Understanding an Exchange SAN Setup

I just took a new position with a firm that has a medium-sized Exchange Server SAN. All the disks are 18GB, which surprises me. Wouldn't it make more sense to use larger disks, given that the number of slots in the SAN is fixed?

The use of relatively small disks in RAID sets seems counterintuitive. After all, given the relatively low cost of disk storage, using three 400GB disks instead of dozens of 18GB disks would make more sense, right? As it turns out, there are several good reasons for using smaller disks in large RAID sets.

First is speed, or, more precisely, rotational latency. The number of I/O operations per second (IOPS) that a disk can generate is an important factor in how that disk will perform in a SAN or RAID array. A few years ago, the IOPS-per-disk sweet spot (at least for 10PS per gigabyte) was an 18GB or 36GB disk, so that's what people tended to use. As the data density of disks increased, that sweet spot moved upwards, but your company's SAN was probably put in a place before that trend occurred. . . .