There are a variety of database writer-related wait events. One way to categorize the contention situation is to understand if the database writer is having "push-to-disk" issues or "pull-from-write-list" issues. Most of the issues are pushing-that is, writing to disk issues. But there is a very common pull situation that I'll detail in the next section.

All wait events related to database writer push-to-disk issues start with db file. As with other IO events, just before and after the IO call, a gettimeofday call is made, and the difference is what we see through Oracle's wait interface. Here are the two common database writer push-to-disk wait events:

* db file parallel write: By the far the most common database writer wait event, a parallel write is simply a multiblock write, like most of the calls shown in Figure 6-24. This is the result of the database writer feeding off the write list and writing a batch of dirty blocks to disk. I like to see the wait time below 5 ms, but every organization has its own budget and service level requirements. A write less than 5 ms indicates write caching is working well. The write calls in Figure 6-24 range from around 1.5 ms to nearly 40 ms!