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

From: Eric Blake
Subject: Re: [Qemu-devel] [RFC PATCH 0/5] asynchronous migration state change handlers
Date: Tue, 05 Jun 2012 07:38:02 -0600
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On 06/05/2012 07:15 AM, Gerd Hoffmann wrote:
>   Hi,
>> Absolutely not.  This is hideously ugly and affects a bunch of code.
>> Spice is *not* getting a hook in migration where it gets to add
>> arbitrary amounts of downtime to the migration traffic.  That's a
>> terrible idea.

> 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.

But there was recently a proposal for a new monitor command that will
let libvirt query which events a given qemu supports, and therefore
libvirt can at least know in advance whether to expect and wait for the
event, or to fall back to some other option.  Just because libvirt would
require a change doesn't necessarily rule out this option.

> Option (3): Your suggestion?
> thanks,
>   Gerd

Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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