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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Kevin Wolf (kwolf@redhat.com) wrote:
> Am 28.03.2018 um 19:02 hat Dr. David Alan Gilbert (git) geschrieben:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Activating the block devices causes the locks to be taken on
> > the backing file.  If we're running with -S and the destination libvirt
> > hasn't started the destination with 'cont', it's expecting the locks are
> > still untaken.
> > 
> > Don't activate the block devices if we're not going to autostart the VM;
> > 'cont' already will do that anyway.
> > 
> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=1560854
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> I'm not sure that this is a good idea. Going back to my old writeup of
> the migration phases...
> 
> https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg07917.html
> 
> ...the phase between migration completion and 'cont' is described like
> this:
> 
>     b) Migration converges:
>        Both VMs are stopped (assuming -S is given on the destination,
>        otherwise this phase is skipped), the destination is in control of
>        the resources
> 
> This patch changes the definition of the phase so that neither side is
> in control of the resources. We lose the phase where the destination is
> in control, but the VM isn't running yet. This feels like a problem to
> me.

But see Jiri's writeup on that bz;  libvirt is hitting the opposite
problem;   in this corner case they can't have the destination taking
control yet.

> Consider a case where the management tool keeps a mirror job with
> sync=none running to expose all I/O requests to some external process.
> It needs to shut down the old block job on the source in the
> 'pre-switchover' state, and start a new block job on the destination
> when the destination controls the images, but the VM doesn't run yet (so
> that it doesn't miss an I/O request). This patch removes the migration
> phase that the management tool needs to implement this correctly.
> 
> If we need a "neither side has control" phase, we might need to
> introduce it in addition to the existing phases rather than replacing a
> phase that is still needed in other cases.

This is yet another phase to be added.
IMHO this needs the managment tool to explicitly take control in the
case you're talking about.

> Kevin

Dave (out this week)

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK