[libvirt] high outage times for qemu virtio network links during live migration, trying to debug

[Chris Friesen]

Hi,

I'm using libvirt (1.2.12) with qemu (2.2.0) in the context of OpenStack.

If I live-migrate a guest with virtio network interfaces, I see a ~1200msec 
delay in processing the network packets, and several hundred of them get 
dropped.  I get the dropped packets, but I'm not sure why the delay is there.

I instrumented qemu and libvirt, and the strange thing is that this delay seems 
to happen before qemu actually starts doing any migration-related work.  (i.e. 
before qmp_migrate() is called)

Looking at my timestamps, the start of the glitch seems to coincide with 
libvirtd calling qemuDomainMigratePrepareTunnel3Params(), and the end of the 
glitch occurs when the migration is complete and we're up and running on the 
destination.

My question is, why doesn't qemu continue processing virtio packets while the 
dirty page scanning and memory transfer over the network is proceeding?

Thanks,
Chris

(Please CC me on responses, I'm not subscribed to the lists.)