Re: [Xen-users] What makes live migration so slow?

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Wenda Ni wrote:

> I have now shfited to Centos 5.5, and I am testing live migration 
> between 2 physical hosts with XEN 3.1.2. XEN 3.1.2 (virtualization) 
> is included in centOS 5.5 during the installation phase, so 
> everything is handled by default. A third host with Ubuntu OS serves 
> as the network file system. The three hosts are connected by one 
> D-link gigabit switch (DGS-2205).
> The downtime of live migration is still very long. If we measure the 
> downtime by Ping command,  one migration event of a guest VM (domU) 
> gives a loss of 30-50 packets. (should be over half a minute).
> Could anyone know the potential problem? Thank you in advance!
I would suggest you try with something like arping and see if you get 
any different results. I can think of one thing that would make the 
VM "disappear" from the network for a while during/after migration 
apart from the downtime while the migration actually happens :
Every switch (bridge) on the network will have learned the port to 
which the node is attached and cached it in it's MAC <-> Port table. 
When you move a machine, these switches will continue to forward 
unicast packets via the port it has in it's tables UNTIL it has 
reason to update the table entry. Thus packets may get sent to the 
wrong place for a while - and so the migrated machine just doesn't 
get any traffic.
Most switches will update the table as soon as it sees a packet from 
the device arrive on a different port. Depending on what's going on, 
that may take a second, or minutes.
Arping uses ARP request packets to probe a device rather than unicast 
ICP-Ping packets. These being broadcast will be flooded to the entire 
network and so will reach the device even though it's now on a 
different port. When it replies, the intermediate switches will 
update their MAC table accordingly.
Also, I've found some switches can be slow to update. We've some HP 
1800-24G switches at work, and they seem to have a 5 minute timeout 
before they'll update the MAC table. I guess it's probably 
configurable via CLI - but I haven't looked.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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