Re: [Nbd] [Qemu-devel] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?
- To: Paolo Bonzini <pbonzini@...696...>
- Cc: "nbd-general@lists.sourceforge.net" <nbd-general@lists.sourceforge.net>, "qemu-devel@...530..." <qemu-devel@...530...>
- Subject: Re: [Nbd] [Qemu-devel] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?
- From: Kevin Wolf <kwolf@...696...>
- Date: Wed, 6 Apr 2016 15:50:31 +0200
- Message-id: <20160406135031.GJ5098@...2331...>
- In-reply-to: <57050F1A.9070604@...696...>
- References: <56FD7B7E.4060004@...696...> <64B326DA-CDF4-4537-B38A-46E7B57C319C@...872...> <56FD8069.7020101@...696...> <FC6D7B08-B1C0-4ED7-ABD0-FE409C43782C@...872...> <56FE26B2.6090903@...696...> <20160405050926.GC4183@...2331...> <5703BD8F.1080409@...696...> <20160406131448.GH5098@...2331...> <57050F1A.9070604@...696...>
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
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