Capacity versus I/O Throughput and Latency

After my blog entry about why "RAID-DP doesn't waste extra disk space", I got a comment from a reader saying that he thought performance was usually the limiting factor in storage systems rather than capacity:

In my experience, I/O throughput/latency is much more important than storage capacity and is the main driver for the number of spindles. If RAID-DP does not decrease I/O throughput, I don't care if it decreases my available disk space by 15%. Modern disk drives have a completely unbalanced capacity to throughput ratio, in any case, as ever-lengthening backup windows show (hence the value of snapshots, of course). —Fazal Majid

It's true that disk capacity is going up much, much faster than seek time. As a result, for many applications, the number of spindles required is determined by disk performance and not by capacity. One of the Exchange folks at Microsoft told me that one of their focus areas in development is to reduce disk traffic, for exactly this reason. Since people buy more capacity than they need in order to support the disk I/O, driving down disk traffic is a great way for Microsoft to help lower customers' costs.

I think this is one of the reasons that snapshots have become practical. We've got all of this disk space without enough performance attached to it, so we might as well fill it up with something that we don't look at very often, like snapshots.

On the other hand, there certainly are applications—especially archival apps—where capacity is the limiting factor and not performance. In order to correctly size a storage array, you really have to understand the performance requirements of your application, and not just the amount of storage used.