openstack team mailing list archive

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Thu, Aug 09, 2012 at 07:10:17AM -0700, Vishvananda Ishaya wrote:
> 
> On Aug 9, 2012, at 1:03 AM, Blair Bethwaite <blair.bethwaite@xxxxxxxxx> wrote:
> 
> > Hi Daniel,
> > 
> > Thanks for following this up!
> > 
> > On 8 August 2012 19:53, Daniel P. Berrange <berrange@xxxxxxxxxx> wrote:
> >> not tune this downtime setting, I don't see how you'd see 4 mins
> >> downtime unless it was not truely live migration, or there was
> > 
> > Yes, quite right. It turns out Nova is not passing/setting libvirt's
> > VIR_MIGRATE_LIVE when it is asked to "live-migrate" a guest, so it is
> > not proper live-migration. That is the default behaviour unless the
> > flag is added to the migrate flags in nova.conf, unfortunately that
> > flag isn't currently mentioned in the OpenStack docs either.
> 
> Can you file a bug on this to change the default? I don't see any
> reason why this should be off.

With non-live migration, the migration operation is guaranteed to
complete. With live migration, you can get into a non-convergence
scenario where the guest is dirtying data faster than it can be
migrated. With the way Nova currently works the live migration
will just run forever with no way to stop it. So if you want to
enable live migration by default, we'll need todo more than
simply set the flag. Nova will need to be able to monitor the
migration, and either cancel it after some time, or tune the
max allowed downtime to let it complete


Daniel
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