Re: [Qemu-devel] [Nbd] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?

[Kevin Wolf]

Am 06.04.2016 um 15:28 hat Paolo Bonzini geschrieben:
> 
> 
> On 06/04/2016 15:14, Kevin Wolf wrote:
> >>> > > 
> >>> > > I think it does make sense. It means that on completion there is no
> >>> > > pending discard operation (i.e. either there wasn't a discard or if
> >>> > > there was, it has completed) and other readers will see the final 
> >>> > > state
> >>> > > of the blocks.
> >> > 
> >> > This is what already happens though, isn't it?
> > You mean because in practice discard requests aren't even cached, so we
> > always behave as if FUA were specified? That's probably right, but is
> > there a fundamental reason why some storage backend couldn't have a
> > writeback cache for discards?
> 
> No, there isn't.  Does qcow2's discard get cached?  I wouldn't be
> surprised (and SCSI actually says nowhere that WRITE SAME is durable
> without a subsequent SYNCHRONIZE CACHE!).

For qcow2, yes and no. It has a cache for coalescing discards within a
single request, but that cache is flushed before completing the
operation. So it's not visible for the caller.

> > It probably wouldn't make sense to introduce FUA for this if it didn't
> > already exist elsewhere, but now that we do have it, I'd allow it for
> > TRIM, too, for the sake of consistency and symmetry.
> 
> Yes, that's fine.

Kevin