Apple Xsan 2 (Third Edition) - Manual (Page 86)

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To increase SAN security and redundancy, you can add,
switch, and monitor Xsan metadata controllers.
Every SAN volume you set up is managed by a metadata controller. To be sure that
the volume is available to clients even if the primary metadata controller becomes
unresponsive, you can set up standby controllers, one of which will assume control of
the volume if the primary controller fails.
This chapter shows you how to add metadata controllers, set their failover priority,
and force volume failover from the primary controller to a standby controller.
6
Managing Metadata Controllers