.. 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.