Figure 8-12 is a good example of how having more than just the average wait can create a more informative and persuasive argument. The OSM swhistx.sql report is based on an interval of time (you simply run the report once, pause as long as you want, and then run it again) and clearly shows only 11.5% of all log writer background process write requests completed in 16 ms or less. If your IO write goal is 5 ms or less, or perhaps 10 ms or less, you're not even close! If you're feeling feisty, you can even say that only around half of all log writer background process writes completed in 64 ms or less (just make sure you didn't cause the problem in the first place).

If you start telling people write calls are taking over 200 ms, you're asking for a fight, so be prepared. Even if you are correct, you should expect to be aggressively challenged. So make sure you trace the log writer background process in summary, as shown in Figure 8-13, and in detail, as shown in Figure 8-10. Figure 8-10 highlights the log writer background process is issuing 1MB write calls, Oracle is gathering its timing information from the gettimeofday call and also gathering log writer background process CPU consumption details from the times call. If you run an IO subsystem-focused command, such as iostat, its numbers could be significantly different based on IO caching and physical configuration. So focus on how long it takes the Oracle log writer background process to have its writes serviced.

Figure 8-12. Shown is a v$event_histogram view, allowing us to glean more than the average wait time. For example, we can say that only 3.3% of all the log writer background process writes completed in 8 ms or less. Even more shocking, only 12% of log writer writes completed in less than 16 ms or less.