Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 14, 2011 at 11:37:27AM +0000, Daniel P. Berrange wrote:
> On Mon, Nov 14, 2011 at 01:34:15PM +0200, Michael S. Tsirkin wrote:
> > On Mon, Nov 14, 2011 at 11:29:18AM +0000, Daniel P. Berrange wrote:
> > > 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.
> > 
> > Do you run qemu with -S, then give a 'cont' command to start it?
> 
> Yes
> 
> Daniel

Probably in an attempt to improve reliability :)

So this is in fact unrelated to migration.  So we can either ignore this
bug (assuming no distros ship cutting edge qemu with an old libvirt), or
special-case -S and do an open/close cycle on startup.


> -- 
> |: 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 :|