SSDs, power loss protection and fsync latency

It seems pointless to issue flush commands when writing to an NVMe drive with a direct IO implementation that functions properly. The NVMe spec says:

> 6.8 Flush command

> …

> If a volatile write cache is not present or not enabled, then Flush commands shall complete successfully and have no effect.

And:

> 5.21.1.6 Volatile Write Cache

> …

> Note: If the controller is able to guarantee that data present in a write cache is written to non-volatile media on loss of power, then that write cache is considered non-volatile and this feature does not apply to that write cache.

Did you check that the drives actually honor the flush? Half of drives tested lose FLUSH'd data on power loss.

Very appropriate topic, after yesterday's High-Performance DBMSs with io_uring: When and How to use it. https://news.ycombinator.com/item?id=46517319 https://arxiv.org/abs/2512.04859

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