As you might know XSAN is a spinoff of Quantum's (formerly ADICs) Filesystem called StorNEXT.
StorNEXT (only when running Linux MDC, upgrade exists from Apple MDC) allows multi tier storage through an option called storage manager. Xsan clients are compatible with StorNEXT as a server. XSAN clients would see a big volume off ex 20 TB (even when you have 10 TB of disk and 20TB of tape storage. Definable data migration rules defeine when data has to be moved to tape or back.
See:
http://www.quantum.com/pdf/PB00033.pdf
Regards from Belgium.