I learned that RAID5 has a hidden gotcha in it's implementation: it maintains just one parity drive, no matter how big the array is. This is a bit counter intuitive; you might expect it to instead maintain 1/3 of the available drive space in parity. I found this out when I ran across a customer maintaining a 23-member RAID5 array. It had tons of usable space of course, but could only tolerate a single disk failure, and worse, rebuilding that disk required fully reading all 22 remaining drives to recalculate parity across the whole set. Somehow it hadn't caught fire yet when we discovered it, but it was a disaster waiting to happen.