When I did a total cost of ownership calculation for SAN-attached disk arrays during a recent assignment, the biggest factor in favour of SAN was the usage factor. With direct-attached disk on servers, you typically over-allocate the disk by a wide margin, because adding disk at a later time is a pain. With SAN-attached storage you can pop some more disks into the disk array and, depending on the operating system, you can increase the storage available in a relatively easy way.

Therefore if you manage your SAN-attached storage on a just-in-time basis, you can achieve perhaps 80 percent utilization of the disk, whereas in a typical group of servers using direct attached storage you might have 20 percent utilization. This four-t0-one price difference is significant.

Earlier I calculated that there's roughly a ten to one difference between consumer disk and the top-of-the-line SAN disk in at least one manufacturer's offerings. So a four-to-one price difference goes some way to fixing that, but not all the way. And to chip away further at the disk usage argument, a lot of the disks that contribute to the 20 percent utilization number are the system disks on small servers. These days you can't get much less the 72 GB on a system disk, and most servers need far, far less than that. My technical experts recommend that you don't boot from SAN, so you'll still have that low utilization rate even after installing a SAN.

I'm not making much of a case for SAN, am I?