filesystems/ext4/allocators.rst

1 .. SPDX-License-Identifier: GPL-2.0
3 Block and Inode Allocation Policy
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9 that the head actuator and disk must perform to access a data block,
13 effect of concentrating writes on a single erase block, which can speed
17 The first tool that ext4 uses to combat fragmentation is the multi-block
18 allocator. When a file is first created, the block allocator
23 files) then the file data gets written out in a single multi-block
33 file's data blocks in the same block group as its inode. This cuts down
39 same block group as the directory, when feasible. The working assumption
43 The fifth trick is that the disk volume is cut up into 128MB block
44 groups; these mini-containers are used as outlined above to try to
45 maintain data locality. However, there is a deliberate quirk -- when a
47 the block groups and puts that directory into the least heavily loaded
48 block group that it can find. This encourages directories to spread out
49 over a disk; as the top-level directory/file blobs fill up one block
50 group, the allocators simply move on to the next block group. Allegedly
51 this scheme evens out the loading on the block groups, though the author
53 the end of a spinning drive get a raw deal performance-wise.