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

["Daniel P. Berrange" <berrange@redhat.com>]

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

How about 'switchover-cleanup'


> (qemu) migrate -d ...
> (qemu) info migrate
> ...
> Migration status: pause-before-device

and 'switchover'

> ...
> << issue commands to clean up any block jobs>>
> 
> (qemu) migrate_continue pause-before-device
> (qemu) info migrate
> ...
> Migration status: completed
> 
> This set has been _very_ lightly tested just at the normal migration
> code, without the addition of the drive mirror; so this is a first
> cut.  I'd appreciate some feedback from libvirt whether the inteface
> is OK and ideally a hack to test it in a full libvirt setup to see
> if we hit any other issues.
> 
> The precopy flow is:
> active->pause-before-device->completed
> 
> The postcopy flow is:
> active->pause-before-device->postcopy-active->completed
> 
> Although the behaviour with postcopy only gets interesting when
> we add something like Max's active-sync.
> 
> Please argue about the command and state naming.

Argued above :-)

Regards,
Daniel
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