[Engine-devel] live migration and different technologies

[Daniel P. Berrange]

On Sat, Jun 09, 2012 at 03:57:40PM +0300, Itamar Heim wrote:
> On 06/08/2012 06:54 PM, Daniel P. Berrange wrote:
> >On Wed, Jun 06, 2012 at 05:15:53PM +0300, Itamar Heim wrote:
> >>Hi Daniel,
> >>
> >>on the quantum-ovirt call today the question of live migration
> >>between multiple technologies was raised.
> >>
> >>iirc, you implemented the abstraction in libvirt between what the
> >>guest sees and the actual host networking implementation for live
> >>migration.
> >>
> >>can you please share if there are any considerations around live
> >>migrations across different network implementations (bridge, sr-iov,
> >>ovs, qbg, openflow, etc.)
> >
> >Yes, we added the ability to use libvirt's 'virtual network' APIs
> >(virNetworkXXXXX) to define host networks using bridging, macvtap,
> >etc, etc. A guest's NICs can then be configured solely using
> ><interface type='network'>. This means that the guest XML will
> >not have any host-specific data in it, as you see when using
> ><interface type='bridge'>  or<interface type='direct'>
> >
> >This means you can migrate between machines where the bridges have
> >different names (eg br0 on host A and br7 on host B), without any
> >limitations.
> >
> >You can also migrate between different impls of the same technology
> >(eg traditional software bridging vs macvtap bridging without
> >limitations.
> >
> >Finally, you can migrate between completely different technologies
> >(eg bridging vs vepa), but you will likely loose connectivity in
> >the guests, since the technologies are not compatible at the ethernet
> >layer.
> 
> can you please explain this point - how would packet going out of
> the host or arriving to the guest would be different between a
> bridged and a vepa implemtnaiton?

I'm not the expert on VEPA - I'm just relaying what I have been told
wrt VEPA modes in the past.

IIUC, with VEPA modes there is quite alot of extra traffic due to a
handshake negotiation between the host & switch, before any guest
traffic can pass, and there needs to be a special synchronization
done with VEPA during migration to maintain state in the switch.

Regards,
Daniel
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