Re: [Xen-users] Live migration: 2500ms downtime

[mail4dla@xxxxxxxxxxxxxx]

Hi,

On 8/10/07, Marconi Rivello <marconirivello@xxxxxxxxx> wrote:

I did ping from a third physical machine. The result doesn't vary much.

I followed your advice on analyzing the traffic. But I don't see why to look for ICMPs, since the DomU does answer the ping, it just has a 2.5s gap after stopping on one machine and starting on the other.

Well, I asked you to this (in your original test setup where the ping was performed from the source Dom0) in order to see whether the packets are actually sent out of the machine, or the Dom0 tries to send it through the bridge and the no-longer existing virtual interface.
 

Here follows 2 scenarios:

 

That happens when the physical machines are connected to the switch. I started tcpdump on both Dom0's to see if the DomU would send the unsolicited arp reply to update the switch's tables. And there is none. So, unless there is already traffic going out from the domU, there isn't anything to tell the switch the machine changed from one port to another.

This is excactly the anticipated behaviour. AFAIR, it is a known issue and more recent builds of Xen do send the unsolicited arp reply after migration.
With a switch, you are quite lucky to have only 2.5s outtime. Depending on the switch and it's ageing algorithm, this can be significantly higher, i.e., 30s or so.

Do you also have the 2.5s outtime when pinging from a 3rd machine and having the machines connected by a hub (actually, it's sufficient to connect the Xen machines via a hub to the same switch port)?

Just to emphasize: I'm running CentOS 5, with Xen 3.0.3 (which comes with it), and applied the Xen related official CentOS (same as redhat's) updates.

My experiences are based on the Xen that is shipped with Ubuntu 7.04, which is also a 3.0.3 and when the machines are connected through a hub, the unavailability period is within the same orders as in the paper that you quoted in your first email.


hth
dla

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users