Please see Apple knowledge base article regarding this configuration. http://support.apple.com/kb/HT1110
In order to achieve maximum performance when integrating Promise VTrak RAIDs into an existing Xsan 2 volume that is composed of Apple Xserve RAIDs, the Promise VTrak RAIDs should not be included in storage pools that are composed of Apple Xserve RAIDs. Instead, create new storage pools to be exclusively composed of Promise VTrak RAIDs.
Note: Promise VTrak RAIDs are only supported with Mac OS X 10.5.x based systems.