From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1O9NR6-0006ol-Fw
	for qemu-devel@nongnu.org; Tue, 04 May 2010 15:05:40 -0400
Received: from [140.186.70.92] (port=51236 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1O9NR5-0006mo-1x
	for qemu-devel@nongnu.org; Tue, 04 May 2010 15:05:39 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <mst@redhat.com>) id 1O9NR3-00014k-IU
	for qemu-devel@nongnu.org; Tue, 04 May 2010 15:05:38 -0400
Received: from mx1.redhat.com ([209.132.183.28]:42564)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <mst@redhat.com>) id 1O9NR3-00014Z-BK
	for qemu-devel@nongnu.org; Tue, 04 May 2010 15:05:37 -0400
Date: Tue, 4 May 2010 22:01:24 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH
Message-ID: <20100504190124.GA29764@redhat.com>
References: <20100218222220.GA14847@redhat.com>
	<20100420014635.GE21899@shareable.org>
	<20100504185614.GB24998@lst.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20100504185614.GB24998@lst.de>
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Christoph Hellwig <hch@lst.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org

On Tue, May 04, 2010 at 08:56:14PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 20, 2010 at 02:46:35AM +0100, Jamie Lokier 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.
> 
> No.  The Linux virtio_blk guest driver either supports data integrity
> by using FLUSH or can send down BARRIER requests which aren't much
> help at all.

It seems we use BARRIER when we get REQ_HARDBARRIER, right?
What does the REQ_HARDBARRIER flag in request mean and when is it set?

>  Qemu only implements FLUSH anyway.