http://support.apple.com/kb/HT1200
Configure Promise VTrak RAIDs for best performance using the guidelines in this article. Although Promise VTrak RAIDs may be manually configured, it is recommended that they be configured using the scripts described in this article. Both script and manual configuration steps are provided below.
Note: Xsan-specific configuration scripts configure Revertible Spare Drives. In the event that a drive is replaced, data will be migrated from the Spare Drive to the replacement drive. Performance may be impacted for the duration of the migration procedure.