DRBD 8.0.6 brings full live migration for Xen on DRBD

Please note: since I first posted this entry, documentation on this subject has evolved considerably. Please check out the corresponding section in the DRBD User’s Guide for more details.

Undoubtedly, many of you out there are using Xen as the virtualization platform of your choice. Many of you have asked us about doing Xen live migration for DRBD-backed domU’s. Ask no longer, we’ve heard your call.

DRBD 8.0.6, released today, contains a block-drbd device helper script for use with Xen. This greatly simplifies the task of integrating Xen with DRBD.

Xen integration with DRBD: the old way

Previously, if you wanted to use DRBD as a Xen virtual block device (VBD), you would have added entry like this to your domU config file:

disk = [ 'phy:/dev/drbd0,xvda,w',
         'phy:/dev/drbd1,xvdb,w' ]

… and you would have had to make sure by yourself that those devices were in the Primary role on the node where you were starting your domain. Heartbeat could assist you with managing your DRBD, but Xen couldn’t do it on its own. It was all just a wee bit clumsy.

Xen integration with DRBD: the new way

With the new helper script, you do just this:

disk = [ 'drbd:myresource,xvda,w',
         'drbd:myotherresource,xvdb,w']

Now, when you fire up your domain, the helper script will put your DRBD resources named myresource and myotherresource in the Primary role, and hand the corresponding device nodes over to the Xen daemon for use as VBDs for the newly started domain. Likewise, when you issue a domain shutdown or destroy command, Xen will restore your DRBDs to the Secondary role.

The extra plus: Xen live migration with DRBD

And, what’s even better, with this DRBD directly hooks into Xen’s live migration capabilities. Just include these entries in your DRBD resource configuration:

net {
allow-two-primaries;
after-sb-0pri discard-zero-changes;
after-sb-1pri discard-secondary;
}

Add a resource configured like that to your Xen disk configuration, and voilà, you’ll be able to do xm migrate and even xm migrate --live with that domU.

The added bonus: simplified Xen clustering

Use a Xen domain as a Heartbeat cluster service? No problem, just add it to your Heartbeat V2 CRM configuration. The Xen OCF Resource Agent is your friend. Configure DRBD for Heartbeat? Not necessary, Xen will take care of everything all by itself!