| Andrew Scull | b4b6d4a | 2019-01-02 15:54:55 +0000 | [diff] [blame] | 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 |
| 12 | `bigalloc <Bigalloc>`__ feature provides exactly this ability. The |
| 13 | administrator can set a block cluster size at mkfs time (which is stored |
| 14 | in the s\_log\_cluster\_size field in the superblock); from then on, the |
| 15 | block bitmaps track clusters, not individual blocks. This means that |
| 16 | block groups can be several gigabytes in size (instead of just 128MiB); |
| 17 | however, the minimum allocation unit becomes a cluster, not a block, |
| 18 | even for directories. TaoBao had a patchset to extend the “use units of |
| 19 | clusters instead of blocks” to the extent tree, though it is not clear |
| 20 | where those patches went-- they eventually morphed into “extent tree v2” |
| 21 | but that code has not landed as of May 2015. |
| 22 | |