Clustering: Designate a Hot-Standby Unit (Raid-5 style)
If one is concerned about up-time, it doesn't make sense to have a cluster where all units must be functional to handle the load.
Clustering should allow the use of a Hot-Standby unit for the Cluster, for example running two 320's as active with a spare 320 (thus more affordable) in reserve. Allows for true N+1 setups without paying always for the +1!
Bob Alfson commented
I meant "RAID 6" style = RAID 5 + standby disk