| .. 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. |
| |
