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!

Be careful about making assertions when there are not very many samples. For example, during the 30-second interval between reports, single-block read requests (event db file sequential read) took an average of 8.2 ms, but Oracle server processes issued less than 1,000 of them. Another more pronounced example is log writer multiblock writes (event log file parallel write) took an average of 49.0 ms! Normally, that would be unacceptable, but they occurred less than 1,000 times during the report interval, and their combined time accounts for only 0.45% of the wait time. So this is nothing to be concerned about.