[Openstack-operators] KVM live block migration: stability, future, docs

Blair Bethwaite blair.bethwaite at gmail.com
Mon Aug 6 23:56:41 UTC 2012

Apologies for any duplicates, signed up to the list with the wrong
address initially...

Hi all,

KVM block migration support in OpenStack
seems to be somewhat of a secret - there's almost nothing in the
docs/guides (which to the contrary state that live migration is only
possible with shared storage) and only a couple of mentions on list,
yet it's been around since Diablo. Should this be taken to mean it's
considered unstable, or just that no-one interested in documenting it
understands the significance of such a feature to deployment
architects? After all, decent shared storage is an expensive prospect
with a pile of associated design and management overhead!

I'd be happy to contribute some documentation patches (starting with
the admin guide) that cover this. But first I'd like to get some
confirmation that it's here to stay, which will be significant for our
own large deployment. We've tested with Essex on Ubuntu Precise and
seen a bit of weird file-system behaviour, which we currently suspect
might be a consequence of using ext3 in the guest. But also, there
seems to be some associated lag with interactive services (e.g. active
VNC session) in the guest, not yet sure how this compares to the
non-block live migration case.

We'd really appreciate anybody actively using this feature to speak up
and comment on their mileage, especially with respect to ops.

I'm slightly concerned that KVM may drop this going forward
http://www.spinics.net/lists/kvm/msg72228.html), though that would be
unlikely to affect anybody deploying on Precise.


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