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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | Bigalloc | |
4 | -------- | |
5 | ||
6 | At the moment, the default size of a block is 4KiB, which is a commonly | |
7 | supported page size on most MMU-capable hardware. This is fortunate, as | |
8 | ext4 code is not prepared to handle the case where the block size | |
9 | exceeds the page size. However, for a filesystem of mostly huge files, | |
10 | it is desirable to be able to allocate disk blocks in units of multiple | |
11 | blocks to reduce both fragmentation and metadata overhead. The | |
e8552640 AR |
12 | bigalloc feature provides exactly this ability. |
13 | ||
14 | The bigalloc feature (EXT4_FEATURE_RO_COMPAT_BIGALLOC) changes ext4 to | |
15 | use clustered allocation, so that each bit in the ext4 block allocation | |
16 | bitmap addresses a power of two number of blocks. For example, if the | |
17 | file system is mainly going to be storing large files in the 4-32 | |
18 | megabyte range, it might make sense to set a cluster size of 1 megabyte. | |
19 | This means that each bit in the block allocation bitmap now addresses | |
20 | 256 4k blocks. This shrinks the total size of the block allocation | |
21 | bitmaps for a 2T file system from 64 megabytes to 256 kilobytes. It also | |
22 | means that a block group addresses 32 gigabytes instead of 128 megabytes, | |
23 | also shrinking the amount of file system overhead for metadata. | |
24 | ||
25 | The administrator can set a block cluster size at mkfs time (which is | |
26 | stored in the s\_log\_cluster\_size field in the superblock); from then | |
27 | on, the block bitmaps track clusters, not individual blocks. This means | |
28 | that block groups can be several gigabytes in size (instead of just | |
29 | 128MiB); however, the minimum allocation unit becomes a cluster, not a | |
30 | block, even for directories. TaoBao had a patchset to extend the “use | |
31 | units of clusters instead of blocks” to the extent tree, though it is | |
32 | not clear where those patches went-- they eventually morphed into | |
33 | “extent tree v2” but that code has not landed as of May 2015. | |
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