From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47874 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OvFFG-0003lM-37 for qemu-devel@nongnu.org; Mon, 13 Sep 2010 16:03:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OvFFE-0004g8-M2 for qemu-devel@nongnu.org; Mon, 13 Sep 2010 16:03:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61677) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OvFFE-0004g3-D2 for qemu-devel@nongnu.org; Mon, 13 Sep 2010 16:03:16 -0400 Message-ID: <4C8E8390.6050607@redhat.com> Date: Mon, 13 Sep 2010 22:03:28 +0200 From: Kevin Wolf MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts 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> <4C8E2747.9090806@linux.vnet.ibm.com> <4C8E2981.7000304@redhat.com> <4C8E2A52.1000708@linux.vnet.ibm.com> <4C8E319A.4090103@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Anthony Liguori , Anthony Liguori , qemu-devel@nongnu.org, Stefan Hajnoczi , Juan Quintela Am 13.09.2010 21:29, schrieb Stefan Hajnoczi: > On Mon, Sep 13, 2010 at 3:13 PM, Kevin Wolf wrote: >> Am 13.09.2010 15:42, schrieb Anthony Liguori: >>> On 09/13/2010 08:39 AM, Kevin Wolf wrote: >>>>> 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. >>>>> >>>> We would catch most of them if we try to open the image when migration >>>> starts and immediately close it again until migration is (almost) >>>> completed, so that no other code can possibly use it before the source >>>> has really closed it. >>>> >>> >>> I think the only real advantage is that we fix NFS migration, right? >> >> That's the one that we know about, yes. >> >> The rest is not a specific scenario, but a strong feeling that having an >> image opened twice at the same time feels dangerous. As soon as an >> open/close sequence writes to the image for some format, we probably >> have a bug. For example, what about this mounted flag that you were >> discussing for QED? > > There is some room left to work in, even if we can't check in open(). > One idea would be to do the check asynchronously once I/O begins. It > is actually easy to check L1/L2 tables as they are loaded. > > The only barrier relationship between I/O and checking is that an > allocating write (which will need to update L1/L2 tables) is only > allowed after check completes. Otherwise reads and non-allocating > writes may proceed while the image is not yet fully checked. We can > detect when a table element is an invalid offset and discard it. I'm not even talking about such complicated things. You wanted to have a dirty flag in the header, right? So when we allow opening an image twice, you get this sequence with migration: Source: open Destination: open (with dirty image) Source: close The image is now marked as clean, even though the destination is still working on it. Kevin