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

[Kevin Wolf <kwolf@redhat.com>]

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.

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.

Kevin