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



On Wed, Apr 22, 2015 at 06:01:56PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange 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.

I didn't mean the admin would literally configure stuff manually. I really
just meant that the guest OS itself should decide how it is done, whether
NetworkManager magically does the right thing, or the person building the
cloud disk image provides a magic udev rule, or $something else. I just
don't think that the QEMU guest agent should be involved, as that will
definitely trample all over other things that manage networking in the
guest.  I could see this being solved in the cloud disk images by using
cloud-init metadata to mark the NICs as being in a set, or perhaps there
is some magic you could define in SMBIOS tables, or something else again.
A cloud-init based solution wouldn't need any QEMU work, but an SMBIOS
solution might.

Regards,
Daniel
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