linux/Documentation/filesystems/ext4/bitmaps.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3Block and inode Bitmaps
   4-----------------------
   5
   6The data block bitmap tracks the usage of data blocks within the block
   7group.
   8
   9The inode bitmap records which entries in the inode table are in use.
  10
  11As with most bitmaps, one bit represents the usage status of one data
  12block or inode table entry. This implies a block group size of 8 \*
  13number\_of\_bytes\_in\_a\_logical\_block.
  14
  15NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
  16of the kernel and e2fsprogs code pretends that the block bitmap contains
  17zeros (i.e. all blocks in the group are free). However, it is not
  18necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
  19the bitmaps and group descriptor live inside the group. Unfortunately,
  20ext2fs\_test\_block\_bitmap2() will return '0' for those locations,
  21which produces confusing debugfs output.
  22
  23Inode Table
  24-----------
  25Inode tables are statically allocated at mkfs time.  Each block group
  26descriptor points to the start of the table, and the superblock records
  27the number of inodes per group.  See the section on inodes for more
  28information.
  29