Unlike when I need to gather CPU capacity, if I must determine an IO subsystem's capacity, I ask the IO administrator. As detailed in the "Gathering IO Requirements" section earlier, we have the information needed to determine Oracle's IO requirements, but determining IO capacity with authority is best done by the IO subsystem team. If your IO subsystem is simply a series of SCSI drives daisy-chained together, as was done in the 1980s and early 1990s, then simple math can be used to predict the IO subsystem's capacity. However, the combination of read and write caching and batching from both Oracle and the operating system virtually eliminates the possibility of deriving a reliable IO capacity figure. Surely, we can gather and even predict IO requirements, but predicting IO capacity is something I simply will no longer attempt.

When talking with your IO administrator about capacity, ask for both read and write capacity in either MB/s or IOPS. While the IO administrator may not classify the read and write requirements, because Oracle systems have very unique read and write characteristics, just to be safe, it is always best to ask for the requirements of both. Since we can gather IO requirements in both MB/s and IOPS, it really doesn't make much difference to us in which form capacity is delivered or expressed.

With both requirements and capacity defined and the data collection covered, let's use them together. The classic requirements-versus-capacity indicator is utilization. It can be applied in a wide variety of situations-from river water flow and factory production to Oracle performance analysis.