One controller is always acting as the Open Directory Master. You can change which controller that is.
Open Server on the first SAN controller computer (acting as the Open Directory Master), then select Open Directory.
Select the Open Directory Master from the list of servers.
Click the Action pop-up menu , then choose Archive Open Directory Master.
Follow the assistant instructions, then save the archive to the disk.
Open Server on the standby controller computer, then select Open Directory.
Select the Open Directory Replica from the list of servers.
Click the Action pop-up menu , then choose Promote Replica to Master.
Follow the assistant instructions, then provide the archive file when prompted.
In the Xsan pane of the Server app, select the Controllers tab.
Select the old controller that was previously the Open Directory Master.
Click Remove , then click OK to confirm.
The computer that’s a new Open Directory Master becomes the primary controller for the SAN.