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xen-devel
[Xen-devel] lost gARP after live migration
Hi,
with reference to RHBZ#713585:
It seems when a RHEL-6.1 or F-15 Xen PV guest is live migrated, the
gratuitous ARP packet is not forwarded to the affected "networking
equipment". The netback vif is added to a routed bridge in the host(s)
and external hosts are expeted to have connection to the guest at all
times, no matter the current Xen host.
I experimented a bit with tcpdump, and the gARP does appear on the
netfront interface. It also appears on the host bridge if sufficient
time passes between completing the xenbus handshake and sending the gARP.
When the guest queues eg. three gARPs in rapid succession, a variable
number of them gets lost. (When all such packets disappear, then the
migrated guest becomes invisible to the outside world, until it
initiates network traffic on its own.)
When the guest waits for about half a second before sending (queueing),
the very first gARP packet successfully appears on the host bridge.
I suspect it's a timing race against the netback vif being added to the
host bridge. What would be a good countermeasure?
- Adding two modparams to xen-netfront (gARP requeue count & number of
msecs to wait between queueing the gARPs).
- (Paolo's idea:) watching the "hotplug-status" xenstore node and
sending a single gARP when the watch fires with "connected". This node
belongs to the backend xenstore subtree, thus watching it from the guest
doesn't please the architecture astronaut in me.
- Something else.
Sorry for the naivety / verbiage.
Thanks,
lacos
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