I have a server with 3 hard drives installed, and a total capacity of 6. We're planning to max it out, but our consultant also suggested getting a second RAID controller "for redundancy" to support the new drives. To me, this doesn't make much sense. Even with a second RAID controller running half of the disks, we're still stuck with only half of our disks/programs/data if one of the controllers dies (which isn't much better than running with none). We're putting vmware on the server and he vaguely mentioned some advanced fault tolerance/failover features, but if the disks are inaccessable due to a failed controller, how is it supposed to work?
Counting only reasons for redundancy, not performance, why would I want to have a second RAID controller in my server?