Use a Mac OS X Server system running the same versions of Xsan and Mac OS X Server as existing metadata controllers.
When using the Server Setup Assistant to set up your primary metadata controller, you can choose to manage SAN users and groups with Xsan Admin. If you choose this option, the Server Setup Assistant makes the primary metadata controller an Open Directory master server. When you add additional metadata controllers, they must run the same version of Mac OS X Server because they will become Open Directory Replicas to help manage the users and groups.
All systems in the Xsan must also be online when converting an Xsan client to metadata controller.