From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Daniel P. Berrange" Subject: Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions Date: Mon, 14 Nov 2011 11:29:18 +0000 Message-ID: <20111114112918.GC32392@redhat.com> References: <4EBAB9FA.3070601@codemonkey.ws> <4EBB919B.7040605@redhat.com> <4EBC1792.3030004@codemonkey.ws> <4EBC4260.1090405@codemonkey.ws> <4EBCF5DA.1000605@redhat.com> <4EBE499E.4030100@redhat.com> <20111114101610.GA32392@redhat.com> <20111114102421.GE16454@redhat.com> <20111114110802.GB32392@redhat.com> <4EC0F9D1.3060505@redhat.com> Reply-To: "Daniel P. Berrange" Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: "Michael S. Tsirkin" , Avi Kivity , Anthony Liguori , Lucas Meneghel Rodrigues , KVM mailing list , Marcelo Tosatti , QEMU devel , Juan Jose Quintela Carreira , "libvir-list@redhat.com" To: Kevin Wolf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:14841 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753426Ab1KNL3c (ORCPT ); Mon, 14 Nov 2011 06:29:32 -0500 Content-Disposition: inline In-Reply-To: <4EC0F9D1.3060505@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Nov 14, 2011 at 12:21:53PM +0100, Kevin Wolf wrote: > Am 14.11.2011 12:08, schrieb Daniel P. Berrange: > > On Mon, Nov 14, 2011 at 12:24:22PM +0200, Michael S. Tsirkin wrote: > >> On Mon, Nov 14, 2011 at 10:16:10AM +0000, Daniel P. Berrange wrote: > >>> On Sat, Nov 12, 2011 at 12:25:34PM +0200, Avi Kivity wrote: > >>>> On 11/11/2011 12:15 PM, Kevin Wolf wrote: > >>>>> Am 10.11.2011 22:30, schrieb Anthony Liguori: > >>>>>> Live migration with qcow2 or any other image format is just not going to work > >>>>>> right now even with proper clustered storage. I think doing a block level flush > >>>>>> cache interface and letting block devices decide how to do it is the best approach. > >>>>> > >>>>> I would really prefer reusing the existing open/close code. It means > >>>>> less (duplicated) code, is existing code that is well tested and doesn't > >>>>> make migration much of a special case. > >>>>> > >>>>> If you want to avoid reopening the file on the OS level, we can reopen > >>>>> only the topmost layer (i.e. the format, but not the protocol) for now > >>>>> and in 1.1 we can use bdrv_reopen(). > >>>>> > >>>> > >>>> Intuitively I dislike _reopen style interfaces. If the second open > >>>> yields different results from the first, does it invalidate any > >>>> computations in between? > >>>> > >>>> What's wrong with just delaying the open? > >>> > >>> If you delay the 'open' until the mgmt app issues 'cont', then you loose > >>> the ability to rollback to the source host upon open failure for most > >>> deployed versions of libvirt. We only fairly recently switched to a five > >>> stage migration handshake to cope with rollback when 'cont' fails. > >>> > >>> Daniel > >> > >> I guess reopen can fail as well, so this seems to me to be an important > >> fix but not a blocker. > > > > If if the initial open succeeds, then it is far more likely that a later > > re-open will succeed too, because you have already elminated the possibility > > of configuration mistakes, and will have caught most storage runtime errors > > too. So there is a very significant difference in reliability between doing > > an 'open at startup + reopen at cont' vs just 'open at cont' > > > > Based on the bug reports I see, we want to be very good at detecting and > > gracefully handling open errors because they are pretty frequent. > > Do you have some more details on the kind of errors? Missing files, > permissions, something like this? Or rather something related to the > actual content of an image file? Missing files due to wrong/missing NFS mounts, or incorrect SAN / iSCSI setup. Access permissions due to incorrect user / group setup, or read only mounts, or SELinux denials. Actual I/O errors are less common and are not so likely to cause QEMU to fail to start any, since QEMU is likely to just report them to the guest OS instead. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|