| |
|
 |
|
 |
|
|
|
| |
|
|
xen-users
Re: [Xen-users] What makes live migration so slow?
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
Christmas stocking fillers. Some available as e-books.
_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users
|
|
|
| |
|
Home