From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=33562 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Ov96R-0002qV-Oz for qemu-devel@nongnu.org; Mon, 13 Sep 2010 09:29:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1Ov96Q-0002uN-MF for qemu-devel@nongnu.org; Mon, 13 Sep 2010 09:29:47 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:52953) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1Ov96Q-0002uB-C9 for qemu-devel@nongnu.org; Mon, 13 Sep 2010 09:29:46 -0400 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e33.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id o8DDOh3P026867 for ; Mon, 13 Sep 2010 07:24:43 -0600 Received: from d03av02.boulder.ibm.com (d03av02.boulder.ibm.com [9.17.195.168]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id o8DDTjET212598 for ; Mon, 13 Sep 2010 07:29:45 -0600 Received: from d03av02.boulder.ibm.com (loopback [127.0.0.1]) by d03av02.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id o8DDTjjD003470 for ; Mon, 13 Sep 2010 07:29:45 -0600 Message-ID: <4C8E2747.9090806@linux.vnet.ibm.com> Date: Mon, 13 Sep 2010 08:29:43 -0500 From: Anthony Liguori MIME-Version: 1.0 References: <1284213896-12705-1-git-send-email-aliguori@us.ibm.com> <1284213896-12705-4-git-send-email-aliguori@us.ibm.com> <4C8DE19B.9090309@redhat.com> In-Reply-To: <4C8DE19B.9090309@redhat.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Anthony Liguori , Juan Quintela , qemu-devel@nongnu.org, Stefan Hajnoczi On 09/13/2010 03:32 AM, Kevin Wolf wrote: > Am 11.09.2010 16:04, schrieb Anthony Liguori: > >> This fixes a couple nasty problems relating to live migration. >> >> 1) When dealing with shared storage with weak coherence (i.e. NFS), even if >> we re-read, we may end up with undesired caching. By delaying any reads >> until we absolutely have to, we decrease the likelihood of any undesirable >> caching. >> >> 2) When dealing with copy-on-read, the local storage acts as a cache. We need >> to make sure to avoid any reads to avoid polluting the local cache. >> >> Signed-off-by: Anthony Liguori >> > I think we should also delay even opening the image file at all to the > latest possible point to avoid that new problems of this kind are > introduced. Ideally, opening the image would be the very last thing we > do before telling the migration source that we're set and it should > close the images. > There's a lot of possible failure scenarios that opening an image file can introduce. Fortunately, I don't think we have a strict requirement for it provided that we make a couple of reasonable changes. > Even better would be to only open the image when the source has already > closed it (we could completely avoid the invalidation/reopen then), but > I think you were afraid that we might lose the image on both ends. > Yeah, one of the key design points of live migration is to minimize the number of failure scenarios where you lose a VM. If someone typed the wrong command line or shared storage hasn't been mounted yet and we delay failure until live migration is in the critical path, that would be terribly unfortunate. Regards, Anthony Liguori > Kevin >