Open Stack

On Fri, Apr 29, 2016 at 10:32:09PM +0000, Murray, Paul (HP Cloud) wrote:
> The following summarizes status of the main topics relating to live migration
> after the Newton design summit. Please feel free to correct any inaccuracies
> or add additional information.

> Post copy
> 
> The spec to add post copy migration support in the libvirt driver was
> discussed in the live migration session. Post copy guarantees completion
> of a migration in linear time without needing to pause the VM. This can
> be used as an alternative to pausing in live-migration-force-complete.
> Pause or complete could also be invoked automatically under some
> circumstances. The issue slowing these specs is how to decide which
> method to use given they provide a different user experience but we
> don't want to expose virt specific features in the API. Two additional
> specs listed below suggest possible generic ways to address the issue.
> 
> There was no conclusions reached in the session so the debate will
> continue on the specs. The first below is the main spec for the feature.
> 
> https://review.openstack.org/#/c/301509 : Adds post-copy live migration support to Nova
> https://review.openstack.org/#/c/305425 : Define instance availability profiles
> https://review.openstack.org/#/c/306561 : Automatic Live Migration Completion

There are currently many options for live migration with QEMU that can
assist in completion

 - Pause the VM
 - Auto-converge
 - XBZRLE compression
 - Multi-thread compression
 - Post-copy

Combined with tunables such as max-bandwidth and max-downtime. It is
absolutely clear as mud which of these work best for ensuring completion,
and what kind of impact they have on the guest performance.

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.

Regards,
Daniel
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