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