Re: [Qemu-devel] [RFC PATCH 0/5] asynchronous migration state change handlers

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/06/2012 08:01 PM, Yonit Halperin wrote:
> On 06/06/2012 12:22 PM, Anthony Liguori wrote:
>>
>> So send a QMP event and call it a day.
>>
> Using a QMP event is making spice seamless migration dependent on libvirt
> version.

That is not an acceptable justification.

> Delaying the status change to "migration completed", (1) doesn't affect
> qemu migration time, the migration has already completed, and (2) will allow
> spice to seamlessly migrate, no matter which libvirt version is used.

(1) libvirt starts the destination with -S and starts it manually IIUC.  It 
waits for the migration completed event to do this.

Seriously, just add the event.  Async notifiers are not an option.

Regards,

Anthony Liguori

>
> Yonit.
>> Regards,
>>
>> Anthony Liguori
>>
>>>
>>> Yonit.
>>>
>>>>
>>>> Very short version: The requirement is simply to not kill qemu on the
>>>> source side until the source spice-server has finished session handover
>>>> to the target spice-server.
>>>>
>>>> Long version: spice-client connects automatically to the target
>>>> machine, so the user ideally doesn't notice that his virtual machine was
>>>> just migrated over to another host.
>>>>
>>>> Today this happens via "switch-host", which is a simple message asking
>>>> the spice client to connect to the new host.
>>>>
>>>> We want move to "seamless migration" model where we don't start over
>>>> from scratch, but hand over the session from the source to the target.
>>>> Advantage is that various state cached in spice-client will stay valid
>>>> and doesn't need to be retransmitted. It also requires a handshake
>>>> between spice-servers on source and target. libvirt killing qemu on the
>>>> source host before the handshake is done isn't exactly helpful.
>>>>
>>>> [ Side note: In theory this issue exists even today: in case the data
>>>> pipe to the client is full spice-server will queue up the switch-host
>>>> message and qemu might be killed before it is sent out. In practice
>>>> it doesn't happen though because it goes through the low-traffic main
>>>> channel so the socket buffers usually have enougth space. ]
>>>>
>>>> So, the big question is how to tackle the issue?
>>>>
>>>> Option (1): Wait until spice-server is done before signaling completion
>>>> to libvirt. This is what this patch series implements.
>>>>
>>>> Advantage is that it is completely transparent for libvirt, thats why I
>>>> like it.
>>>>
>>>> Disadvantage is that it indeed adds a small delay for the spice-server
>>>> handshake. The target qemu doesn't process main loop events while the
>>>> incoming migration is running, and because of that the spice-server
>>>> handshake doesn't run in parallel with the final stage of vm migration,
>>>> which it could in theory.
>>>>
>>>> BTW: There will be no "arbitrary amounts of downtime". Seamless spice
>>>> client migration is pretty pointless if it doesn't finish within a
>>>> fraction of a second, so we can go with a very short timeout there.
>>>>
>>>> Option (2): Add a new QMP event which is emmitted when spice-server is
>>>> done, then make libvirt wait for it before killing qemu.
>>>>
>>>> Obvious disadvantage is that it requires libvirt changes.
>>>>
>>>> Option (3): Your suggestion?
>>>>
>>>> thanks,
>>>> Gerd
>>>>
>>>
>>
>