design: fix computation of buffer log item bitmap size

Wengang Wang was trying to work through a buffer log item by consulting
the ondisk format documentation, and was confused by the formula given
in section 14.3.14 regarding the size of blf_data_map, aka the dirty
bitmap for buffer log items.  We noticed that the documentation doesn't
match what the kernel actually does, so let's fix this.

Reported-by: Wengang Wang <>
Signed-off-by: Darrick J. Wong <>
Reviewed-by: Chandan Babu R <>
diff --git a/design/XFS_Filesystem_Structure/journaling_log.asciidoc b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
index 8421a53..ddcb87f 100644
--- a/design/XFS_Filesystem_Structure/journaling_log.asciidoc
+++ b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
@@ -867,7 +867,7 @@
 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.
+area.  Each bit represents +XFS_BLF_CHUNK+ (i.e. 128) bytes.
 === Buffer Data Log Item