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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>
37 lines
1.7 KiB
ReStructuredText
37 lines
1.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Inline Data
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-----------
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The inline data feature was designed to handle the case that a file's
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data is so tiny that it readily fits inside the inode, which
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(theoretically) reduces disk block consumption and reduces seeks. If the
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file is smaller than 60 bytes, then the data are stored inline in
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``inode.i_block``. If the rest of the file would fit inside the extended
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attribute space, then it might be found as an extended attribute
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“system.data” within the inode body (“ibody EA”). This of course
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constrains the amount of extended attributes one can attach to an inode.
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If the data size increases beyond i\_block + ibody EA, a regular block
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is allocated and the contents moved to that block.
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Pending a change to compact the extended attribute key used to store
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inline data, one ought to be able to store 160 bytes of data in a
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256-byte inode (as of June 2015, when i\_extra\_isize is 28). Prior to
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that, the limit was 156 bytes due to inefficient use of inode space.
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The inline data feature requires the presence of an extended attribute
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for “system.data”, even if the attribute value is zero length.
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Inline Directories
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~~~~~~~~~~~~~~~~~~
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The first four bytes of i\_block are the inode number of the parent
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directory. Following that is a 56-byte space for an array of directory
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entries; see ``struct ext4_dir_entry``. If there is a “system.data”
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attribute in the inode body, the EA value is an array of
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``struct ext4_dir_entry`` as well. Note that for inline directories, the
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i\_block and EA space are treated as separate dirent blocks; directory
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entries cannot span the two.
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Inline directory entries are not checksummed, as the inode checksum
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should protect all inline data contents.
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