Re: [Qemu-devel] question: I found a qemu crash about migration

[Dr. David Alan Gilbert]

* Matthew Schumacher (address@hidden) wrote:
> Am 28.09.2017 um 19:01 hat Dr. David Alan Gilbert geschrieben:
> > Hi,
> >   This is a 'fun' bug;  I had a good chat to kwolf about it earlier.
> > A proper fix really needs to be done together with libvirt so that we
> > can sequence:
> >    a) The stopping of the CPU on the source
> >    b) The termination of the mirroring block job
> >    c) The inactivation of the block devices on the source
> >        (bdrv_inactivate_all)
> >    d) The activation of the block devices on the destination
> >        (bdrv_invalidate_cache_all)
> >    e) The start of the CPU on the destinationOn 01/12/2018 03:21 PM,
> qemu-devel-confirm+7e23769bf079599cf1f3db6b00d347e8675d87f
> address@hidden wrote:
> >
> >
> > It looks like you're hitting a race between b/c;  we've had races
> > between c/d in the past and moved the bdrv_inactivate_all.
> >
> > During the discussion we ended up with two proposed solutions;
> > both of them require one extra command and one extra migration
> > capability.
> >
> > The block way
> > -------------
> >    1) Add a new migration capability pause-at-complete
> >    2) Add a new migration state almost-complete
> >    3) After saving devices, if pause-at-complete is set,
> >       transition to almost-complete
> >    4) Add a new command (migration-continue) that
> >       causes the migration to inactivate the devices (c)
> >       and send the final EOF to the destination.
> >
> > You set pause-at-complete, wait until migrate hits almost-complete;
> > cleanup the mirror job, and then do migration-continue.  When it
> > completes do 'cont' on the destination.
> >
> > The migration way
> > -----------------
> >    1) Stop doing (d) when the destination is started with -S
> >       since it happens anyway when 'cont' is issued
> >    2) Add a new migration capability ext-manage-storage
> >    3) When 'ext-manage-storage' is set, we don't bother doing (c)
> >    4) Add a new command 'block-inactivate' on the source
> >
> > You set ext-manage-storage, do the migrate and when it's finished
> > clean up the block job, block-inactivate on the source, and
> > then cont on the destination.
> >
> >
> > My worry about the 'block way' is that the point at which we
> > do the pause seems pretty interesting;  it probably is best
> > done after the final device save but before the inactivate,
> > but could be done before it.  But it probably becomes API
> > and something might become dependent on where we did it.
> >
> > I think Kevin's worry about the 'migration way' is that
> > it's a bit of a block-specific fudge; which is probably right.
> >
> >
> > I've not really thought what happens when you have a mix of shared and
> > non-shared storage.
> >
> > Could we do any hack that isn't libvirt-visible for existing versions?
> > I guess maybe hack drive-mirror so it interlocks with the migration
> > code somehow to hold off on that inactivate?
> >
> > This code is visible probalby from 2.9.ish with the new locking code;
> > but really that b/c race has been there for ever - there's maybe
> > always the chance that the last few blocks of mirroring might have
> > happened too late ?
> >
> > Thoughts?
> > What are the libvirt view on the preferred solution.
> >
> > Dave
> 
> Devs,
> 
> Did this issue ever get addressed?  I'm looking at the history for
> mirror.c at https://github.com/qemu/qemu/commits/master/block/mirror.c
> and I don't see anything that leads me to believe this was fixed.
> 
> I'm still unable to live migrate storage without risking corruption on
> even a moderately loaded vm.

Yes, there's now a 'pause-before-switchover' which gives libvirt
a chance to quiesce the block devices.
That went in my 93fbd0314^..0331c8cabf6168 back in October.
I believe libvirt uses that; see libvirt commit 6addde2.

Dave

> Thanks,
> schu
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK