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

[Jens Freimann]

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 


> --
> Eduardo