Documentation/filesystems/ext4/allocators.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 Block and Inode Allocation Policy
   4 ---------------------------------
   5
   6 ext4 recognizes (better than ext3, anyway) that data locality is
   7 generally a desirably quality of a filesystem. On a spinning disk,
   8 keeping related blocks near each other reduces the amount of movement
   9 that the head actuator and disk must perform to access a data block,
  10 thus speeding up disk IO. On an SSD there of course are no moving parts,
  11 but locality can increase the size of each transfer request while
  12 reducing the total number of requests. This locality may also have the
  13 effect of concentrating writes on a single erase block, which can speed
  14 up file rewrites significantly. Therefore, it is useful to reduce
  15 fragmentation whenever possible.
  16
  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
  19 speculatively allocates 8KiB of disk space to the file on the assumption
  20 that the space will get written soon. When the file is closed, the
  21 unused speculative allocations are of course freed, but if the
  22 speculation is correct (typically the case for full writes of small
  23 files) then the file data gets written out in a single multi-block
  24 extent. A second related trick that ext4 uses is delayed allocation.
  25 Under this scheme, when a file needs more blocks to absorb file writes,
  26 the filesystem defers deciding the exact placement on the disk until all
  27 the dirty buffers are being written out to disk. By not committing to a
  28 particular placement until it's absolutely necessary (the commit timeout
  29 is hit, or sync() is called, or the kernel runs out of memory), the hope
  30 is that the filesystem can make better location decisions.
  31
  32 The third trick that ext4 (and ext3) uses is that it tries to keep a
  33 file's data blocks in the same block group as its inode. This cuts down
  34 on the seek penalty when the filesystem first has to read a file's inode
  35 to learn where the file's data blocks live and then seek over to the
  36 file's data blocks to begin I/O operations.
  37
  38 The fourth trick is that all the inodes in a directory are placed in the
  39 same block group as the directory, when feasible. The working assumption
  40 here is that all the files in a directory might be related, therefore it
  41 is useful to try to keep them all together.
  42
  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
  46 directory is created in the root directory, the inode allocator scans
  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
  52 suspects that the directories which are so unlucky as to land towards
  53 the end of a spinning drive get a raw deal performance-wise.
  54
  55 Of course if all of these mechanisms fail, one can always use e4defrag
  56 to defragment files.