Multiblock writes are more efficient than single-block writes, so the database writer builds up a dirty list before it writes. Over the years, Oracle has changed the instance parameters and algorithm to control the minimum dirty list batch size. In fact, Oracle has even used a self-adjusting algorithm to account for bursts in dirty buffers. When the database writer is in the midst of a multiblock write, it will post the wait event db file parallel write.

The v$session_wait parameters are documented to provide the number of blocks being written, but I have personally not found this accurate. Once again, I turn to operating system tracing. Figure 6-22 shows the result of tracing an active database writer process. There are three columns: the pwrite command, the number of bytes written, and the number of 8KB Oracle blocks written. Your system will look different, but don't be surprised to see the database writer submit a write request of more than 100 Oracle 8KB blocks!

Figure 6-22. Operating system tracing is the reliable way to see how many Oracle blocks the database writer is actually submitting for each request. The far-right column is the number of 8KB Oracle blocks written. The largest multiblock database writer write shown in this figure is 36.