alt Hacker NewsIt 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.
Replies
IOCTLs can tell you if write caching is enabled or not. Can they reliably tell you whether the write cache is volatile, though? Many drives with PLPs still report volatile write caches, or at least did when I was testing this a few years back.
What SSDs are reasonably performant without a volatile write cache? The standards you quote specify why it is necessary to issue flush!
If you know your application will only ever run against enterprise SSDs with power loss protection, then sending flush commands to the drive itself would indeed be pointless no-ops. But it if it's a flush command that has effects somewhere between the application layer and the NVMe drive (eg. if you're not using direct IO) or if there's any possibility of the code being run on a consumer SSD (eg. a developer's laptop) then the flush commands are probably worth including; the performance hit on enterprise drives will be very small.