We can creatively use Oracle's instrumentation to monitor IO subsystem response time. Since Oracle times (instruments) each and every IO request, we simply query from the wait event views to get a very accurate performance view. Regardless of how the IO devices are performing, how the IO subsystem is configured, or how the network-attached storage (NAS) or storage area network (SAN) is performing, we can easily tell how long an Oracle IO request is taking.

Figure 4-19 is an OraPub wait interface report based on v$system_event. I ran the report once (not shown), waited 30 seconds, and then ran the report again. The second run, which contains what occurred during the 30-second interval, is shown in Figure 4-19. The top wait event is db file scattered read, which is what Oracle calls a multiblock read request emanating from an Oracle server process. During the 30-second interval, the average IO subsystem response time was 2.8 ms. That's very good, and I suspect some of the blocks were in a cache, but remember that these blocks are not in Oracle's buffer cache.6

Figure 4-19. A classic instance-level interval (30 seconds) wait event report based on v$system_event. Over the duration of this report, the IO subsystem responded to Oracle's multiblock read requests on average in 2.8 ms. That's pretty good!