When we perform an ORTA, we classify all IO time as queue time subclassification. This may seem like an unfortunate and desperate abstraction, but it actually fits perfectly from a database perspective. If no IO occurs, Oracle satisfies all requests consuming only CPU time. But as the workload increases and some of this work requires IO, response time begins to increase; that is, queue time begins to increase; that is, IO time begins to increase. The pattern fits very nicely into an ORTA.

In summary, both our CPU service time and IO queue time abstractions fit very nicely into an ORTA, providing us with the opportunity to apply predictive mathematics to evaluate alternative firefighting solutions.

The classic response-time curve in Figure 9-8 highlights the differences between CPU and IO subsystems. It turns out that real Oracle systems operate somewhere between the two. The dotted line in Figure 9-8 represents an Oracle system that operates completely and only with CPU. In others words, there is no physical IO, only logical IO activity. In contrast, the solid line represents an IO-centric Oracle system. No Oracle system can operate with only IO, because there must be CPU resources consumed to run processes, which includes processing the IO once it has been read from disk.