openstack team mailing list archive

["Daniel P. Berrange" <dan@xxxxxxxxxxxx>]

On Wed, Apr 24, 2013 at 11:48:35AM -0400, Lorin Hochstein wrote:
> In the docs, we describe how to configure KVM block-based live migration,
> and it has the advantage of avoiding the need for shared storage of
> instances.
> 
> However, there's this email from Daniel Berrangé from back in Aug 2012:
> http://osdir.com/ml/openstack-cloud-computing/2012-08/msg00293.html
> 
> "Block migration is a part of the KVM that none of the upstream developers
> really like, is not entirely reliable, and most distros typically do not
> want to support it due to its poor design (eg not supported in RHEL).
> 
> It is quite likely that it will be removed in favour of an alternative
> implementation. What that alternative impl will be, and when I will
> arrive, I can't say right now."
> 
> Based on this info, the OpenStack Ops guide currently recommends against
> using block-based live migration, but the Compute Admin guide has no
> warnings about this.
> 
> I wanted to sanity-check against the mailing list to verify that this was
> still the case. What's the state of block-based live migration with KVM?
> Should we say be dissuading people from using it, or is it reasonable for
> people to use it?

What I wrote above about the existing impl is still accurate. The new
block migration code is now merged into libvirt and makes use of an
NBD server built-in to the QMEU process todo block migration. API
wise it should actually work in the same way as the existing deprecated
block migration code.  So if you have new enough libvirt and new enough
KVM, it probably ought to 'just work' with openstack without needing
any code changes in nova. I have not actually tested this myself
though.

So we can probably update the docs - but we'd want to checkout just
what precise versions of libvirt + qemu are needed, and have someone
check that it does in fact work.

Regards,
Daniel
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