force-unit-access · shipping · also FUA, REQ_FUA

Force Unit Access

Force Unit Access is a write-request property requiring the flagged data to reach nonvolatile storage before that request is reported complete.

Linux represents this requirement with REQ_FUA. It is distinct from REQ_PREFLUSH, which makes earlier completed writes persistent before the accompanying I/O begins; FUA alone is not that ordering guarantee. The flags can be combined. For devices without native FUA, Linux can implement the requirement with a post-write flush. The name describes completion semantics, not a universal physical bypass of every cache or a guarantee that a multi-block write is atomic.

Relations

related

cache-flush, write-back-cache, write-through-cache, persistence-ordering, power-loss-protection

Advantages

• Allows persistence to be required for a specific write's completion. [1]

Limitations

• Does not replace the preflush guarantee for previously completed writes. [1]

Citations

1. Linux kernel documentation. Explicit volatile write back cache control: REQ_PREFLUSH and REQ_FUA. (accessed 2026-10-12)
2. NFS stable writes: what COMMIT, flush and FUA actually promise (accessed 2026-10-12)

JSON: /data/terms/force-unit-access.json