Like it says: Why do arrays have even numbers of disks?
Most hardware disk arrays - certainly the ones that Sun sell - currently have an even number of drives in them. The StorEdge 3x00 series have 12, while the 6130 has 14.
The problem I have is that if you take away one drive to act as a hot spare, you're then left with 11 or 13. Not only is this an odd number, it's also prime.
So, what sort of sensible grouping of the drives can you come up with? I often punt and simply create a huge raid-5 volume spanning all the drives I've got left, which is simple. But there are cases when I really want to configure 2 identical sets of disks - either to mirror or to give to 2 hosts. To make this work, I have one drive left over (so I use it as a second hot spare).
Wouldn't it be neat to add an extra drive?