[libvirt] sending openvswitch "port db" info during libvirt migration

[Daniel P. Berrange]

On Wed, Aug 08, 2012 at 10:58:28PM -0400, Laine Stump wrote:
> On 08/08/2012 12:30 PM, Kyle Mestery (kmestery) wrote:
> > On Aug 8, 2012, at 9:49 AM, Daniel P. Berrange wrote:
> >> On Tue, Aug 07, 2012 at 03:50:20PM -0400, Laine Stump wrote:
> >>> Someone asked on IRC the other day about sending openvswitch per-port
> >>> data (normally stored in the switch) to the destination host during a
> >>> migration. I suggested maybe this could be handled by encoding the
> >>> information into the interface's <virtualport> prior to migration, and
> >>> then writing it back out to openvswitch on the destination when the
> >>> interface was reconnected there.
> >>>
> >>> I think there's a problem with that, though - they don't want to save
> >>> the port data on the source until the instant that the interface is
> >>> disconnected (since it is constantly changing), but by then the domain
> >>> XML has already long ago been formatted and sent to the destination.
> >>>
> >>> So is there some other way within the confines of the current migration
> >>> protocol that this information can be sent from migration source to
> >>> destination?
> >> I'd be interested to know more about just what sort of data it is that
> >> needs to be passed around. From what you describe though, the only
> >> way to pass that info would be to store it in the migration cookie
> >> when the 'Perform' step completes (ie QEMU now paused, no network
> >> I/O taking place), whereupon it will be passed into the 'Finish'
> >> step on the dest (which can set it and then resume QEMU)
> >>
> > This is per-port data, tied to the specific virtual port associated with the VM
> > itself. This data is stored in the OVS DB, and is used/could be used by an
> > entity controlling Open vSwitch on the host. For example, stats may be kept
> > here.
> >
> > >From what you indicate above, can you point me at the code which handles the
> > migration cookie?
> 
> Look in qemu_migration.c. I notice there is also code there that calls
> the 802.1QbX-oriented virtualport associate function (search for
> qemuMigrationVPAssociatePortProfiles), and it's likely there where the
> port data should be unloaded from the cookie into the destination host.
> As for when/where to load the port data *into* the cookie on the source,
> I'm new to that code too, and didn't see it right away :-)
> 
> I do see that the entire persistent definition can be sent in the
> cookie. It seems to me that "last minute (last instant) updates" to the
> live XML could be an issue not just for interfaces connected to ovs
> switches, but for other circumstances as well. Does it maybe make sense
> to solve this in a more generalized fashion, by resending the complete
> live XML? (I'm assuming that during migration libvirt is preventing any
> modification to the live XML that would result in invalidating the vm
> state contained in the memory image being transferred).

No, at that point the VM is already started on the dest host, so you
can't just re-send the live XML there. You can put any other data into
the cookie though, it doesn't have to be passed in the persistent XML
directly.


Daniel
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