Due to the enormous cost of selecting and migrating to a completely new primary storage infrastructure, most organizations try to wring every last drop of functionality out of their storage resources. That's one reason why most storage deployments are viewed as five-year investments.
Yet with corporate data growing at geometric rates, the notion of deploying a platform that can scale out for such a long time -- not to mention the idea you can plan that far into the future accurately -- is becoming a joke. Many "long-term" storage investments have hit the wall much earlier than anticipated, incurring uncomfortable trips to the corner office. Hey, didn't you say those big, expensive hunks of hardware were going to last?
Face it -- upgrading your storage infrastructure is going to happen more often than you'd like. But at least server virtualization has dramatically decreased the pain involved in making a midstream migration from one storage platform to another or of running more than one system in parallel. The truth is your ability to predict your future needs is more difficult than ever. In fact, you'll probably be wrong -- and that's OK.
The old approach
Most enterprise SANs are built around a controller and disk-shelf architecture. Typically, the controller resources are sized based on the total amount of host I/O required on the front end and the amount of disk resources addressed on the back end.