summaryrefslogtreecommitdiffstats
path: root/Documentation/filesystems/ext4/bitmaps.rst
diff options
context:
space:
mode:
Diffstat (limited to '')
-rw-r--r--Documentation/filesystems/ext4/bitmaps.rst28
1 files changed, 28 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/bitmaps.rst b/Documentation/filesystems/ext4/bitmaps.rst
new file mode 100644
index 000000000..91c45d86e
--- /dev/null
+++ b/Documentation/filesystems/ext4/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.