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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 <Bigalloc>`__ feature provides exactly this ability. 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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