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Re: [libvirt] [RFC v1 0/6] Live Migration with ephemeral host NIC devices



On Wed, May 13, 2015 at 09:08:39 +0100, Dr. David Alan Gilbert wrote:
> * Peter Krempa (pkrempa redhat com) wrote:
> > On Wed, May 13, 2015 at 11:36:26 +0800, Chen Fan wrote:
> > > my main goal is to add support migration with host NIC
> > > passthrough devices and keep the network connectivity.
> > > 
> > > this series patch base on Shradha's patches on
> > > https://www.redhat.com/archives/libvir-list/2012-November/msg01324.html
> > > which is add migration support for host passthrough devices.
> > > 
> > >  1) unplug the ephemeral devices before migration
> > > 
> > >  2) do native migration
> > > 
> > >  3) when migration finished, hotplug the ephemeral devices
> > 
> > IMHO this algorithm is something that an upper layer management app
> > should do. The device unplug operation is complex and it might not
> > succeed which will make the current migration thread hang or fail in an
> > intermediate state that will not be recoverable.
> 
> However you wouldn't want each of the upper layer management apps implementing
> their own hacks for this; so something somewhere needs to standardise
> what the guest sees.

The guest still will see an PCI device unplug request and will have to
respond to it, then will be paused and after resume a new PCI device
will appear. This is standardised. The nonstandardised part (which can't
really be standardised) is how the bonding or other guest-dependant
stuff will be handled, but that is up to the guest OS to handle.

From libvirt's perspective this is only something that will trigger the
device unplug and plug the devices back. And there are a lot of issues
here:

1) the destination of the migration might not have the desired devices

    This will trigger a lot of problems as we will not be able to guarantee
    that the devices reappear on the destination and if we'd wanted to check
    we'd need a new migration protocol AFAIK.

2) The guest OS might refuse to detach the PCI device (it might be stuck
before PCI code is loaded)

    In that case the migration will be stuck forever and abort attempts
    will make the domain state basically undefined depending on the
    phase where it failed.

Since we can't guarantee that the unplug of the PCI host devices will be
atomic or that it will succeed we basically can't guarantee in any way
in which state the VM will end up later after (a possibly failed)
migration. To recover such state there are too many option that could be
desired by the user that would be hard to implement in a way that would
be flexible enough.

Peter

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