From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NoZYo-0004nj-Mz for qemu-devel@nongnu.org; Mon, 08 Mar 2010 04:47:38 -0500 Received: from [199.232.76.173] (port=50962 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NoZYo-0004nD-7w for qemu-devel@nongnu.org; Mon, 08 Mar 2010 04:47:38 -0500 Received: from Debian-exim by monty-python.gnu.org with spam-scanned (Exim 4.60) (envelope-from ) id 1NoZYl-0004XJ-RZ for qemu-devel@nongnu.org; Mon, 08 Mar 2010 04:47:38 -0500 Received: from mail2.shareable.org ([80.68.89.115]:44753) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1NoZYl-0004XB-HT for qemu-devel@nongnu.org; Mon, 08 Mar 2010 04:47:35 -0500 Date: Mon, 8 Mar 2010 09:47:32 +0000 From: Jamie Lokier Subject: Re: [Qemu-devel] bs->enable_write_cache and the guest ABI Message-ID: <20100308094732.GB2869@shareable.org> References: <4B93BB4A.8090605@redhat.com> <20100308093908.GA1914@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100308093908.GA1914@lst.de> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Christoph Hellwig Cc: Avi Kivity , qemu-devel Christoph Hellwig wrote: > On Sun, Mar 07, 2010 at 04:42:18PM +0200, Avi Kivity wrote: > > This means that if I start a guest with cache=writethrough and then > > restart (or live migrate) it with cache=none, then the guest will see a > > change, even though the user only changed the drive's backing, not > > something guest visible. In the case of live migration, the guest will > > not even notice the change and we may be at risk of data loss. > > > > For 0.13 I propose setting enable_write_cache to true unconditionally. > > For 0.12 the question is more difficult, since we'll be changing the > > guest ABI. Given that guests are unlikely not to be able to cope with > > write caches, and that the alternative is data loss, I believe that's > > also the right solution there. > > Setting it to true unconditionally will cause performance degradation > for cache=writethrough devices, as we now have to drain the queue in > the guest for no reason at all. > > I think the better option would be to move the cache setting to qdev > property on the block device at it's a device visible setting. Assuming the outcome is that it becomes a qdev property, and stays preserved across migrations, even if the backing device access changes, then I think the right thing is to dynamically decide to set O_DSYNC and/or call fdatasync before completing writes from qemu when the guest thinks enable_write_cache=0 (or sets it to 0). With cache=none, that would set O_DSYNC|O_DIRECT if the two flags do work properly together on our favourite hosts. Thus enable_write_cache won't always have the default value for the different backing device access type, but it will match the guest's expectations and be actually safe. Moreover more, by responding to the guest changing that, it's closer to behaving like real harware. -- Jamie