57 lines
3.1 KiB
ReStructuredText
57 lines
3.1 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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Block and Inode Allocation Policy
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---------------------------------
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ext4 recognizes (better than ext3, anyway) that data locality is
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generally a desirably quality of a filesystem. On a spinning disk,
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keeping related blocks near each other reduces the amount of movement
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that the head actuator and disk must perform to access a data block,
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thus speeding up disk IO. On an SSD there of course are no moving parts,
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but locality can increase the size of each transfer request while
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reducing the total number of requests. This locality may also have the
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effect of concentrating writes on a single erase block, which can speed
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up file rewrites significantly. Therefore, it is useful to reduce
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fragmentation whenever possible.
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The first tool that ext4 uses to combat fragmentation is the multi-block
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allocator. When a file is first created, the block allocator
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speculatively allocates 8KiB of disk space to the file on the assumption
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that the space will get written soon. When the file is closed, the
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unused speculative allocations are of course freed, but if the
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speculation is correct (typically the case for full writes of small
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files) then the file data gets written out in a single multi-block
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extent. A second related trick that ext4 uses is delayed allocation.
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Under this scheme, when a file needs more blocks to absorb file writes,
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the filesystem defers deciding the exact placement on the disk until all
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the dirty buffers are being written out to disk. By not committing to a
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particular placement until it's absolutely necessary (the commit timeout
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is hit, or sync() is called, or the kernel runs out of memory), the hope
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is that the filesystem can make better location decisions.
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The third trick that ext4 (and ext3) uses is that it tries to keep a
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file's data blocks in the same block group as its inode. This cuts down
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on the seek penalty when the filesystem first has to read a file's inode
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to learn where the file's data blocks live and then seek over to the
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file's data blocks to begin I/O operations.
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The fourth trick is that all the inodes in a directory are placed in the
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same block group as the directory, when feasible. The working assumption
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here is that all the files in a directory might be related, therefore it
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is useful to try to keep them all together.
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The fifth trick is that the disk volume is cut up into 128MB block
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groups; these mini-containers are used as outlined above to try to
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maintain data locality. However, there is a deliberate quirk -- when a
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directory is created in the root directory, the inode allocator scans
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the block groups and puts that directory into the least heavily loaded
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block group that it can find. This encourages directories to spread out
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over a disk; as the top-level directory/file blobs fill up one block
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group, the allocators simply move on to the next block group. Allegedly
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this scheme evens out the loading on the block groups, though the author
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suspects that the directories which are so unlucky as to land towards
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the end of a spinning drive get a raw deal performance-wise.
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Of course if all of these mechanisms fail, one can always use e4defrag
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to defragment files.
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