On Wed, May 26, 2010 at 09:54:22AM -0500, Anthony Liguori wrote: > On 05/26/2010 05:33 AM, Daniel P. Berrange wrote: > >>>I'm not sure why you would need a notification of when migration > >>>starts (since you know when you've started migration). > >>> > >>But you don't know if the other end "knows" that it has also started. > >> > >>started is needed only in incoming part, because .... we don't have a > >>monitor to ask if migration has started. > >> > >If we ever want to get closer to allowing multiple monitors, or allowing > >apps to issue QMP commands directly via libvirt, then we still need the > >'migration started' event on the source, because something else can > >have issued the 'migrate' command without the mgmt app knowing. > > > > Migration started doesn't help multiple monitors. You need locking of > some sort. > > Part of the problem is the QMP migrate command is implemented as a > synchronous command. It really ought to be an asynchronous command. > That tells you when the migration has actually completed without polling.
Handling asynchronous commands is alot more complicated and error prone for client apps, than providing a asynchronous event notification of the lifecycle stages. If you want to also query status while waiting for the completion, it means you can have to deal with overlapping command execute+return pairs within a single monitor connection. AFAICT this requires a change to QMP to require a unique ID to be sent with the {'execute'..} command and be sent back with the later corresponding {'return'...} data, so you can actually correlate reliably. > On the destination side, we're really limited by the fact that we don't > do live incoming migrations. The monitor doesn't get a chance to run at > all with exec: migration, for instance. If QEMU let you issue a monitor command for starting incoming migration, instead of using -incoming this wouldn't such a bad problem. eg you can launch QEMU in the desired config, with CPUs stopped, do the normal QMP handshake + whatever else is required then issue 'migrate_incoming URI' which blocked the caller for the duration, to allow completion to be detected. > For tcp: and unix:, a CONNECTED event absolutely makes sense (every > socket server should emit a CONNECTED event). Unfortunately, after > CONNECTED you lose the monitor until migration is complete. If > something bad happens, you have to exit qemu so once the monitor > returns, migration has completed successfully. > > If we introduce live incoming migration, we'll need to rethink things. > I would actually suggest that we deprecate the incoming command if we do > that and make incoming migration a monitor command. I would think it > should have the same semantics as migrate (as an asynchronous command). > A CONNECTED event still makes sense for tcp and unix protocols but I > don't think events make sense for start stop vs. an asynchronous command > completion. Do you actually mean 'deprecate -incoming arg' here ? Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|