ext4-bigalloc · shipping · also bigalloc, ext4 clustered allocation

ext4 bigalloc

Ext4 bigalloc allocates space in power-of-two clusters of filesystem blocks, with each allocation-bitmap bit tracking a cluster.

Related Observatory article: ext4 bigalloc on hesela.com

Cluster size and filesystem block size are different parameters. A larger cluster reduces the bitmap bits needed to describe a given address space, while making allocation coarser. This is not a change to the logical sector size reported by a drive. [1]

Mke2fs selects block size with -b and cluster size with -C; -C requires bigalloc, and bigalloc requires extents. Record both the explicit options and the resulting feature set, rather than assuming defaults are unchanged across systems. [2] [3]

Hesela's offline e2fsprogs 1.47.0 controls used 4 KiB blocks in every image. A dense 65,537-byte file had 68 KiB of inode-reported allocation without bigalloc, versus 128 KiB with 64 KiB clusters. This is a userspace image observation, not a kernel allocation or performance benchmark. [4]

Relations

related

Allocation bitmap, Free-space fragmentation, File preallocation, Filesystem aging

Limitations

• Coarser allocation can increase slack for small, non-inline files; bitmap savings are not total-filesystem savings. [1]
• Successful image creation does not validate every kernel, repair, resize or defragmentation workflow. [3]

Citations

1. Linux kernel documentation: ext4 bigalloc and clustered allocation (accessed 2026-10-22)
2. e2fsprogs 1.47.0 mke2fs(8): block size, cluster size and directory import (accessed 2026-10-22)
3. e2fsprogs 1.47.0 ext4(5): bigalloc requires extents (accessed 2026-10-22)
4. Hesela: ext4 bigalloc offline image controls, observations and boundaries (accessed 2026-10-22)

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