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Re: [Xen-devel] Using xen and drbd/migration

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Subject: Re: [Xen-devel] Using xen and drbd/migration
From: daniele patoner <daniele.patoner@xxxxxxxxxxxxxxx>
Date: Fri, 10 Jun 2005 13:02:03 +0200
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I have successfully migrated domU with gnbd, but with a third host C that
export the root filesystem, but IMHO support for drbd is a nice feature for High-Affidability purpose with xen.



Michael Paesold wrote:

short introduction, my name is Michael Paesold, I am student from Austria/Europe and I am currently evaluating high availability and virtualization technologies for Linux/Unix.

What I want to do is to use xen with a drbd[1] device as root for a domU and then be able to migrate the domU between two host systems. I have already read the documenation on xen and some papers[2] and searched through the archives about drbd/xen usage. Unfortunatly my questions are not covered anywhere, so I hope you can help me.

drbd would run in the domain 0 to replicate a block device, say /dev/drbd0 on host A and host B. Xen would be set up on both hosts.
On host A I would create a new VM with /dev/drbd0 as root.

drbd will now sync /dev/drbd0 from host A to host B and keep those two consistent. Nevertheless in a drbd setup only on host can be primary and only one host may access a device (read/write).

So a live migration of a VM from host A to B would have to look like this:

pre-copy migration
suspend VM on A
stop-and-copy phase
--> on A: drbdadm secondary drbd0 # make A secondary for /dev/drbd0
--> on B: drbdadm primary drbd0 # make B primary for /dev/drbd0
resume VM on B

(with the root now pointing to the local /dev/drbd0 device)

Is there a way to do this with xen 2.0 or 3.0?

If no, I think a way would be to implement a module interface for the migration so that one can setup the environment after the VM is stop, but before the VM is resumed on the new host.


Best Regards,
Michael Paesold

[1] distributed remote block device - http://www.drbd.org/
[2] e.g. http://www.cl.cam.ac.uk/Research/SRG/netos/papers/2005-migration-nsdi-pre.pdf

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