Figure 5-11 is a good example of how a histogram perspective adds important additional information to your analysis. While not shown, a standard wait event report showed the top wait event to be db file scattered read, with an average wait time of 14.9 ms. While the average multiblock read request is greater than my rule-of-thumb of 10 ms, some IO administrators will argue this is not significant enough to motivate change. But what most people miss is that over 25% of the multiblock read requests took more than 16 ms, and 18% of the requests took an excess of 32 ms! That is shocking and makes the argument to reduce IO response time stronger and more urgent.

Figure 5-11. Based on the v$event_histogram view and during the report interval, while the average wait time is 14.9 ms (not shown), it can seen that about 25% of the scattered read waits took more than 16 ms. The far-right column is a running total.

Unfortunately, the histogram granularity is limited. The millisecond increment shown in Figure 5-11 is the most detailed time breakdown we can get. This is particularly vexing, because most of the really interesting Oracle wait activity happens around 10 ms. When working in the millisecond neighborhood, a jump from 8 ms to 16 ms is massive, so we lose some valuable diagnostic information. However, the information available is helpful.