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