allocation-bitmap · shipping · also space allocation bitmap

Allocation bitmap

An allocation bitmap encodes the allocation state of storage units as bits.

Related Observatory article: Allocation bitmap on hesela.com

The unit depends on the format: ext4 has inode and block-allocation bitmaps, but bigalloc makes each block-bitmap bit represent a cluster of multiple blocks. ExFAT uses cluster allocation bits. Ext4's uninitialized-group handling also means a raw bitmap need not be interpreted without its metadata context. [1] [5] [4]

Linux v6.12's exFAT implementation searches bitmap bits for free clusters and counts set bits for used-cluster accounting. Neither operation traverses a file's ordered chain. When freeing a cluster, clearing its bit and automatically issuing discard are separate steps; this discard depends on the mount option. The separate trim operation can also issue discard. [2]

Relations

related

exFAT, FAT cluster chain, Free-space fragmentation, File preallocation, TRIM

Limitations

• A space map alone cannot identify which file owns a unit or reconstruct the order of its contents. [2, 3]

Citations

1. Linux kernel documentation: ext4 block and inode bitmaps (accessed 2026-10-20)
2. Linux v6.12: exFAT allocation bitmap operations, fs/exfat/balloc.c (accessed 2026-10-20)
3. Linux v6.12: exFAT chain traversal and allocation, fs/exfat/fatent.c (accessed 2026-10-20)
4. Microsoft exFAT specification: sections 3.1.10, 6.3.4.2, 7.1 and 7.6 (accessed 2026-10-20)
5. Linux kernel documentation: ext4 bigalloc and clustered allocation (accessed 2026-10-20)

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