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[Xen-users] Live migration on top of DRBD 8.3.1 & heartbeat floating IP

To: xen-users <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-users] Live migration on top of DRBD 8.3.1 & heartbeat floating IP stacked devices
From: Adam Wilbraham <adam.wilbraham@xxxxxxxxxxxxxxxx>
Date: Wed, 15 Jul 2009 16:58:29 +0100
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Now I'm trying to build a pretty complex setup and so far I believe what I'm trying to do is impossible. This may be better posted to the DRBD users list, but since it is a Xen related question too I thought I'd start here.

My current setup is a 2 node Xen cluster using DRBD on top of LVM allowing me to live migrate at will between the nodes making use of the "allow-two-primaries" feature of DRBD. I had to work around some stuff to enable HVM based live migration, but it all works well now.

I want to take advantage of the DRBD 8.3.1 stacked devices feature so that I can replicate my DRBD devices off site, which means using heartbeat to manage a floating IP address (or number of IP addresses) between the two boxes, managing your DRBD device using the drbdupper resource agent and referencing the stacked device in your Xen domU config rather than the underlying DRBD device. I've had to create a "block-drbdstacked" helper script to handle accessing stacked devices which works fine and handles promoting to primary and dropping back to secondary in the same way that the standard block-drbd script does, however I am now at the point where I need to be able to do live migrations.

Currently if I do a "xm migrate test-domu host2 --live" then the running domU gets migrated but the storage backend doesn't which is to be expected in the setups current state. If I manually force a failover using heartbeat then the drbd device moves over, but its too late by this point as the running domU has already lost the storage. What I'm trying to establish is whether its theoretically possible to do what I'm trying to do - trigger a migration which doesn't complete until the underlying device has been migrated over by heartbeat. I believe that live migration relies on the allow-two-primaries functionality of DRBD which is physically impossible using a floating resource like in my setup. Has anyone had any experience with configuring a system like this?

Part of me is trying to tell me its one step too far but the other part of me says that if its possible to save the state of a domU, switch the resource over to the second node and then restore the state of the domU onto there then it should definitely be possible to make this work even if its not using the actual live migration functionality.

Thanks for any of your thoughts in advance,

Adam Wilbraham - Systems Administrator
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