1.. SPDX-License-Identifier: GPL-2.0 2 3Bigalloc 4-------- 5 6At the moment, the default size of a block is 4KiB, which is a commonly 7supported page size on most MMU-capable hardware. This is fortunate, as 8ext4 code is not prepared to handle the case where the block size 9exceeds the page size. However, for a filesystem of mostly huge files, 10it is desirable to be able to allocate disk blocks in units of multiple 11blocks to reduce both fragmentation and metadata overhead. The 12`bigalloc <Bigalloc>`__ feature provides exactly this ability. The 13administrator can set a block cluster size at mkfs time (which is stored 14in the s\_log\_cluster\_size field in the superblock); from then on, the 15block bitmaps track clusters, not individual blocks. This means that 16block groups can be several gigabytes in size (instead of just 128MiB); 17however, the minimum allocation unit becomes a cluster, not a block, 18even for directories. TaoBao had a patchset to extend the “use units of 19clusters instead of blocks” to the extent tree, though it is not clear 20where those patches went-- they eventually morphed into “extent tree v2” 21but that code has not landed as of May 2015. 22 23