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- .. 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.
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