[PATCH 1/2] design: document the new inode btree counter feature

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

Update the ondisk format documentation to discuss the inode btree
counter feature.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 .../allocation_groups.asciidoc                     |   21 ++++++++++++++++++++
 1 file changed, 21 insertions(+)


diff --git a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
index 992615d..2e78f56 100644
--- a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
+++ b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
@@ -405,6 +405,13 @@ reference counts of AG blocks.  This enables files to share data blocks safely.
 See the section about xref:Reflink_Deduplication[reflink and deduplication] for
 more details.
 
+| +XFS_SB_FEAT_RO_COMPAT_INOBTCNT+ |
+Inode B+tree block counters.  Each allocation group's inode (AGI) header
+tracks the number of blocks in each of the inode B+trees.  This allows us
+to have a slightly higher level of redundancy over the shape of the inode
+btrees, and decreases the amount of time to compute the metadata B+tree
+preallocations at mount time.
+
 |=====
 
 *sb_features_incompat*::
@@ -928,6 +935,10 @@ struct xfs_agi {
 
      __be32              agi_free_root;
      __be32              agi_free_level;
+
+     __be32              agi_iblocks;
+     __be32              agi_fblocks;
+
 }
 ----
 *agi_magicnum*::
@@ -984,6 +995,16 @@ B+tree.
 *agi_free_level*::
 Specifies the number of levels in the free inode B+tree.
 
+*agi_iblocks*::
+The number of blocks in the inode B+tree, including the root.
+This field is zero if the +XFS_SB_FEAT_RO_COMPAT_INOBTCNT+ feature is not
+enabled.
+
+*agi_fblocks*::
+The number of blocks in the free inode B+tree, including the root.
+This field is zero if the +XFS_SB_FEAT_RO_COMPAT_INOBTCNT+ feature is not
+enabled.
+
 [[Inode_Btrees]]
 == Inode B+trees