1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 Bigalloc 3 Bigalloc
4 -------- 4 --------
5 5
6 At the moment, the default size of a block is 6 At the moment, the default size of a block is 4KiB, which is a commonly
7 supported page size on most MMU-capable hardwa 7 supported page size on most MMU-capable hardware. This is fortunate, as
8 ext4 code is not prepared to handle the case w 8 ext4 code is not prepared to handle the case where the block size
9 exceeds the page size. However, for a filesyst 9 exceeds the page size. However, for a filesystem of mostly huge files,
10 it is desirable to be able to allocate disk bl 10 it is desirable to be able to allocate disk blocks in units of multiple
11 blocks to reduce both fragmentation and metada 11 blocks to reduce both fragmentation and metadata overhead. The
12 bigalloc feature provides exactly this ability !! 12 `bigalloc <Bigalloc>`__ feature provides exactly this ability. The
13 !! 13 administrator can set a block cluster size at mkfs time (which is stored
14 The bigalloc feature (EXT4_FEATURE_RO_COMPAT_B !! 14 in the s\_log\_cluster\_size field in the superblock); from then on, the
15 use clustered allocation, so that each bit in !! 15 block bitmaps track clusters, not individual blocks. This means that
16 bitmap addresses a power of two number of bloc !! 16 block groups can be several gigabytes in size (instead of just 128MiB);
17 file system is mainly going to be storing larg !! 17 however, the minimum allocation unit becomes a cluster, not a block,
18 megabyte range, it might make sense to set a c !! 18 even for directories. TaoBao had a patchset to extend the “use units of
19 This means that each bit in the block allocati !! 19 clusters instead of blocks” to the extent tree, though it is not clear
20 256 4k blocks. This shrinks the total size of !! 20 where those patches went-- they eventually morphed into “extent tree v2”
21 bitmaps for a 2T file system from 64 megabytes !! 21 but that code has not landed as of May 2015.
22 means that a block group addresses 32 gigabyte <<
23 also shrinking the amount of file system overh <<
24 <<
25 The administrator can set a block cluster size <<
26 stored in the s_log_cluster_size field in the <<
27 on, the block bitmaps track clusters, not indi <<
28 that block groups can be several gigabytes in <<
29 128MiB); however, the minimum allocation unit <<
30 block, even for directories. TaoBao had a patc <<
31 units of clusters instead of blocks” to the <<
32 not clear where those patches went-- they even <<
33 “extent tree v2” but that code has not lan <<
34 22
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