From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paul Brook <paul@codesourcery.com>
Cc: kvm@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>,
qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org,
hch@lst.de
Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH
Date: Wed, 21 Apr 2010 13:39:59 +0300 [thread overview]
Message-ID: <20100421103959.GA31461@redhat.com> (raw)
In-Reply-To: <201004201422.58456.paul@codesourcery.com>
On Tue, Apr 20, 2010 at 02:22:58PM +0100, Paul Brook wrote:
> > Does this mean that virtio-blk supports all three combinations?
> >
> > 1. FLUSH that isn't a barrier
> > 2. FLUSH that is also a barrier
> > 3. Barrier that is not a flush
> >
> > 1 is good for fsync-like operations;
> > 2 is good for journalling-like ordered operations.
> > 3 sounds like it doesn't mean a lot as the host cache provides no
> > guarantees and has no ordering facility that can be used.
>
> (3) allows the guest to queue overlapping transfers with well defined results.
> I have no idea how useful this is in practice, but it's certainly plausible.
>
> Paul
In theory, yes.
At the moment, qemu only implements FLUSH and lguest only
implements barrier without FLUSH.
If you think it's useful, maybe start by using FLUSH+barrier
in linux guest driver, that'd demonstrate how it's used.
--
MST
next prev parent reply other threads:[~2010-04-21 10:44 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-02-18 22:22 [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH Michael S. Tsirkin
2010-04-19 21:26 ` [Qemu-devel] " Michael S. Tsirkin
2010-04-28 15:52 ` Michael S. Tsirkin
2010-04-20 1:46 ` [Qemu-devel] " Jamie Lokier
2010-04-20 13:22 ` Paul Brook
2010-04-21 10:39 ` Michael S. Tsirkin [this message]
2010-05-04 18:56 ` Christoph Hellwig
2010-05-04 19:01 ` Michael S. Tsirkin
2010-05-04 4:38 ` [Qemu-devel] " Rusty Russell
2010-05-04 6:56 ` Stefan Hajnoczi
2010-05-04 8:34 ` Avi Kivity
2010-05-04 8:41 ` Jens Axboe
2010-05-04 20:17 ` Jamie Lokier
2010-05-05 4:58 ` Rusty Russell
2010-05-05 6:03 ` Neil Brown
2010-05-06 6:05 ` Rusty Russell
2010-05-06 14:57 ` Jamie Lokier
2010-05-06 15:25 ` Jamie Lokier
2010-05-04 10:05 ` Christoph Hellwig
2010-05-04 20:32 ` Jamie Lokier
2010-05-04 18:54 ` Christoph Hellwig
2010-05-04 18:56 ` Michael S. Tsirkin
2010-05-04 18:58 ` Michael S. Tsirkin
2010-05-05 5:00 ` Rusty Russell
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