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8a98ec7c7b
Move the ext4 data structures book to Documentation/filesystems/ext4/ since the administrative information moved elsewhere. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
29 lines
1.1 KiB
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29 lines
1.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Block and inode Bitmaps
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-----------------------
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The data block bitmap tracks the usage of data blocks within the block
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group.
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The inode bitmap records which entries in the inode table are in use.
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As with most bitmaps, one bit represents the usage status of one data
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block or inode table entry. This implies a block group size of 8 \*
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number\_of\_bytes\_in\_a\_logical\_block.
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NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
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of the kernel and e2fsprogs code pretends that the block bitmap contains
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zeros (i.e. all blocks in the group are free). However, it is not
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necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
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the bitmaps and group descriptor live inside the group. Unfortunately,
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ext2fs\_test\_block\_bitmap2() will return '0' for those locations,
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which produces confusing debugfs output.
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Inode Table
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-----------
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Inode tables are statically allocated at mkfs time. Each block group
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descriptor points to the start of the table, and the superblock records
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the number of inodes per group. See the section on inodes for more
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information.
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