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