Referencing the Load Profile's user calls per second metric shown in Figure 9-1, notice it closely matches our calculation of 414.84 uc/s. The difference is due to the two-digit precision of 149.97 minutes. Knowing that an Oracle transaction contains both commits (statistic user commits) and rollbacks (statistic user rollbacks), you can calculate the transaction rate and compare that to the load profile. You'll find the Statspack report does the math correctly.

The arrival rate is one of the most fundamental aspects of expressing what is occurring in a system-whether it's Oracle, a river, or an expressway. As you'll see in the following sections, when combined with other metrics, the arrival rate can be used to convey part of the performance situation and also provides clues about how our proposed performance solutions will affect the system.

If I were standing in front of you right now, I would have in my hands an empty glass and a pitcher of water. I would hold out the empty glass and say over and over, "capacity." Then I would hold out the pitcher and say repeatedly, "requirements." Then I would ask you, "Is the water going to fit in the glass? Are the requirements going to exceed the capacity?" In IT, what usually occurs is the water is poured in the glass, and we all look away, hoping it will fit. After a while, we start feeling the water dripping down our arm, and we have a mess. That mess is the result of the requirements exceeding the available capacity. When this occurs, we have a performance firefighting situation.