1 .. SPDX-License-Identifier: GPL-2.0
2
3 Bigalloc
4 --------
5
6 At the moment, the default size of a block is
7 supported page size on most MMU-capable hardwa
8 ext4 code is not prepared to handle the case w
9 exceeds the page size. However, for a filesyst
10 it is desirable to be able to allocate disk bl
11 blocks to reduce both fragmentation and metada
12 bigalloc feature provides exactly this ability
13
14 The bigalloc feature (EXT4_FEATURE_RO_COMPAT_B
15 use clustered allocation, so that each bit in
16 bitmap addresses a power of two number of bloc
17 file system is mainly going to be storing larg
18 megabyte range, it might make sense to set a c
19 This means that each bit in the block allocati
20 256 4k blocks. This shrinks the total size of
21 bitmaps for a 2T file system from 64 megabytes
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
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