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authorDarrick J. Wong <darrick.wong@oracle.com>2018-07-29 22:41:00 +0300
committerTheodore Ts'o <tytso@mit.edu>2018-07-29 22:41:00 +0300
commit33dfadc747a88243d616d8cb5c59ec3031dc60dd (patch)
tree2e031b6fbd31f3ac399828c8a5c7d522f649e657 /Documentation/filesystems/ext4
parent3c6ba09dd3f8081c2da2878ee275c0c2213d2d43 (diff)
downloadlinux-33dfadc747a88243d616d8cb5c59ec3031dc60dd.tar.xz
ext4: import bitmaps chapter from wiki page
Import the chapter about bitmaps from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'Documentation/filesystems/ext4')
-rw-r--r--Documentation/filesystems/ext4/ondisk/bitmaps.rst28
-rw-r--r--Documentation/filesystems/ext4/ondisk/globals.rst1
2 files changed, 29 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/ondisk/bitmaps.rst b/Documentation/filesystems/ext4/ondisk/bitmaps.rst
new file mode 100644
index 000000000000..c7546dbc197a
--- /dev/null
+++ b/Documentation/filesystems/ext4/ondisk/bitmaps.rst
@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Block and inode Bitmaps
+-----------------------
+
+The data block bitmap tracks the usage of data blocks within the block
+group.
+
+The inode bitmap records which entries in the inode table are in use.
+
+As with most bitmaps, one bit represents the usage status of one data
+block or inode table entry. This implies a block group size of 8 \*
+number\_of\_bytes\_in\_a\_logical\_block.
+
+NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
+of the kernel and e2fsprogs code pretends that the block bitmap contains
+zeros (i.e. all blocks in the group are free). However, it is not
+necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
+the bitmaps and group descriptor live inside the group. Unfortunately,
+ext2fs\_test\_block\_bitmap2() will return '0' for those locations,
+which produces confusing debugfs output.
+
+Inode Table
+-----------
+Inode tables are statically allocated at mkfs time. Each block group
+descriptor points to the start of the table, and the superblock records
+the number of inodes per group. See the section on inodes for more
+information.
diff --git a/Documentation/filesystems/ext4/ondisk/globals.rst b/Documentation/filesystems/ext4/ondisk/globals.rst
index c0dea6297e7f..5891ee137749 100644
--- a/Documentation/filesystems/ext4/ondisk/globals.rst
+++ b/Documentation/filesystems/ext4/ondisk/globals.rst
@@ -8,3 +8,4 @@ have static metadata at fixed locations.
.. include:: super.rst
.. include:: group_descr.rst
+.. include:: bitmaps.rst