HP StorageWorks NAS 8000 - Version 1.6.X Manual page 16

Network and Storage Planning
Active spare is similar to the dedicated hot spare disks used in some conventional arrays.
However, those arrays let the hot spare remain idle until it is needed. This array uses the active
spare for RAID 1+0 storage until the spare is needed. This provides the added benefit of
enhancing performance while also protecting against disk failure.
16 HP StorageWorks NAS 8000 SAN High Availability, Solution Integration Manual (SIM)