Prior to DRBD 9, promoting a resource would be done with the
drbdadm primary command. With DRBD
9, DRBD will automatically promote a resource to primary role when the
option is enabled, and one of its volumes is
mounted or opened for writing. As soon as all volumes are unmounted or closed, the role
of the resource changes back to secondary.
Automatic promotion will only succeed if the cluster state allows it (that is, if an
drbdadm primary command would succeed). Otherwise, mounting or opening
the device fails as it did prior to DRBD 9.