From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1KoCq5-0007Zv-Ah
	for qemu-devel@nongnu.org; Fri, 10 Oct 2008 03:55:09 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1KoCq2-0007Zj-UI
	for qemu-devel@nongnu.org; Fri, 10 Oct 2008 03:55:08 -0400
Received: from [199.232.76.173] (port=43363 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1KoCq2-0007Zg-P9
	for qemu-devel@nongnu.org; Fri, 10 Oct 2008 03:55:06 -0400
Received: from mx20.gnu.org ([199.232.41.8]:17004)
	by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32)
	(Exim 4.60) (envelope-from <kraxel@redhat.com>) id 1KoCq3-0000ul-55
	for qemu-devel@nongnu.org; Fri, 10 Oct 2008 03:55:07 -0400
Received: from mx2.redhat.com ([66.187.237.31])
	by mx20.gnu.org with esmtp (Exim 4.60)
	(envelope-from <kraxel@redhat.com>) id 1KoCq0-0001du-SD
	for qemu-devel@nongnu.org; Fri, 10 Oct 2008 03:55:05 -0400
Message-ID: <48EF0A26.90209@redhat.com>
Date: Fri, 10 Oct 2008 09:54:14 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
References: <48EE38B9.2050106@codemonkey.ws>
In-Reply-To: <48EE38B9.2050106@codemonkey.ws>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Reply-To: qemu-devel@nongnu.org
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/pipermail/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: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, Ryan Harper <ryanh@us.ibm.com>, Laurent Vivier <Laurent.Vivier@bull.net>, kvm-devel <kvm-devel@lists.sourceforge.net>

  Hi,

> Read performance should be unaffected by using O_DSYNC.  O_DIRECT will
> significantly reduce read performance.  I think we should use O_DSYNC by
> default and I have sent out a patch that contains that.  We will follow
> up with benchmarks to demonstrate this.

So O_SYNC on/off is pretty much equivalent to disk write caching being
on/off, right?  So we could make that guest-controlled, i.e. toggeling
write caching in the guest (using hdparm) toggles O_SYNC in qemu?  This
together with disk-flush command support (mapping to fsync on the host)
should allow guests to go into barrier mode for better write performance
without loosing data integrity.

cheers,
  Gerd