[Xen-devel] xen, drbd, live migration

[Michael Paesold <michael@xxxxxxxxxx>]

Hello,

I would like to enable live migration of vm's between two hosts using a 
block device synced via drbd between the two hosts.
Google doesn't help much. Searching for
> xen drbd "live migration"
gives 2 related posts on xen-users.

Myself asking about the same topic, in June:
http://lists.xensource.com/archives/html/xen-users/2005-06/msg00206.html

This one is about "non-live" migration, including a hack of libxs:
http://lists.xensource.com/archives/html/xen-users/2005-06/msg00264.html


DRBD allows to syncronize a block device between two hosts, it can be 
seen as raid-1 over network. The most importent issue for live migration 
is that you must access the drbd block device only from either of the 
hosts, never from both.
I have created a block-drbd script to make a /dev/drbd device accessable 
at vbd "bind" and release the resource on "unbind" (drbdadm primary and 
secondary in DRBD terms).
Now the sequence of events in Xen live migration seems to be this:
1) send configuration to target, create resources there (including vbd 
bind!)
2) copy memory
3) destroy vm on source (unbind vbds)
4) unpause vm on destination

Right?

Now (1) is were it fails already, because with drbd you have to first 
"unbind" the device on the old host and then "bind" it on the new one. 
There is no way around this.
So the sequence would have to look like this:
1) send configuration to target, don't create vbd resources yet (only 
place holders perhaps?)
2) copy memory
3) destroy vm on source (unbind vbds)
4) bind vbds on target
5) unpause vm on destination

DRBD is a very reliable and well performing solution. It would be great 
to be able to use it with xen and would be an outstanding solution for 
highly available xen, at least in a two-hosts setup.
To those developers with insight in the migration stuff: is it possible 
to change this in Xen so it will work as above? Do you have time or 
interest? Could you help me to find the right places so I can try myself?
Best Regards,
Michael Paesold


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