On Sat, 12 Jun 2010 13:20:54 +0200 Juan Quintela <quint...@redhat.com> wrote:
> Luiz Capitulino <lcapitul...@redhat.com> wrote: > > On Fri, 11 Jun 2010 09:38:42 -0500 > > Anthony Liguori <anth...@codemonkey.ws> wrote: > > > >> > 1. QMP only returns the response when the command is finished, eg: > >> > > >> > C: { "execute": "migrate", "id": "foo" ... } > >> > /* nothing is returned, other commands are issued, after several > >> > hours... */ > >> > S: { "return": ... "id": "foo" } > >> > > >> > >> This is how just about every RPC mechanism works... > > > > Let's go for it then. > > > >> >> - MIGRATION_STARTED: somebody started a migration, it is emited on > >> >> source and target, all monitors receive this event. > >> >> > >> > The client has started the migration, it knows it. Why is the event > >> > needed? > >> > > >> > >> I think we've more or less agreed that MIGRATION_CONNECTED is really the > >> event we want. > > > > Is it useful in the source or in the target? > > Both. What does it report in the source? > >> >> - MIGRATION_ENDED: migration ended with sucess, all needed data is in > >> >> target machine. Also emitted in all monitors on source and target. > >> >> > >> >> - MIGRATION_CANCELED: in one of the source monitors somebody typed: > >> >> migrate_cancel. It is only emmited on the source monitors, target > >> >> monitors will receive a MIGRATION_FAILED event. > >> >> > >> >> - MIGRATION_FAILED (with this error). At this point we don't have > >> >> neither the QMP infraestructure for sending (with this error) nor > >> >> migration infrastructure to put there anything different than -1. > >> >> > >> > Aren't all the three events above duplicating the async response? > >> > > >> > >> Yes. Today, we should just generate a MIGRATION_DONE event and let a > >> client poll for failure status. > > > > [...] > > > >> MIGRATION_DONE gets deprecated for 0.14. > > > > Yeah, this removes the need for polling in 0.13, but I was wondering if > > it's worth it. If I'm not mistaken, libvirt does the polling when working > > with the text Monitor and I believe it's not a big deal to keep it until > > 0.14. > > It makes things slower for no good reason. > > This reasoning is sneaky at least: > - qemu didn't give interfaces to libvirt for do what libvirt wanted > - libvirt uses workarounds > - qemu tells libvirt that they are using workarounds that they shouldn't > - libvirt tells qemu why they need the new interface > - qemu tells libvirt that they could continue to use its workarounds. > > I am losing something? The whole point of live migration is that they > need to be as fast as possible. For some scenaries (shared storage with > funny locking) libvirt needs to move from shared locks to normal locks > as far as migration ends on target. We are telling them to do > workarounds becauese qemu don't want to tell libvirt on destination when > migration has ended. > > Why can't we just tell them that migration has ended with success as > fast as possible? > > I can't understand what I am missing here. I can't believe that > libvirt(management app in general) could came with a simple request that > would make its live better. And to make things worse, it is _trivial_ > to implement, semantics are clear, has other uses, ..... I'd be ok in having the done event if the polling is shown to be that problematic, but from some private talks I had with libvirt guys it seemed to me that it's more like a wish have. For us, having this event means that we'll have to carry it for (at least) several releases. I really would like to avoid adding one-time workarounds in a stable interface like QMP.