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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC PATCH 0/5] asynchronous migration state change handlers
Date: Wed, 06 Jun 2012 05:37:37 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 06/05/2012 09:38 PM, Eric Blake wrote:
On 06/05/2012 07:15 AM, Gerd Hoffmann wrote:

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.

Right, this approach sounds much, much better to me too because it doesn't affect migration downtime.


Anthony Liguori

Option (3): Your suggestion?


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