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

Tue May 19 15:03:04 UTC 2015

* Daniel P. Berrange (berrange at redhat.com) wrote:
> On Tue, May 19, 2015 at 10:15:17AM -0400, Laine Stump wrote:
> > On 05/19/2015 05:07 AM, Michael S. Tsirkin wrote:
> > > On Wed, Apr 22, 2015 at 10:23:04AM +0100, Daniel P. Berrange 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.
> > > There should not be a conflict.
> > > guest agent should just give NM the information, and have  NM do
> > > the right thing.
> > 
> > That assumes the guest will have NM running. Unless you want to severely
> > limit the scope of usefulness, you also need to handle systems that have
> > NM disabled, and among those the different styles of system network
> > config. It gets messy very fast.
> 
> Also OpenStack already has a way to pass guest information about the
> required network setup, via cloud-init, so it would not be interested
> in any thing that used the QEMU guest agent to configure network
> manager. Which is really just another example of why this does not
> belong anywhere in libvirt or lower.  The decision to use NM is a
> policy decision that will always be wrong for a non-negligble set
> of use cases and as such does not belong in libvirt or QEMU. It is
> the job of higher level apps to make that kind of policy decision.

This is exactly my worry though; why should every higher level management
system have it's own way of communicating network config for hotpluggable
devices.  You shoudln't need to reconfigure a VM to move it between them.

This just makes it hard to move it between management layers; there needs
to be some standardisation (or abstraction) of this;  if libvirt isn't the place
to do it, then what is?

Dave

> > > Users are actually asking for this functionality.
> > >
> > > Configuring everything manually is possible but error
> > > prone.
> > 
> > Yes, but attempting to do it automatically is also error prone (due to
> > the myriad of different guest network config systems, even just within
> > the seemingly narrow category of "Linux guests"). Pick your poison :-)
> 
> Also note I'm not debating the usefulness of the overall concept
> or the need for automation. It simply doesn't belong in libvirt or
> lower - it is a job for the higher level management applications to
> define a policy for that fits in with the way they are managing the
> virtual machines and the networking.
> 
> Regards,
> Daniel
> -- 
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> 
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK