[openstack-dev] [Nova] Live Migration: Austin summit update

Daniel P. Berrange berrange at redhat.com
Wed May 4 08:48:49 UTC 2016

On Tue, May 03, 2016 at 04:16:43PM -0600, Chris Friesen wrote:
> On 05/03/2016 03:14 AM, Daniel P. Berrange wrote:
> >There are currently many options for live migration with QEMU that can
> >assist in completion
> <snip>
> >Given this I've spent the last week creating an automated test harness
> >for QEMU upstream which triggers migration with an extreme guest CPU
> >load and measures the performance impact of these features on the guest,
> >and whether the migration actually completes.
> >
> >I hope to be able to publish the results of this investigation this week
> >which should facilitate us in deciding which is best to use for OpenStack.
> >The spoiler though is that all the options are pretty terrible, except for
> >post-copy.
> Just to be clear, it's not really CPU load that's the issue though, right?
> Presumably it would be more accurate to say that the issue is the rate at
> which unique memory pages are being dirtied and the total number of dirty
> pages relative to your copy bandwidth.
> This probably doesn't change the results though...at a high enough dirty
> rate you either pause the VM to keep it from dirtying more memory or you
> post-copy migrate and dirty the memory on the destination.

Yes that's correct - I should have been more explicit. A high rate of
dirtying memory implies high CPU load, but high CPU load does not imply
high rate of dirtying memory. My stress test used for benchmarking is
producing a high rate of dirtying memory.

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