Re: [Qemu-devel] [PATCH] migration: Don't activate block devices if using -S

From: Jiri Denemark <jdenemar@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	qemu-devel@nongnu.org, quintela@redhat.com, famz@redhat.com,
	peterx@redhat.com
Subject: Re: [Qemu-devel] [PATCH] migration: Don't activate block devices if using -S
Date: Tue, 10 Apr 2018 09:36:35 +0200	[thread overview]
Message-ID: <20180410073635.GA91107@orkuz.home> (raw)
In-Reply-To: <20180409134003.GG5294@localhost.localdomain>

On Mon, Apr 09, 2018 at 15:40:03 +0200, Kevin Wolf wrote:
> Am 09.04.2018 um 12:27 hat Dr. David Alan Gilbert geschrieben:
> > It's a fairly hairy failure case they had; if I remember correctly it's:
> >   a) Start migration
> >   b) Migration gets to completion point
> >   c) Destination is still paused
> >   d) Libvirt is restarted on the source
> >   e) Since libvirt was restarted it fails the migration (and hence knows
> >      the destination won't be started)
> >   f) It now tries to resume the qemu on the source
> > 
> > (f) fails because (b) caused the locks to be taken on the destination;
> > hence this patch stops doing that.  It's a case we don't really think
> > about - i.e. that the migration has actually completed and all the data
> > is on the destination, but libvirt decides for some other reason to
> > abandon migration.
> 
> If you do remember correctly, that scenario doesn't feel tricky at all.
> libvirt needs to quit the destination qemu, which will inactivate the
> images on the destination and release the lock, and then it can continue
> the source.
> 
> In fact, this is so straightforward that I wonder what else libvirt is
> doing. Is the destination qemu only shut down after trying to continue
> the source? That would be libvirt using the wrong order of steps.

There's no connection between the two libvirt daemons in the case we're
talking about so they can't really synchronize the actions. The
destination daemon will kill the new QEMU process and the source will
resume the old one, but the order is completely random.

...
> > Yes it was a 'block-activate' that I'd wondered about.  One complication
> > is that if this now under the control of the management layer then we
> > should stop asserting when the block devices aren't in the expected
> > state and just cleanly fail the command instead.
> 
> Requiring an explicit 'block-activate' on the destination would be an
> incompatible change, so you would have to introduce a new option for
> that. 'block-inactivate' on the source feels a bit simpler.

As I said in another email, the explicit block-activate command could
depend on a migration capability similarly to how pre-switchover state
works.

Jirka