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

[mail4dla@xxxxxxxxxxxxxx]

Hi,

from my own experience, I can confirm that the actual downtime is very low and the limiting factor is propagation of the new location in the network.

As also the Dom0 itself caches MAC adresses, you should try to do the ping from a 3rd machine to rule out that the Dom0 does not send the packets out to the network.
If this is not an option, you can use something like 'tcpdump -i eth0 "proto ICMP"' to see what's actually going on on your network and correlate this to the output of your ping command.

Cheers
dla

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

Hi there,

I've read the paper on Xen live migration, and it shows some very impressive figures, like 165ms downtime on a running web server, and 50ms for a quake3 server.

I installed CentOS 5 on 2 servers, each with 2x Xeon E5335 (quad-core), 2x Intel 80003ES2LAN Gb NICs. Then I installed 2 DomUs, also with CentOS 5.

One NIC is connected to the LAN (on the same switch and VLAN), the other interconnects the 2 servers with a cross cable.

Then I start pinging the DomU that is going to be migrated with 100ms interval, from within the Dom0 that is currently hosting it. And migrate the VM. The pinging is done on the LAN interface, while the migration occurs on the cross cabled one.

64 bytes from 10.10.241.44: icmp_seq=97 ttl=64 time=0.044 ms
64 bytes from 10.10.241.44: icmp_seq=98 ttl=64 time=0.039 ms
64 bytes from 10.10.241.44: icmp_seq=99 ttl=64 time=0.039 ms
64 bytes from 10.10.241.44: icmp_seq=125 ttl=64 time=0.195 ms
64 bytes from 10.10.241.44: icmp_seq=126 ttl=64 time= 0.263 ms
64 bytes from 10.10.241.44: icmp_seq=127 ttl=64 time=0.210 ms

As you can see, the response time before the migration is around 40us, and after, it's 200us, which is understandable since the VM is now in another physical host.

The problem is the 25 lost packets between the last phase of the migration. Don't get me wrong: 2.5s is a very good time, but 50 times higher than what it is told to be, isn't.

I tried the same test connecting both machines on a hub, and got the same results.

Did anybody try to measure the downtime during a live migration? How are the results?

Any thoughts and suggestions are very appreciated.

Thanks,
Marconi.

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