen;
+ __u8 namelen;
+ __u8 nameval[1];
+ } xfs_attr_leaf_name_local_t;
+
+**valuelen**
+ Length of the value, in bytes.
+
+**namelen**
+ Length of the name, in bytes.
+
+**nameval**
+ The name and the value. String values are not zero-terminated.
+
+.. code:: c
+
+ typedef struct xfs_attr_leaf_name_remote {
+ __be32 valueblk;
+ __be32 valuelen;
+ __u8 namelen;
+ __u8 name[1];
+ } xfs_attr_leaf_name_remote_t;
+
+**valueblk**
+ The logical block in the attribute map where the value is located.
+
+**valuelen**
+ Length of the value, in bytes.
+
+**namelen**
+ Length of the name, in bytes.
+
+**nameval**
+ The name. String values are not zero-terminated.
+
+.. code:: c
+
+ typedef struct xfs_attr_leafblock {
+ xfs_attr_leaf_hdr_t hdr;
+ xfs_attr_leaf_entry_t entries[1];
+ xfs_attr_leaf_name_local_t namelist;
+ xfs_attr_leaf_name_remote_t valuelist;
+ } xfs_attr_leafblock_t;
+
+**hdr**
+ Attribute block header.
+
+**entries**
+ A variable-length array of attribute entries.
+
+**namelist**
+ A variable-length array of descriptors of local attributes. The location
+ and size of these entries is determined dynamically.
+
+**valuelist**
+ A variable-length array of descriptors of remote attributes. The location
+ and size of these entries is determined dynamically.
+
+On a v5 filesystem, the header becomes xfs\_da3\_blkinfo\_t to accomodate the
+extra metadata integrity fields:
+
+.. code:: c
+
+ typedef struct xfs_attr3_leaf_hdr {
+ xfs_da3_blkinfo_t info;
+ __be16 count;
+ __be16 usedbytes;
+ __be16 firstused;
+ __u8 holes;
+ __u8 pad1;
+ xfs_attr_leaf_map_t freemap[3];
+ __be32 pad2;
+ } xfs_attr3_leaf_hdr_t;
+
+
+ typedef struct xfs_attr3_leafblock {
+ xfs_attr3_leaf_hdr_t hdr;
+ xfs_attr_leaf_entry_t entries[1];
+ xfs_attr_leaf_name_local_t namelist;
+ xfs_attr_leaf_name_remote_t valuelist;
+ } xfs_attr3_leafblock_t;
+
+Each leaf header uses the magic number XFS\_ATTR\_LEAF\_MAGIC (0xfbee). On a
+v5 filesystem, the magic number is XFS\_ATTR3\_LEAF\_MAGIC (0x3bee).
+
+The hash/index elements in the entries[] array are packed from the top of the
+block. Name/values grow from the bottom but are not packed. The freemap
+contains run-length-encoded entries for the free bytes after the entries[]
+array, but only the three largest runs are stored (smaller runs are dropped).
+When the freemap doesn’t show enough space for an allocation, the name/value
+area is compacted and allocation is tried again. If there still isn’t enough
+space, then the block is split. The name/value structures (both local and
+remote versions) must be 32-bit aligned.
+
+For attributes with small values (ie. the value can be stored within the
+leaf), the XFS\_ATTR\_LOCAL flag is set for the attribute. The entry details
+are stored using the xfs\_attr\_leaf\_name\_local\_t structure. For large
+attribute values that cannot be stored within the leaf, separate filesystem
+blocks are allocated to store the value. They use the
+xfs\_attr\_leaf\_name\_remote\_t structure. See `Remote
+Values <#remote-attribute-values>`__ for more information.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/69.png
+ :alt: Leaf attribute layout
+
+ Leaf attribute layout
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/69.png
+ :scale: 45%
+ :alt: Leaf attribute layout
+
+ Leaf attribute layout
+
+Both local and remote entries can be interleaved as they are only addressed by
+the hash/index entries. The flag is stored with the hash/index pairs so the
+appropriate structure can be used.
+
+Since duplicate hash keys are possible, for each hash that matches during a
+lookup, the actual name string must be compared.
+
+An "incomplete" bit is also used for attribute flags. It shows that an
+attribute is in the middle of being created and should not be shown to the
+user if we crash during the time that the bit is set. The bit is cleared when
+attribute has finished being set up. This is done because some large
+attributes cannot be created inside a single transaction.
+
+xfs\_db Leaf Attribute Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A single 30KB extended attribute is added to an inode:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.nblocks = 9
+ core.nextents = 0
+ core.naextents = 1
+ core.forkoff = 15
+ core.aformat = 2 (extents)
+ ...
+ a.bmx[0] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,37535,9,0]
+ xfs_db> ablock 0
+ xfs_db> p
+ hdr.info.forw = 0
+ hdr.info.back = 0
+ hdr.info.magic = 0xfbee
+ hdr.count = 1
+ hdr.usedbytes = 20
+ hdr.firstused = 4076
+ hdr.holes = 0
+ hdr.freemap[0-2] = [base,size] 0:[40,4036] 1:[0,0] 2:[0,0]
+ entries[0] = [hashval,nameidx,incomplete,root,secure,local]
+ 0:[0xfcf89d4f,4076,0,0,0,0]
+ nvlist[0].valueblk = 0x1
+ nvlist[0].valuelen = 30692
+ nvlist[0].namelen = 8
+ nvlist[0].name = "big_attr"
+
+Attribute blocks 1 to 8 (filesystem blocks 37536 to 37543) contain the raw
+binary value data for the attribute.
+
+Index 4076 (0xfec) is the offset into the block where the name/value
+information is. As can be seen by the value, it’s at the end of the block:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+
+ 000: 00 00 00 00 00 00 00 00 fb ee 00 00 00 01 00 14 ................
+ 010: 0f ec 00 00 00 28 0f c4 00 00 00 00 00 00 00 00 ................
+ 020: fc f8 9d 4f 0f ec 00 00 00 00 00 00 00 00 00 00 ...O............
+ 030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ ...
+ fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 ................
+ ff0: 00 00 77 e4 08 62 69 67 5f 61 74 74 72 00 00 00 ..w..big.attr...
+
+A 30KB attribute and a couple of small attributes are added to a file:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.nblocks = 10
+ core.extsize = 0
+ core.nextents = 1
+ core.naextents = 2
+ core.forkoff = 15
+ core.aformat = 2 (extents)
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,81857,1,0]
+ a.bmx[0-1] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,81858,1,0]
+ 1:[1,182398,8,0]
+ xfs_db> ablock 0
+ xfs_db> p
+ hdr.info.forw = 0
+ hdr.info.back = 0
+ hdr.info.magic = 0xfbee
+ hdr.count = 3
+ hdr.usedbytes = 52
+ hdr.firstused = 4044
+ hdr.holes = 0
+ hdr.freemap[0-2] = [base,size] 0:[56,3988] 1:[0,0] 2:[0,0]
+ entries[0-2] = [hashval,nameidx,incomplete,root,secure,local]
+ 0:[0x1e9d3934,4044,0,0,0,1]
+ 1:[0x1e9d3937,4060,0,0,0,1]
+ 2:[0xfcf89d4f,4076,0,0,0,0]
+ nvlist[0].valuelen = 6
+ nvlist[0].namelen = 5
+ nvlist[0].name = "attr2"
+ nvlist[0].value = "value2"
+ nvlist[1].valuelen = 6
+ nvlist[1].namelen = 5
+ nvlist[1].name = "attr1"
+ nvlist[1].value = "value1"
+ nvlist[2].valueblk = 0x1
+ nvlist[2].valuelen = 30692
+ nvlist[2].namelen = 8
+ nvlist[2].name = "big_attr"
+
+As can be seen in the entries array, the two small attributes have the local
+flag set and the values are printed.
+
+A raw disk dump shows the attributes. The last attribute added is highlighted
+(offset 4044 or 0xfcc):
+
+::
+
+ 000: 00 00 00 00 00 00 00 00 fb ee 00 00 00 03 00 34 ...............4
+ 010: 0f cc 00 00 00 38 0f 94 00 00 00 00 00 00 00 00 .....8..........
+ 020: 1e 9d 39 34 0f cc 01 00 1e 9d 39 37 0f dc 01 00 ..94......97....
+ 030: fc f8 9d 4f 0f ec 00 00 00 00 00 00 00 00 00 00 ...0............
+ 040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00.................
+ ...
+ fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 06 05 61 ...............a
+ fd0: 74 74 72 32 76 61 6c 75 65 32 00 00 00 06 05 61 ttr2value2.....a
+ fe0: 74 74 72 31 76 61 6c 75 65 31 00 00 00 00 00 01 ttr1value1......
+ ff0: 00 00 77 e4 08 62 69 67 5f 61 74 74 72 00 00 00 ..w..big.attr...
+
+Node Attributes
+~~~~~~~~~~~~~~~
+
+When the number of attributes exceeds the space that can fit in one filesystem
+block (ie. hash, flag, name and local values), the first attribute block
+becomes the root of a B+tree where the leaves contain the hash/name/value
+information that was stored in a single leaf block. The inode’s attribute
+format itself remains extent based. The nodes use the xfs\_da\_intnode\_t or
+xfs\_da3\_intnode\_t structures introduced in the section about
+`directories <#directory-attribute-internal-node>`__.
+
+The location of the attribute leaf blocks can be in any order. The only way to
+find an attribute is by walking the node block hash/before values. Given a
+hash to look up, search the node’s btree array for the first hashval in the
+array that exceeds the given hash. The entry is in the block pointed to by the
+before value.
+
+Each attribute node block has a magic number of XFS\_DA\_NODE\_MAGIC (0xfebe).
+On a v5 filesystem this is XFS\_DA3\_NODE\_MAGIC (0x3ebe).
+
+.. figure:: images/72.png
+ :alt: Node attribute layout
+
+ Node attribute layout
+
+xfs\_db Node Attribute Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+An inode with 1000 small attributes with the naming "attribute\_n" where
+'n' is a number:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.nblocks = 15
+ core.nextents = 0
+ core.naextents = 1
+ core.forkoff = 15
+ core.aformat = 2 (extents)
+ ...
+ a.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,525144,15,0]
+ xfs_db> ablock 0
+ xfs_db> p
+ hdr.info.forw = 0
+ hdr.info.back = 0
+ hdr.info.magic = 0xfebe
+ hdr.count = 14
+ hdr.level = 1
+ btree[0-13] = [hashval,before]
+ 0:[0x3435122d,1]
+ 1:[0x343550a9,14]
+ 2:[0x343553a6,13]
+ 3:[0x3436122d,12]
+ 4:[0x343650a9,8]
+ 5:[0x343653a6,7]
+ 6:[0x343691af,6]
+ 7:[0x3436d0ab,11]
+ 8:[0x3436d3a7,10]
+ 9:[0x3437122d,9]
+ 10:[0x3437922e,3]
+ 11:[0x3437d22a,5]
+ 12:[0x3e686c25,4]
+ 13:[0x3e686fad,2]
+
+The hashes are in ascending order in the btree array, and if the hash for the
+attribute we are looking up is before the entry, we go to the addressed
+attribute block.
+
+For example, to lookup attribute "attribute\_267":
+
+::
+
+ xfs_db> hash attribute_267
+ 0x3437d1a8
+
+In the root btree node, this falls between 0x3437922e and 0x3437d22a,
+therefore leaf 11 or attribute block 5 will contain the entry.
+
+::
+
+ xfs_db> ablock 5
+ xfs_db> p
+ hdr.info.forw = 4
+ hdr.info.back = 3
+ hdr.info.magic = 0xfbee
+ hdr.count = 96
+ hdr.usedbytes = 2688
+ hdr.firstused = 1408
+ hdr.holes = 0
+ hdr.freemap[0-2] = [base,size] 0:[800,608] 1:[0,0] 2:[0,0]
+ entries[0.95] = [hashval,nameidx,incomplete,root,secure,local]
+ 0:[0x3437922f,4068,0,0,0,1]
+ 1:[0x343792a6,4040,0,0,0,1]
+ 2:[0x343792a7,4012,0,0,0,1]
+ 3:[0x343792a8,3984,0,0,0,1]
+ ...
+ 82:[0x3437d1a7,2892,0,0,0,1]
+ 83:[0x3437d1a8,2864,0,0,0,1]
+ 84:[0x3437d1a9,2836,0,0,0,1]
+ ...
+ 95:[0x3437d22a,2528,0,0,0,1]
+ nvlist[0].valuelen = 10
+ nvlist[0].namelen = 13
+ nvlist[0].name = "attribute_310"
+ nvlist[0].value = "value_316\d"
+ nvlist[1].valuelen = 16
+ nvlist[1].namelen = 13
+ nvlist[1].name = "attribute_309"
+ nvlist[1].value = "value_309\d"
+ nvlist[2].valuelen = 10
+ nvlist[2].namelen = 13
+ nvlist[2].name = "attribute_308"
+ nvlist[2].value = "value_308\d"
+ nvlist[3].valuelen = 10
+ nvlist[3].namelen = 13
+ nvlist[3].name = "attribute_307"
+ nvlist[3].value = "value_307\d"
+ ...
+ nvlist[82].valuelen = 10
+ nvlist[82].namelen = 13
+ nvlist[82].name = "attribute_268"
+ nvlist[82].value = "value_268\d"
+ nvlist[83].valuelen = 10
+ nvlist[83].namelen = 13
+ nvlist[83].name = "attribute_267"
+ nvlist[83].value = "value_267\d"
+ nvlist[84].valuelen = 10
+ nvlist[84].namelen = 13
+ nvlist[84].name = "attribute_266"
+ nvlist[84].value = "value_266\d"
+ ...
+
+Each of the hash entries has XFS\_ATTR\_LOCAL flag set (1), which means the
+attribute’s value follows immediately after the name. Raw disk of the
+name/value pair at offset 2864 (0xb30), highlighted with "value\_267"
+following immediately after the name:
+
+::
+
+ b00: 62 75 74 65 5f 32 36 35 76 61 6c 75 65 5f 32 36 bute.265value.26
+ b10: 35 0a 00 00 00 0a 0d 61 74 74 72 69 62 75 74 65 5......attribute
+ b20: 51 32 36 36 76 61 6c 75 65 5f 32 36 36 0a 00 00 .266value.266...
+ b30: 00 0a 0d 61 74 74 72 69 62 75 74 65 5f 32 36 37 ...attribute.267
+ b40: 76 61 6c 75 65 5f 32 36 37 0a 00 00 00 0a 0d 61 value.267......a
+ b50: 74 74 72 69 62 75 74 65 5f 32 36 38 76 61 6c 75 ttribute.268va1u
+ b60: 65 5f 32 36 38 0a 00 00 00 0a 0d 61 74 74 72 69 e.268......attri
+ b70: 62 75 74 65 5f 32 36 39 76 61 6c 75 65 5f 32 36 bute.269value.26
+
+Each entry starts on a 32-bit (4 byte) boundary, therefore the highlighted
+entry has 2 unused bytes after it.
+
+B+tree Attributes
+~~~~~~~~~~~~~~~~~
+
+When the attribute’s extent map in an inode grows beyond the available space,
+the inode’s attribute format is changed to a "btree". The inode contains
+root node of the extent B+tree which then address the leaves that contains the
+extent arrays for the attribute data. The attribute data itself in the
+allocated filesystem blocks use the same layout and structures as described in
+`Node Attributes <#node-attributes>`__.
+
+Refer to the previous section on `B+tree Data Extents <#b-tree-extent-list>`__
+for more information on XFS B+tree extents.
+
+xfs\_db B+tree Attribute Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Added 2000 attributes with 729 byte values to a file:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ ...
+ core.nblocks = 640
+ core.extsize = 0
+ core.nextents = 1
+ core.naextents = 274
+ core.forkoff = 15
+ core.aformat = 3 (btree)
+ ...
+ a.bmbt.level = 1
+ a.bmbt.numrecs = 2
+ a.bmbt.keys[1-2] = [startoff] 1:[0] 2:[219]
+ a.bmbt.ptrs[1-2] = 1:83162 2:109968
+ xfs_db> fsblock 83162
+ xfs_db> type bmapbtd
+ xfs_db> p
+ magic = 0x424d4150
+ level = 0
+ numrecs = 127
+ leftsib = null
+ rightsib = 109968
+ recs[1-127] = [startoff,startblock,blockcount,extentflag]
+ 1:[0,81870,1,0]
+ ...
+ xfs_db> fsblock 109968
+ xfs_db> type bmapbtd
+ xfs_db> p
+ magic = 0x424d4150
+ level = 0
+ numrecs = 147
+ leftsib = 83162
+ rightsib = null
+ recs[1-147] = [startoff,startblock,blockcount,extentflag]
+ ...
+ (which is fsblock 81870)
+ xfs_db> ablock 0
+ xfs_db> p
+ hdr.info.forw = 0
+ hdr.info.back = 0
+ hdr.info.magic = 0xfebe
+ hdr.count = 2
+ hdr.level = 2
+ btree[0-1] = [hashval,before] 0:[0x343612a6,513] 1:[0x3e686fad,512]
+
+The extent B+tree has two leaves that specify the 274 extents used for the
+attributes. Looking at the first block, it can be seen that the attribute
+B+tree is two levels deep. The two blocks at offset 513 and 512 (ie. access
+using the ablock command) are intermediate xfs\_da\_intnode\_t nodes that
+index all the attribute leaves.
+
+Remote Attribute Values
+~~~~~~~~~~~~~~~~~~~~~~~
+
+On a v5 filesystem, all remote value blocks start with this header:
+
+.. code:: c
+
+ struct xfs_attr3_rmt_hdr {
+ __be32 rm_magic;
+ __be32 rm_offset;
+ __be32 rm_bytes;
+ __be32 rm_crc;
+ uuid_t rm_uuid;
+ __be64 rm_owner;
+ __be64 rm_blkno;
+ __be64 rm_lsn;
+ };
+
+**rm\_magic**
+ Specifies the magic number for the remote value block: "XARM"
+ (0x5841524d).
+
+**rm\_offset**
+ Offset of the remote value data, in bytes.
+
+**rm\_bytes**
+ Number of bytes used to contain the remote value data.
+
+**rm\_crc**
+ Checksum of the remote value block.
+
+**rm\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**rm\_owner**
+ The inode number that this remote value block belongs to.
+
+**rm\_blkno**
+ Disk block number of this remote value block.
+
+**rm\_lsn**
+ Log sequence number of the last write to this block.
+
+Filesystems formatted prior to v5 do not have this header in the remote block.
+Value data begins immediately at offset zero.
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 21/22] docs: add XFS symlink structures to the DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (18 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 20/22] docs: add XFS extended attributes structures " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-04 4:20 ` [PATCH 22/22] docs: add XFS metadump structure to " Darrick J. Wong
2018-10-06 0:51 ` [PATCH v2 00/22] xfs-4.20: major documentation surgery Dave Chinner
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/dynamic.rst | 1
.../xfs-data-structures/symbolic_links.rst | 140 ++++++++++++++++++++
2 files changed, 141 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/symbolic_links.rst
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 16755381d0f8..68837d0f477e 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -7,3 +7,4 @@ Dynamic Allocated Structures
.. include:: data_extents.rst
.. include:: directories.rst
.. include:: extended_attributes.rst
+.. include:: symbolic_links.rst
diff --git a/Documentation/filesystems/xfs-data-structures/symbolic_links.rst b/Documentation/filesystems/xfs-data-structures/symbolic_links.rst
new file mode 100644
index 000000000000..9206fd44b108
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/symbolic_links.rst
@@ -0,0 +1,140 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Symbolic Links
+--------------
+
+Symbolic links to a file can be stored in one of two formats: "local" and
+"extents". The length of the symlink contents is always specified by the
+inode’s di\_size value.
+
+Short Form Symbolic Links
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Symbolic links are stored with the "local" di\_format if the symbolic link
+can fit within the inode’s data fork. The link data is an array of characters
+(di\_symlink array in the data fork union).
+
+.. figure:: images/61.png
+ :alt: Symbolic link short form layout
+
+ Symbolic link short form layout
+
+xfs\_db Short Form Symbolic Link Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A short symbolic link to a file is created:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0120777
+ core.version = 1
+ core.format = 1 (local)
+ ...
+ core.size = 12
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u.symlink = "small_target"
+
+Raw on-disk data with the link contents highlighted:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e a1 ff 01 01 00 01 00 00 00 00 00 00 00 00 IN..............
+ 10: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 01 ................
+ 20: 44 be e1 c7 03 c4 d4 18 44 be el c7 03 c4 d4 18 D.......D.......
+ 30: 44 be e1 c7 03 c4 d4 18 00 00 00 00 00 00 00 Oc D...............
+ 40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 73 6d 61 6c 6c 5f 74 61 72 67 65 74 ....small.target
+ 70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+Extent Symbolic Links
+~~~~~~~~~~~~~~~~~~~~~
+
+If the length of the symbolic link exceeds the space available in the inode’s
+data fork, the link is moved to a new filesystem block and the inode’s
+di\_format is changed to "extents". The location of the block(s) is
+specified by the data fork’s di\_bmx[] array. In the significant majority of
+cases, this will be in one filesystem block as a symlink cannot be longer than
+1024 characters.
+
+On a v5 filesystem, the first block of each extent starts with the following
+header structure:
+
+.. code:: c
+
+ struct xfs_dsymlink_hdr {
+ __be32 sl_magic;
+ __be32 sl_offset;
+ __be32 sl_bytes;
+ __be32 sl_crc;
+ uuid_t sl_uuid;
+ __be64 sl_owner;
+ __be64 sl_blkno;
+ __be64 sl_lsn;
+ };
+
+**sl\_magic**
+ Specifies the magic number for the symlink block: "XSLM" (0x58534c4d).
+
+**sl\_offset**
+ Offset of the symbolic link target data, in bytes.
+
+**sl\_bytes**
+ Number of bytes used to contain the link target data.
+
+**sl\_crc**
+ Checksum of the symlink block.
+
+**sl\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**sl\_owner**
+ The inode number that this symlink block belongs to.
+
+**sl\_blkno**
+ Disk block number of this symlink.
+
+**sl\_lsn**
+ Log sequence number of the last write to this block.
+
+Filesystems formatted prior to v5 do not have this header in the remote block.
+Symlink data begins immediately at offset zero.
+
+.. figure:: images/62.png
+ :alt: Symbolic link extent layout
+
+ Symbolic link extent layout
+
+xfs\_db Symbolic Link Extent Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A longer link is created (greater than 156 bytes):
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0120777
+ core.version = 1
+ core.format = 2 (extents)
+ ...
+ core.size = 182
+ core.nblocks = 1
+ core.extsize = 0
+ core.nextents = 1
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,37530,1,0]
+ xfs_db> dblock 0
+ xfs_db> type symlink
+ xfs_db> p
+ "symlink contents..."
^ permalink raw reply related [flat|nested] 31+ messages in thread* [PATCH 22/22] docs: add XFS metadump structure to DS&A book
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (19 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 21/22] docs: add XFS symlink " Darrick J. Wong
@ 2018-10-04 4:20 ` Darrick J. Wong
2018-10-06 0:51 ` [PATCH v2 00/22] xfs-4.20: major documentation surgery Dave Chinner
21 siblings, 0 replies; 31+ messages in thread
From: Darrick J. Wong @ 2018-10-04 4:20 UTC (permalink / raw)
To: darrick.wong; +Cc: linux-xfs, linux-doc, corbet
From: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
.../filesystems/xfs-data-structures/auxiliary.rst | 2 +
.../filesystems/xfs-data-structures/metadump.rst | 72 ++++++++++++++++++++
2 files changed, 74 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/metadump.rst
diff --git a/Documentation/filesystems/xfs-data-structures/auxiliary.rst b/Documentation/filesystems/xfs-data-structures/auxiliary.rst
index d2fd2f88ad0e..7ed970f0bc28 100644
--- a/Documentation/filesystems/xfs-data-structures/auxiliary.rst
+++ b/Documentation/filesystems/xfs-data-structures/auxiliary.rst
@@ -2,3 +2,5 @@
Auxiliary Data Structures
=========================
+
+.. include:: metadump.rst
diff --git a/Documentation/filesystems/xfs-data-structures/metadump.rst b/Documentation/filesystems/xfs-data-structures/metadump.rst
new file mode 100644
index 000000000000..51bc966c1f76
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/metadump.rst
@@ -0,0 +1,72 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Metadata Dumps
+--------------
+
+The xfs\_metadump and xfs\_mdrestore tools are used to create a sparse
+snapshot of a live file system and to restore that snapshot onto a block
+device for debugging purposes. Only the metadata are captured in the snapshot,
+and the metadata blocks may be obscured for privacy reasons.
+
+A metadump file starts with a xfs\_metablock that records the addresses of the
+blocks that follow. Following that are the metadata blocks captured from the
+filesystem. The first block following the first superblock must be the
+superblock from AG 0. If the metadump has more blocks than can be pointed to
+by the xfs\_metablock.mb\_daddr area, the sequence of xfs\_metablock followed
+by metadata blocks is repeated.
+
+**Metadata Dump Format.**
+
+.. code:: c
+
+ struct xfs_metablock {
+ __be32 mb_magic;
+ __be16 mb_count;
+ uint8_t mb_blocklog;
+ uint8_t mb_reserved;
+ __be64 mb_daddr[];
+ };
+
+**mb\_magic**
+ The magic number, "XFSM" (0x5846534d).
+
+**mb\_count**
+ Number of blocks indexed by this record. This value must not exceed (1 <<
+ mb\_blocklog) - sizeof(struct xfs\_metablock).
+
+**mb\_blocklog**
+ The log size of a metadump block. This size of a metadump block 512 bytes,
+ so this value should be 9.
+
+**mb\_reserved**
+ Reserved. Should be zero.
+
+**mb\_daddr**
+ An array of disk addresses. Each of the mb\_count blocks (of size (1 <<
+ mb\_blocklog) following the xfs\_metablock should be written back to the
+ address pointed to by the corresponding mb\_daddr entry.
+
+Dump Obfuscation
+~~~~~~~~~~~~~~~~
+
+Unless explicitly disabled, the xfs\_metadump tool obfuscates empty block
+space and naming information to avoid leaking sensitive information into the
+metadump file. xfs\_metadump does not copy user data blocks.
+
+The obfuscation policy is as follows:
+
+- File and extended attribute names are both considered "names".
+
+- Names longer than 8 characters are totally rewritten with a name that
+ matches the hash of the old name.
+
+- Names between 5 and 8 characters are partially rewritten to match the hash
+ of the old name.
+
+- Names shorter than 5 characters are not obscured at all.
+
+- Names that cross a block boundary are not obscured at all.
+
+- Extended attribute values are zeroed.
+
+- Empty parts of metadata blocks are zeroed.
^ permalink raw reply related [flat|nested] 31+ messages in thread* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-04 4:18 [PATCH v2 00/22] xfs-4.20: major documentation surgery Darrick J. Wong
` (20 preceding siblings ...)
2018-10-04 4:20 ` [PATCH 22/22] docs: add XFS metadump structure to " Darrick J. Wong
@ 2018-10-06 0:51 ` Dave Chinner
2018-10-06 1:01 ` Jonathan Corbet
` (2 more replies)
21 siblings, 3 replies; 31+ messages in thread
From: Dave Chinner @ 2018-10-06 0:51 UTC (permalink / raw)
To: Darrick J. Wong; +Cc: linux-xfs, linux-doc, corbet
On Wed, Oct 03, 2018 at 09:18:11PM -0700, Darrick J. Wong wrote:
> Hi all,
>
> This series converts the existing in-kernel xfs documentation to rst
> format, links it in with the rest of the kernel's rst documetation, and
> then begins pulling in the contents of the Data Structures & Algorithms
> book from the xfs-documentation git tree. No changes are made to the
> text during the import process except to fix things that the conversion
> process (asciidoctor + pandoc) didn't do correctly. The goal of this
> series is to tie together the XFS code with the on-disk format
> documentation for the features supported by the code.
>
> I've built the docs and put them here, in case you hate reading rst:
> https://djwong.org/docs/kdoc/admin-guide/xfs.html
> https://djwong.org/docs/kdoc/filesystems/xfs-data-structures/index.html
>
> I've posted a branch here because the png import patch is huge:
> https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=docs-4.20-merge
>
> The patchset should apply cleanly against 4.19-rc6. Comments and
> questions are, as always, welcome.
Jon,
Can you let us know whether the CC-by-SA 4.0 license is acceptible
or not? That's really the only thing that we need clarified at this
point - if it's OK I'll to pull this into the XFS tree for the 4.20
merge window. If not, we'll go back to the drawing board....
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
^ permalink raw reply [flat|nested] 31+ messages in thread* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-06 0:51 ` [PATCH v2 00/22] xfs-4.20: major documentation surgery Dave Chinner
@ 2018-10-06 1:01 ` Jonathan Corbet
2018-10-06 1:09 ` Dave Chinner
2018-10-06 13:29 ` Matthew Wilcox
2018-10-11 17:27 ` Jonathan Corbet
2 siblings, 1 reply; 31+ messages in thread
From: Jonathan Corbet @ 2018-10-06 1:01 UTC (permalink / raw)
To: Dave Chinner; +Cc: Darrick J. Wong, linux-xfs, linux-doc
On Sat, 6 Oct 2018 10:51:54 +1000
Dave Chinner <david@fromorbit.com> wrote:
> Can you let us know whether the CC-by-SA 4.0 license is acceptible
> or not? That's really the only thing that we need clarified at this
> point - if it's OK I'll to pull this into the XFS tree for the 4.20
> merge window. If not, we'll go back to the drawing board....
I remain pretty concerned about it, to tell the truth. Rather than
continue to guess, though, I've called for help, and will be talking with
the LF lawyer about this next Thursday. Before then, I can't say anything
except "I don't think this works..."
Will let you know what I hear.
jon
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-06 1:01 ` Jonathan Corbet
@ 2018-10-06 1:09 ` Dave Chinner
0 siblings, 0 replies; 31+ messages in thread
From: Dave Chinner @ 2018-10-06 1:09 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: Darrick J. Wong, linux-xfs, linux-doc
On Fri, Oct 05, 2018 at 07:01:20PM -0600, Jonathan Corbet wrote:
> On Sat, 6 Oct 2018 10:51:54 +1000
> Dave Chinner <david@fromorbit.com> wrote:
>
> > Can you let us know whether the CC-by-SA 4.0 license is acceptible
> > or not? That's really the only thing that we need clarified at this
> > point - if it's OK I'll to pull this into the XFS tree for the 4.20
> > merge window. If not, we'll go back to the drawing board....
>
> I remain pretty concerned about it, to tell the truth. Rather than
> continue to guess, though, I've called for help, and will be talking with
> the LF lawyer about this next Thursday. Before then, I can't say anything
> except "I don't think this works..."
>
> Will let you know what I hear.
Thanks for the update, Jon. I'll put this on the backburner until I
hear back from you.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-06 0:51 ` [PATCH v2 00/22] xfs-4.20: major documentation surgery Dave Chinner
2018-10-06 1:01 ` Jonathan Corbet
@ 2018-10-06 13:29 ` Matthew Wilcox
2018-10-06 14:10 ` Jonathan Corbet
2018-10-11 17:27 ` Jonathan Corbet
2 siblings, 1 reply; 31+ messages in thread
From: Matthew Wilcox @ 2018-10-06 13:29 UTC (permalink / raw)
To: Dave Chinner; +Cc: Darrick J. Wong, linux-xfs, linux-doc, corbet
On Sat, Oct 06, 2018 at 10:51:54AM +1000, Dave Chinner wrote:
> On Wed, Oct 03, 2018 at 09:18:11PM -0700, Darrick J. Wong wrote:
> > Hi all,
> >
> > This series converts the existing in-kernel xfs documentation to rst
> > format, links it in with the rest of the kernel's rst documetation, and
> > then begins pulling in the contents of the Data Structures & Algorithms
> > book from the xfs-documentation git tree. No changes are made to the
> > text during the import process except to fix things that the conversion
> > process (asciidoctor + pandoc) didn't do correctly. The goal of this
> > series is to tie together the XFS code with the on-disk format
> > documentation for the features supported by the code.
> >
> > I've built the docs and put them here, in case you hate reading rst:
> > https://djwong.org/docs/kdoc/admin-guide/xfs.html
> > https://djwong.org/docs/kdoc/filesystems/xfs-data-structures/index.html
> >
> > I've posted a branch here because the png import patch is huge:
> > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=docs-4.20-merge
> >
> > The patchset should apply cleanly against 4.19-rc6. Comments and
> > questions are, as always, welcome.
>
> Jon,
>
> Can you let us know whether the CC-by-SA 4.0 license is acceptible
> or not? That's really the only thing that we need clarified at this
> point - if it's OK I'll to pull this into the XFS tree for the 4.20
> merge window. If not, we'll go back to the drawing board....
This is (obviously) not legal advice.
I had an informal chat with Bradley Kuhn at LinuxCon Japan about using
CC-BY-SA-4.0 and he indicated that I was probably better off using the
GPL-2(+) for documentation. I've changed the XArray documentation from
CC-BY-SA-4.0 to GPL-2+.
YMMV, defer to the LF lawyer, but I thought it worth providing a data
point.
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-06 13:29 ` Matthew Wilcox
@ 2018-10-06 14:10 ` Jonathan Corbet
0 siblings, 0 replies; 31+ messages in thread
From: Jonathan Corbet @ 2018-10-06 14:10 UTC (permalink / raw)
To: Matthew Wilcox; +Cc: Dave Chinner, Darrick J. Wong, linux-xfs, linux-doc
On Sat, 6 Oct 2018 06:29:51 -0700
Matthew Wilcox <willy@infradead.org> wrote:
> I had an informal chat with Bradley Kuhn at LinuxCon Japan about using
> CC-BY-SA-4.0 and he indicated that I was probably better off using the
> GPL-2(+) for documentation. I've changed the XArray documentation from
> CC-BY-SA-4.0 to GPL-2+.
Would you consider doing the same for Documentation/core-api/idr.rst?
That's the only CC-SA SPDX tag in the kernel now, and it's a clear
example of just what I'm worried about: it pulls in a nice DOC section
from the code when you run Sphinx on it, so we're not just talking
function prototypes here. I was likely to come knocking on your door
after my upcoming conversation anyway...:)
Thanks,
jon
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-06 0:51 ` [PATCH v2 00/22] xfs-4.20: major documentation surgery Dave Chinner
2018-10-06 1:01 ` Jonathan Corbet
2018-10-06 13:29 ` Matthew Wilcox
@ 2018-10-11 17:27 ` Jonathan Corbet
2018-10-12 1:33 ` Dave Chinner
2018-10-15 9:55 ` Christoph Hellwig
2 siblings, 2 replies; 31+ messages in thread
From: Jonathan Corbet @ 2018-10-11 17:27 UTC (permalink / raw)
To: Dave Chinner; +Cc: Darrick J. Wong, linux-xfs, linux-doc
On Sat, 6 Oct 2018 10:51:54 +1000
Dave Chinner <david@fromorbit.com> wrote:
> Can you let us know whether the CC-by-SA 4.0 license is acceptible
> or not? That's really the only thing that we need clarified at this
> point - if it's OK I'll to pull this into the XFS tree for the 4.20
> merge window. If not, we'll go back to the drawing board....
OK, I've had a long conversation with the LF lawyer, and she said clearly
that we really should not be introducing CC-SA material into the kernel
source tree. It's a pain; I really do like CC-SA better for
documentation, but it's not GPL-compatible, and that creates a problem for
the processed docs.
Sorry,
jon
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-11 17:27 ` Jonathan Corbet
@ 2018-10-12 1:33 ` Dave Chinner
2018-10-15 9:55 ` Christoph Hellwig
1 sibling, 0 replies; 31+ messages in thread
From: Dave Chinner @ 2018-10-12 1:33 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: Darrick J. Wong, linux-xfs, linux-doc
On Thu, Oct 11, 2018 at 11:27:35AM -0600, Jonathan Corbet wrote:
> On Sat, 6 Oct 2018 10:51:54 +1000 Dave Chinner
> <david@fromorbit.com> wrote:
>
> > Can you let us know whether the CC-by-SA 4.0 license is
> > acceptible or not? That's really the only thing that we need
> > clarified at this point - if it's OK I'll to pull this into the
> > XFS tree for the 4.20 merge window. If not, we'll go back to the
> > drawing board....
>
> OK, I've had a long conversation with the LF lawyer, and she said
> clearly that we really should not be introducing CC-SA material
> into the kernel source tree. It's a pain; I really do like CC-SA
> better for documentation, but it's not GPL-compatible, and that
> creates a problem for the processed docs.
Ok, thanks for following upon this, Jon. We'll just keep it all in
the existing external repo and work out what to do from there.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-11 17:27 ` Jonathan Corbet
2018-10-12 1:33 ` Dave Chinner
@ 2018-10-15 9:55 ` Christoph Hellwig
2018-10-15 14:28 ` Jonathan Corbet
1 sibling, 1 reply; 31+ messages in thread
From: Christoph Hellwig @ 2018-10-15 9:55 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: Dave Chinner, Darrick J. Wong, linux-xfs, linux-doc
On Thu, Oct 11, 2018 at 11:27:35AM -0600, Jonathan Corbet wrote:
> On Sat, 6 Oct 2018 10:51:54 +1000
> Dave Chinner <david@fromorbit.com> wrote:
>
> > Can you let us know whether the CC-by-SA 4.0 license is acceptible
> > or not? That's really the only thing that we need clarified at this
> > point - if it's OK I'll to pull this into the XFS tree for the 4.20
> > merge window. If not, we'll go back to the drawing board....
>
> OK, I've had a long conversation with the LF lawyer, and she said clearly
> that we really should not be introducing CC-SA material into the kernel
> source tree. It's a pain; I really do like CC-SA better for
> documentation, but it's not GPL-compatible, and that creates a problem for
> the processed docs.
That was exactly my concern with this patchset.
Btw, I think we have the same issue with idr.rst, and unless we can
relicense it we should drop it from the tree, as it actually includes
kernel source files.
^ permalink raw reply [flat|nested] 31+ messages in thread
* Re: [PATCH v2 00/22] xfs-4.20: major documentation surgery
2018-10-15 9:55 ` Christoph Hellwig
@ 2018-10-15 14:28 ` Jonathan Corbet
0 siblings, 0 replies; 31+ messages in thread
From: Jonathan Corbet @ 2018-10-15 14:28 UTC (permalink / raw)
To: Christoph Hellwig
Cc: Dave Chinner, Darrick J. Wong, linux-xfs, linux-doc, Matthew Wilcox
On Mon, 15 Oct 2018 02:55:49 -0700
Christoph Hellwig <hch@infradead.org> wrote:
> > OK, I've had a long conversation with the LF lawyer, and she said clearly
> > that we really should not be introducing CC-SA material into the kernel
> > source tree. It's a pain; I really do like CC-SA better for
> > documentation, but it's not GPL-compatible, and that creates a problem for
> > the processed docs.
>
> That was exactly my concern with this patchset.
>
> Btw, I think we have the same issue with idr.rst, and unless we can
> relicense it we should drop it from the tree, as it actually includes
> kernel source files.
...and a significant DOC section at that, not just function prototypes.
I'd already asked Willy [CC'd] about this once, haven't gotten an answer
yet. Hopefully we can address this once he comes back around.
Thanks,
jon
^ permalink raw reply [flat|nested] 31+ messages in thread