Session 205 in Figure 2-9 is primarily waiting for the operating system to complete multiblock IO requests. However, the operating system is doing a fantastic job, as the average time to retrieve multiple blocks is 1.12 ms. For self-preservation, I would not recommend confronting the IO subsystem team and implying their IO subsystem is not meeting expectations. In fact, as I'll discuss later, it very likely the operating system bottleneck is related to the CPU!

Sometimes, you will want to know what is occurring at this very moment or what just occurred. When you need to know very specific details about the wait you are observing, v$session_wait shines. This view gives you a real-time view of a session's wait situation. It shows the current wait or the most recent completed wait event.

It is common to catch a session in the middle of a wait. For example, refer back to Figure 2-6 or Figure 2-7. If we just happened to query from v$session_wait while that session was waiting for the operating system to return the multiblock request, we would see that specific Oracle session posting a db file scattered read.