Re: [Qemu-devel] [PATCH 3/4] net/virtio: add failover support

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (address@hidden) wrote:
> On Tue, Jun 04, 2019 at 03:43:21PM +0200, Jens Freimann wrote:
> > On Mon, Jun 03, 2019 at 04:36:48PM -0300, Eduardo Habkost wrote:
> > > On Mon, Jun 03, 2019 at 10:24:56AM +0200, Jens Freimann wrote:
> > > > On Fri, May 31, 2019 at 06:47:48PM -0300, Eduardo Habkost wrote:
> > > > > On Thu, May 30, 2019 at 04:56:45PM +0200, Jens Freimann wrote:
> > > > > > On Tue, May 28, 2019 at 11:04:15AM -0400, Michael S. Tsirkin wrote:
> > > > > > > On Tue, May 21, 2019 at 10:45:05AM +0100, Dr. David Alan Gilbert 
> > > > > > > wrote:
> > > > > > > > * Jens Freimann (address@hidden) wrote:
> > > > Why is it bad to fully re-create the device in case of a failed 
> > > > migration?
> > > 
> > > Bad or not, I thought the whole point of doing it inside QEMU was
> > > to do something libvirt wouldn't be able to do (namely,
> > > unplugging the device while not freeing resources).  If we are
> > > doing something that management software is already capable of
> > > doing, what's the point?
> > 
> > Event though management software seems to be capable of it, a failover
> > implementation has never happened. As Michael says network failover is
> > a mechanism (there's no good reason not to use a PT device if it is
> > available), not a policy. We are now trying to implement it in a
> > simple way, contained within QEMU.
> > 
> > > Quoting a previous message from this thread:
> > > 
> > > On Thu, May 30, 2019 at 02:09:42PM -0400, Michael S. Tsirkin wrote:
> > > | > On Thu, May 30, 2019 at 07:00:23PM +0100, Dr. David Alan Gilbert 
> > > wrote:
> > > | > >  This patch series is very
> > > | > > odd precisely because it's trying to do the unplug itself in the
> > > | > > migration phase rather than let the management layer do it - so 
> > > unless
> > > | > > it's nailed down how to make sure that's really really bullet proof
> > > | > > then we've got to go back and ask the question about whether we 
> > > should
> > > | > > really fix it so it can be done by the management layer.
> > > | > >
> > > | > > Dave
> > > | >
> > > | > management already said they can't because files get closed and
> > > | > resources freed on unplug and so they might not be able to re-add 
> > > device
> > > | > on migration failure. We do it in migration because that is
> > > | > where failures can happen and we can recover.
> > 
> > This is something that I can work on as well, but it doesn't have to
> > be part of this patch set in my opinion. Let's say migration fails and we 
> > can't
> > re-plug the primary device. We can still use the standby (virtio-net)
> > device which would only mean slower networking. How likely is it that
> > the primary device is grabbed by another VM between unplugging and
> > migration failure anyway?
> > 
> > regards,
> > Jens
> 
> I think I agree with Eduardo it's very important to handle this corner
> case correctly. Fast networking outside migration is why people use
> failover at all.  Someone who can live with a slower virtio would use
> just that.
> 
> And IIRC this corner case is exactly why libvirt could not
> implement it correctly itself and had to push it up the stack
> until it fell off the cliff :).

So I think we need to have the code that shows we can cope with the
corner cases - or provide a way for libvirt to handle it (which is
my strong preference).

Dave


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