Re: [Nbd] Question about the expected behaviour of nbd-server for async ops

[Wouter Verhelst <w@...112...>]

On Sun, May 29, 2011 at 02:53:01PM +0200, Goswin von Brederlow wrote:
> > Yes. But it's more than that. If you write to a CoW based filing system,
> > fsync (and even fdatasync) will ensure the CoW metadata is also flushed
> > to the device, whereas sync_file_range won't. Without the CoW metadata
> > being written, the data itself is not really written.
> 
> Which just means that a CoW based filing system or sparse files don't
> support FUA. The idea of a FUA is that it is cheaper than a FLUSH. But
> if nbd-server does fsync() in both cases then it is pointless to
> announce FUA support.

No, that's not entirely true. With a FLUSH, you need to ensure that
whatever the FLUSH would cover is flushed to disk; with a FUA, you need
to ensure the same thing for just one call, which is easier to do.

[...]
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