Figure 4-20 is an AWR report's Top 5 Timed Events table, showing wait events. Notice that multiblock reads (event db file scattered reads) are taking an average of 20 ms to be returned to an Oracle server process. This normally would not be acceptable. Now, I am not implying the IO subsystem is improper in some way and I'm also not implying the Oracle instance or the related SQL is tuned. All we know from this report is that when Oracle server processes submit a multiblock IO request, it takes the IO subsystem 20 ms to respond. Based on the associated OraPub 3-circle analysis, there will most likely be at least three possible, and probably more, solutions.

Figure 4-20. An AWR report's Top 5 Timed Events table. Notice IO-related wait time accounts for about half of the response time, and multiblock reads account for about 28% of the wait time, with an average response time of 20 ms.

When you find that IO response times are only a couple of milliseconds or less, remember that it takes CPU resources to satisfy these IO requests. It is very common for a seemingly IO-bottlenecked system to actually be suffering from a CPU bottleneck. Performing an OraPub 3-circle analysis and the associated ORTA will clearly show this, so there should be no surprises.