[PATCH] erofs: update documentation

[PATCH] erofs: update documentation

[Gao Xiang]

- Refine highlights for main features;

- Add multi-reference pclusters and fragment description.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 Documentation/filesystems/erofs.rst | 35 ++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index 05e03d54af1a..82af67fdaf99 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -30,12 +30,17 @@ It is implemented to be a better choice for the following scenarios:
    especially for those embedded devices with limited memory and high-density
    hosts with numerous containers.
 
-Here is the main features of EROFS:
+Here are the main features of EROFS:
 
  - Little endian on-disk design;
 
- - 4KiB block size and 32-bit block addresses, therefore 16TiB address space
-   at most for now;
+ - Block-based and file-based distribution over fscache are supported;
+
+ - Support multiple devices to refer to external blobs, which can be used
+   for container images;
+
+ - 4KiB block size and 32-bit block addresses for each device, therefore
+   16TiB address space at most for now;
 
  - Two inode layouts for different requirements:
 
@@ -50,28 +55,29 @@ Here is the main features of EROFS:
    Metadata reserved      8 bytes       18 bytes
    =====================  ============  ======================================
 
- - Metadata and data could be mixed as an option;
-
- - Support extended attributes (xattrs) as an option;
+ - Support extended attributes as an option;
 
- - Support tailpacking data and xattr inline compared to byte-addressed
-   unaligned metadata or smaller block size alternatives;
-
- - Support POSIX.1e ACLs by using xattrs;
+ - Support POSIX.1e ACLs by using extended attributes;
 
  - Support transparent data compression as an option:
    LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
    inplace decompression is also supported to avoid bounce compressed buffers
    and page cache thrashing.
 
+ - Support chunk-based data deduplication and rolling-hash compressed data
+   deduplication;
+
+ - Support tailpacking inline compared to byte-addressed unaligned metadata
+   or smaller block size alternatives;
+
+ - Support merging tail-end data into a special inode as fragments.
+
  - Support direct I/O on uncompressed files to avoid double caching for loop
    devices;
 
  - Support FSDAX on uncompressed images for secure containers and ramdisks in
    order to get rid of unnecessary page cache.
 
- - Support multiple devices for multi blob container images;
-
  - Support file-based on-demand loading with the Fscache infrastructure.
 
 The following git tree provides the file system user-space tools under
@@ -259,7 +265,7 @@ By the way, chunk-based files are all uncompressed for now.
 
 Data compression
 ----------------
-EROFS implements LZ4 fixed-sized output compression which generates fixed-sized
+EROFS implements fixed-sized output compression which generates fixed-sized
 compressed data blocks from variable-sized input in contrast to other existing
 fixed-sized input solutions. Relatively higher compression ratios can be gotten
 by using fixed-sized output compression since nowadays popular data compression
@@ -314,3 +320,6 @@ to understand its delta0 is constantly 1, as illustrated below::
 
 If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
 but it's easy to know the size of such pcluster is 1 lcluster as well.
+
+Since Linux v6.1, each pcluster can be used for multiple variable-sized extents,
+therefore it can be used for compressed data deduplication.
-- 
2.24.4

Re: [PATCH] erofs: update documentation

[Yue Hu]

On Wed, 30 Nov 2022 17:56:05 +0800
Gao Xiang <hsiangkao@linux.alibaba.com> wrote:

> - Refine highlights for main features;
> 
> - Add multi-reference pclusters and fragment description.
> 
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Reviewed-by: Yue Hu <huyue2@coolpad.com>

> ---
>  Documentation/filesystems/erofs.rst | 35 ++++++++++++++++++-----------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 

Re: [PATCH] erofs: update documentation

[Chao Yu]

On 2022/11/30 17:56, Gao Xiang wrote:
> - Refine highlights for main features;
> 
> - Add multi-reference pclusters and fragment description.
> 
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,

[PATCH] erofs: update documentation

[Jingbo Xu]

 - update new features like bloom filter and DEFLATE.

 - add documentation for the long xattr name prefixes, which was
   landed upstream since v6.4.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 Documentation/filesystems/erofs.rst | 40 ++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index 4654ee57c1d5..522183737be6 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -58,12 +58,14 @@ Here are the main features of EROFS:
 
  - Support extended attributes as an option;
 
+ - Support a bloom filter that speeds up negative extended attribute lookups;
+
  - Support POSIX.1e ACLs by using extended attributes;
 
  - Support transparent data compression as an option:
-   LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
-   inplace decompression is also supported to avoid bounce compressed buffers
-   and page cache thrashing.
+   LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In
+   addition, inplace decompression is also supported to avoid bounce compressed
+   buffers and unnecessary page cache thrashing.
 
  - Support chunk-based data deduplication and rolling-hash compressed data
    deduplication;
@@ -268,6 +270,38 @@ details.)
 
 By the way, chunk-based files are all uncompressed for now.
 
+Long extended attribute name prefixes
+-------------------------------------
+There are use cases that extended attributes with different values can have
+only a few common prefixes (such as overlayfs xattrs).  The predefined prefixes
+works inefficiently in both image size and runtime performance in such cases.
+
+The long xattr name prefixes feature is introduced to address this issue.  The
+overall idea is that, apart from the existing predefined prefixes, the xattr
+entry could also refer to user specified long xattr name prefixes, e.g.
+"trusted.overlay.".
+
+When referring to a long xattr name prefix, the highest bit (bit 7) of
+erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole
+represents the index of the referred long name prefix among all long name
+prefixes.  Therefore, only the trailing part of the name apart from the long
+xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
+the full xattr name matches exactly as its long xattr name prefix.
+
+All long xattr prefixes are stored one by one in the packed inode as long as
+the packed inode is valid, or meta inode otherwise.  The xattr_prefix_count (of
+on-disk superblock) indicates the total number of the long xattr name prefixes,
+while (xattr_prefix_start * 4) indicates the start offset of long name prefixes
+in the packed/meta inode.  Note that, long extended attribute name prefixes is
+disabled if xattr_prefix_count is 0.
+
+Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
+where len represents the total size of the data part.  The data part is actually
+represented by 'struct erofs_xattr_long_prefix', where base_index represents the
+index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for
+"trusted.overlay." long name prefix, while the infix string kepts the string
+after stripping the short prefix, e.g. "overlay." for the example above.
+
 Data compression
 ----------------
 EROFS implements fixed-sized output compression which generates fixed-sized
-- 
2.19.1.6.gb485710b

Re: [PATCH] erofs: update documentation

[Gao Xiang]

On Thu, Sep 28, 2023 at 09:16:00PM +0800, Jingbo Xu wrote:
>  - update new features like bloom filter and DEFLATE.
> 
>  - add documentation for the long xattr name prefixes, which was
>    landed upstream since v6.4.
> 
> Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
> ---
>  Documentation/filesystems/erofs.rst | 40 ++++++++++++++++++++++++++---
>  1 file changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
> index 4654ee57c1d5..522183737be6 100644
> --- a/Documentation/filesystems/erofs.rst
> +++ b/Documentation/filesystems/erofs.rst
> @@ -58,12 +58,14 @@ Here are the main features of EROFS:
>  
>   - Support extended attributes as an option;
>  
> + - Support a bloom filter that speeds up negative extended attribute lookups;
> +
>   - Support POSIX.1e ACLs by using extended attributes;
>  
>   - Support transparent data compression as an option:
> -   LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
> -   inplace decompression is also supported to avoid bounce compressed buffers
> -   and page cache thrashing.
> +   LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In
> +   addition, inplace decompression is also supported to avoid bounce compressed
> +   buffers and unnecessary page cache thrashing.
>  
>   - Support chunk-based data deduplication and rolling-hash compressed data
>     deduplication;
> @@ -268,6 +270,38 @@ details.)
>  
>  By the way, chunk-based files are all uncompressed for now.
>  
> +Long extended attribute name prefixes
> +-------------------------------------
> +There are use cases that extended attributes with different values can have

                        ^ where

> +only a few common prefixes (such as overlayfs xattrs).  The predefined prefixes
> +works inefficiently in both image size and runtime performance in such cases.

  ^ work

> +
> +The long xattr name prefixes feature is introduced to address this issue.  The
> +overall idea is that, apart from the existing predefined prefixes, the xattr
> +entry could also refer to user specified long xattr name prefixes, e.g.

                             ^ user-specified

> +"trusted.overlay.".
> +
> +When referring to a long xattr name prefix, the highest bit (bit 7) of
> +erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole
> +represents the index of the referred long name prefix among all long name
> +prefixes.  Therefore, only the trailing part of the name apart from the long
> +xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
> +the full xattr name matches exactly as its long xattr name prefix.
> +
> +All long xattr prefixes are stored one by one in the packed inode as long as
> +the packed inode is valid, or meta inode otherwise.  The xattr_prefix_count (of

                                ^ the meta inode

> +on-disk superblock) indicates the total number of the long xattr name prefixes,

^ the on-disk superblock

> +while (xattr_prefix_start * 4) indicates the start offset of long name prefixes
> +in the packed/meta inode.  Note that, long extended attribute name prefixes is

                                                                            ^ are

> +disabled if xattr_prefix_count is 0.
> +
> +Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
> +where len represents the total size of the data part.  The data part is actually
> +represented by 'struct erofs_xattr_long_prefix', where base_index represents the
> +index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for
> +"trusted.overlay." long name prefix, while the infix string kepts the string

                                                               ^ keeps
Thanks,
Gao Xiang

[PATCH] erofs: update documentation

[Gao Xiang]

 - refine the filesystem overview for better description of recent
   new features like FSDAX or Fscache;

 - add the new `fsid' mount option;

 - fix some typos.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 Documentation/filesystems/erofs.rst | 62 +++++++++++++++++++----------
 1 file changed, 41 insertions(+), 21 deletions(-)

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index bef6d3040ce4..4f108a218049 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -1,63 +1,82 @@
 .. SPDX-License-Identifier: GPL-2.0
 
 ======================================
-Enhanced Read-Only File System - EROFS
+EROFS - Enhanced Read-Only File System
 ======================================
 
 Overview
 ========
 
-EROFS file-system stands for Enhanced Read-Only File System. Different
-from other read-only file systems, it aims to be designed for flexibility,
-scalability, but be kept simple and high performance.
+EROFS filesystem stands for Enhanced Read-Only File System.  It aims to form a
+generic read-only filesystem solution for various read-only use cases instead
+of just focusing on storage space saving without considering any side effects
+of runtime performance.
 
-It is designed as a better filesystem solution for the following scenarios:
+It is designed to meet the needs of flexibility, feature extendability and user
+payload friendly, etc.  Apart from those, it is still kept as a simple
+random-access friendly high-performance filesystem to get rid of unneeded I/O
+amplification and memory-resident overhead compared to similar approaches.
+
+It is implemented to be a better choice for the following scenarios:
 
  - read-only storage media or
 
  - part of a fully trusted read-only solution, which means it needs to be
    immutable and bit-for-bit identical to the official golden image for
-   their releases due to security and other considerations and
+   their releases due to security or other considerations and
 
  - hope to minimize extra storage space with guaranteed end-to-end performance
    by using compact layout, transparent file compression and direct access,
    especially for those embedded devices with limited memory and high-density
-   hosts with numerous containers;
+   hosts with numerous containers.
 
 Here is the main features of EROFS:
 
  - Little endian on-disk design;
 
- - Currently 4KB block size (nobh) and therefore maximum 16TB address space;
-
- - Metadata & data could be mixed by design;
+ - 4KiB block size and 32-bit block addresses, therefore 16TiB address space
+   at most for now;
 
- - 2 inode versions for different requirements:
+ - Two inode layouts for different requirements:
 
-   =====================  ============  =====================================
+   =====================  ============  ======================================
                           compact (v1)  extended (v2)
-   =====================  ============  =====================================
+   =====================  ============  ======================================
    Inode metadata size    32 bytes      64 bytes
-   Max file size          4 GB          16 EB (also limited by max. vol size)
+   Max file size          4 GiB         16 EiB (also limited by max. vol size)
    Max uids/gids          65536         4294967296
    Per-inode timestamp    no            yes (64 + 32-bit timestamp)
    Max hardlinks          65536         4294967296
    Metadata reserved      4 bytes       14 bytes
-   =====================  ============  =====================================
+   =====================  ============  ======================================
+
+ - Metadata and data could be mixed as an option;
 
  - Support extended attributes (xattrs) as an option;
 
- - Support xattr inline and tail-end data inline for all files;
+ - Support tailpacking data inline and xattr inline compared to byte-addressed
+   unaligned metadata or smaller block size alternatives;
 
  - Support POSIX.1e ACLs by using xattrs;
 
  - Support transparent data compression as an option:
-   LZ4 algorithm with the fixed-sized output compression for high performance;
+   LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
+   inplace decompression is also supported to avoid bounce compressed buffers
+   and page cache thrashing.
+
+ - Support direct I/O on uncompressed files to avoid double caching for loop
+   devices;
 
- - Multiple device support for multi-layer container images.
+ - Support FSDAX on uncompressed images for secure containers and ramdisks in
+   order to get rid of unnecessary page cache.
+
+ - Support multiple devices for multi blob container images;
+
+ - Support file-based on-demand loading with the Fscache infrastructure.
 
 The following git tree provides the file system user-space tools under
-development (ex, formatting tool mkfs.erofs):
+development, such as a formatting tool (mkfs.erofs), an on-disk consistency &
+compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs):
 
 - git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
 
@@ -91,6 +110,7 @@ dax={always,never}     Use direct access (no page cache).  See
                        Documentation/filesystems/dax.rst.
 dax                    A legacy option which is an alias for ``dax=always``.
 device=%s              Specify a path to an extra device to be used together.
+fsid=%s                Specify a filesystem image ID for Fscache back-end.
 ===================    =========================================================
 
 Sysfs Entries
@@ -226,8 +246,8 @@ Note that apart from the offset of the first filename, nameoff0 also indicates
 the total number of directory entries in this block since it is no need to
 introduce another on-disk field at all.
 
-Chunk-based file
-----------------
+Chunk-based files
+-----------------
 In order to support chunk-based data deduplication, a new inode data layout has
 been supported since Linux v5.15: Files are split in equal-sized data chunks
 with ``extents`` area of the inode metadata indicating how to get the chunk
-- 
2.24.4

Re: [PATCH] erofs: update documentation

[Chao Yu]

On 2022/5/27 15:01, Gao Xiang wrote:
>   - refine the filesystem overview for better description of recent
>     new features like FSDAX or Fscache;
> 
>   - add the new `fsid' mount option;
> 
>   - fix some typos.
> 
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,

[PATCH] erofs: update documentation

[Gao Xiang]

From: Gao Xiang <gaoxiang25@huawei.com>

Some on-disk structures, fields have been renamed in v5.4,
the corresponding document should be updated as well.

Also fix misrespresentation of file time and words about
fixed-sized output compression, data inline, etc.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
Hi,

I plan to update documentation together with
https://lore.kernel.org/r/20191201084040.29275-1-hsiangkao@aol.com/
for this round.

Comments about this are greatly welcomed.

Thanks,
Gao Xiang

 Documentation/filesystems/erofs.txt | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/erofs.txt b/Documentation/filesystems/erofs.txt
index b0c085326e2e..061eb8bd8387 100644
--- a/Documentation/filesystems/erofs.txt
+++ b/Documentation/filesystems/erofs.txt
@@ -24,11 +24,11 @@ Here is the main features of EROFS:
  - Metadata & data could be mixed by design;
 
  - 2 inode versions for different requirements:
-                          v1            v2
+                          compact (v1)  extended (v2)
    Inode metadata size:   32 bytes      64 bytes
    Max file size:         4 GB          16 EB (also limited by max. vol size)
    Max uids/gids:         65536         4294967296
-   File creation time:    no            yes (64 + 32-bit timestamp)
+   File change time:      no            yes (64 + 32-bit timestamp)
    Max hardlinks:         65536         4294967296
    Metadata reserved:     4 bytes       14 bytes
 
@@ -39,7 +39,7 @@ Here is the main features of EROFS:
  - Support POSIX.1e ACLs by using xattrs;
 
  - Support transparent file compression as an option:
-   LZ4 algorithm with 4 KB fixed-output compression for high performance;
+   LZ4 algorithm with 4 KB fixed-sized output compression for high performance.
 
 The following git tree provides the file system user-space tools under
 development (ex, formatting tool mkfs.erofs):
@@ -85,7 +85,7 @@ All data areas should be aligned with the block size, but metadata areas
 may not. All metadatas can be now observed in two different spaces (views):
  1. Inode metadata space
     Each valid inode should be aligned with an inode slot, which is a fixed
-    value (32 bytes) and designed to be kept in line with v1 inode size.
+    value (32 bytes) and designed to be kept in line with compact inode size.
 
     Each inode can be directly found with the following formula:
          inode offset = meta_blkaddr * block_size + 32 * nid
@@ -117,10 +117,10 @@ may not. All metadatas can be now observed in two different spaces (views):
                                                        |-> aligned with 4B
 
     Inode could be 32 or 64 bytes, which can be distinguished from a common
-    field which all inode versions have -- i_advise:
+    field which all inode versions have -- i_format:
 
         __________________               __________________
-       |     i_advise     |             |     i_advise     |
+       |     i_format     |             |     i_format     |
        |__________________|             |__________________|
        |        ...       |             |        ...       |
        |                  |             |                  |
@@ -129,12 +129,13 @@ may not. All metadatas can be now observed in two different spaces (views):
                                         |__________________| 64 bytes
 
     Xattrs, extents, data inline are followed by the corresponding inode with
-    proper alignes, and they could be optional for different data mappings,
-    _currently_ there are totally 3 valid data mappings supported:
+    proper alignment, and they could be optional for different data mappings.
+    _currently_ total 4 valid data mappings are supported:
 
-     1) flat file data without data inline (no extent);
-     2) fixed-output size data compression (must have extents);
-     3) flat file data with tail-end data inline (no extent);
+     0   flat file data without data inline (no extent);
+     1   fixed-sized output data compression (with non-compacted indexes);
+     2   flat file data with tail packing data inline (no extent);
+     3   fixed-sized output data compression (with compacted indexes).
 
     The size of the optional xattrs is indicated by i_xattr_count in inode
     header. Large xattrs or xattrs shared by many different files can be
@@ -182,8 +183,8 @@ introduce another on-disk field at all.
 
 Compression
 -----------
-Currently, EROFS supports 4KB fixed-output clustersize transparent file
-compression, as illustrated below:
+Currently, EROFS supports 4KB fixed-sized output transparent file compression,
+as illustrated below:
 
          |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
          clusterofs                      clusterofs            clusterofs
-- 
2.20.1