[libvirt] [Qemu-devel] [RFC 0/7] Live Migration with Pass-through Devices proposal

Dr. David Alan Gilbert dgilbert at redhat.com
Wed Apr 22 17:01:56 UTC 2015

* Daniel P. Berrange (berrange at redhat.com) wrote:
> On Fri, Apr 17, 2015 at 04:53:02PM +0800, Chen Fan wrote:
> > backgrond:
> > Live migration is one of the most important features of virtualization technology.
> > With regard to recent virtualization techniques, performance of network I/O is critical.
> > Current network I/O virtualization (e.g. Para-virtualized I/O, VMDq) has a significant
> > performance gap with native network I/O. Pass-through network devices have near
> > native performance, however, they have thus far prevented live migration. No existing
> > methods solve the problem of live migration with pass-through devices perfectly.
> > 
> > There was an idea to solve the problem in website:
> > https://www.kernel.org/doc/ols/2008/ols2008v2-pages-261-267.pdf
> > Please refer to above document for detailed information.
> > 
> > So I think this problem maybe could be solved by using the combination of existing
> > technology. and the following steps are we considering to implement:
> > 
> > -  before boot VM, we anticipate to specify two NICs for creating bonding device
> >    (one plugged and one virtual NIC) in XML. here we can specify the NIC's mac addresses
> >    in XML, which could facilitate qemu-guest-agent to find the network interfaces in guest.
> > 
> > -  when qemu-guest-agent startup in guest it would send a notification to libvirt,
> >    then libvirt will call the previous registered initialize callbacks. so through
> >    the callback functions, we can create the bonding device according to the XML
> >    configuration. and here we use netcf tool which can facilitate to create bonding device
> >    easily.
> I'm not really clear on why libvirt/guest agent needs to be involved in this.
> I think configuration of networking is really something that must be left to
> the guest OS admin to control. I don't think the guest agent should be trying
> to reconfigure guest networking itself, as that is inevitably going to conflict
> with configuration attempted by things in the guest like NetworkManager or
> systemd-networkd.
> IOW, if you want to do this setup where the guest is given multiple NICs connected
> to the same host LAN, then I think we should just let the gues admin configure
> bonding in whatever manner they decide is best for their OS install.

I disagree; there should be a way for the admin not to have to do this manually;
however it should interact well with existing management stuff.

At the simplest, something that marks the two NICs in a discoverable way
so that they can be seen that they're part of a set;  with just that ID system
then an installer or setup tool can notice them and offer to put them into
a bond automatically; I'd assume it would be possible to add a rule somewhere
that said anything with the same ID would automatically be added to the bond.

However, I agree that you might be able to avoid having to do anything in the
guest agent.

Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK

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