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Re: Should we discourage KVM block-based live migration?


On Wed, Apr 24, 2013 at 11:59 AM, Daniel P. Berrange <dan@xxxxxxxxxxxx>wrote:

> 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.
Thanks, Daniel. I can update the docs accordingly. How can I find out what
are the minimum versions of libvirt and qemu are needed?

Also, I noticed you said "qemu" and not "kvm", and I see that
http://wiki.qemu.org/KVM says that qemu-kvm fork for x86 is "deprecated,
use upstream QEMU now".  Is it the case now that when using KVM as the
hypervisor for a host, an admin will just install a "qemu" package instead
of a "qemu-kvm" package to get the userspace stuff?


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