Starting the domU. Once you have configured your DRBD-backed domU, you may start it as you would any other domU:
xm create domU
Using config file "/etc/xen/".domU
Started domain domU
In the process, the DRBD resource you configured as the VBD will be promoted to the primary role, and made accessible to Xen as expected.
Stopping the domU. This is equally straightforward:
xm shutdown -w domU
Domain domU terminated.
Again, as you would expect, the DRBD resource is returned to the secondary role after the domU is successfully shut down.
Migrating the domU. This, too, is done using the usual Xen tools:
xm migrate --live domU destination-host
In this case, several administrative steps are automatically taken in rapid succession:
The resource is promoted to the primary role on
destination-host.
Live migration of domU is
initiated on the local host.
When migration to the destination host has completed, the resource is demoted to the secondary role locally.
The fact that both resources must briefly run in the primary role on both hosts is the reason for having to configure the resource in dual-primary mode in the first place.