Re: [libvirt] [Qemu-devel] high outage times for qemu virtio network links during live migration, trying to debug
On Tue, Jan 26, 2016 at 10:41:12AM -0600, Chris Friesen wrote:
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?
The qemuDomainMigratePrepareTunnel3Params() method is responsible for
starting the QEMU process on the target host. This should not normally
have any impact on host networking connectivity, since the CPUs on that
target QEMU wouldn't be running at that point. Perhaps the mere act of
starting QEMU and plugging the TAP dev into the network on the target
host causes some issue though ? eg are you using a bridge that is doing
STP or something like that.
Regards,
Daniel
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