Re: [Qemu-devel] [PATCH 0/7] migration: pause-before-device

[Kevin Wolf <kwolf@redhat.com>]

Am 12.10.2017 um 10:21 hat Daniel P. Berrange geschrieben:
> On Wed, Oct 11, 2017 at 08:13:10PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Hi,
> >   This set attempts to make a race condition between migration and
> > drive-mirror (and other block users) soluble by allowing the migration
> > to be paused after the source qemu releases the block devices but
> > before the serialisation of the device state.
> > 
> > The symptom of this failure, as reported by Wangjie, is a:
> >    _co_do_pwritev: Assertion `!(bs->open_flags & 0x0800)' failed
> > 
> > and the source qemu dieing; so the problem is pretty nasty.
> > This has only been seen on 2.9 onwards, but the theory is that
> > prior to 2.9 it might have been happening anyway and we were
> > perhaps getting unreported corruptions (lost writes); so this
> > really needs fixing.
> > 
> > This flow came from discussions between Kevin and me, and we can't
> > see a way of fixing it without exposing a new state to the management
> > layer.
> > 
> > The flow is now:
> > 
> > (qemu) migrate_set_capability pause-before-device on
> > (qemu) migrate -d ...
> > (qemu) info migrate
> > ...
> > Migration status: pause-before-device
> > ...
> > << issue commands to clean up any block jobs>>
> > 
> > (qemu) migrate_continue pause-before-device
> > (qemu) info migrate
> > ...
> > Migration status: completed
> 
> I'm curious why QEMU doesn't have enough info to clean up the block
> jobs automatically ? What is the key thing that libvirt knows about
> the block jobs, that QEMU is lacking ? If QEMU had the right info it
> could do it automatically & avoid this extra lock-step synchronization
> with libvirt.

The key point is that the block job needs to be completed while the
source VM is stopped, but the source qemu is still in control of the
image files (e.g. still holds the file locks), so that it can do the
remaining writes.

Without the additional migration phase, the only state where both sides
are stopped is when the destination is in control of the image files
(migration has completed, but -S prevents it from automatically
resuming), so the source can't write to the image any more.

Kevin