XFS: the filesystem of the future?

Any device with a volatile write cache tells the OS that the write IO has been completed before it actually is written to stable storage. IO completion is supposed to mean "the IO is complete" and any device with a volatile write cache is actually lying - the write is not yet on stable storage, so is "lying" about the completion status of the IO to the OS. Pretty much all consumer devices ship with a volatile write cache enabled by default for performance reasons.

Barriers and cache flushes were introduced to provide a mechanism that allowed filesystems to force such drives to order writes the way the filesystem wants correctly. The original barrier mechanism was "cache flush, write, cache flush" and could make the drive slower than not caching in the first place depending on the workload. More recently we just use the FUA mechanism if the drive supports that, and that has neglible performance overhead.

> As far as I can tell, some consumer drives do lie about when writes
> have been flushed, and write back caching is the default anyway

If drives lie about cache flush or FUA completion on volatile writeback caches, then that's a bug in the disk firmware.

FWIW, the difference with server storage (SAS drives) is that most ship with the volatile write cache turned off by default. They don't need it for performance because the SCSI/SAS protocol is much more efficient than SATA and so in most cases a write cache isn't necessary. You can turn it on, but you don't need to to reach full disk performance....

Indeed, it's not just filesystems that don't like volatile write caches. if you turn on volatile write caching on disks behind a RAID controller, the disk will now violate the write ordering guarantees that the RAID controller relies on to maintain data safety (exactly the same as for filesystems). You still lose data or corrupt filesystems on power loss in this case, even though the OS and RAID controller are behaving correctly.

Dave.