On most Oracle systems, unless extremely rapid commits are occurring, the log writer background process will have a chance to batch 1MB of redo before the 1MB flush rule triggers the sequential multiblock write. Figure 8-10 shows a snippet of a typical active log writer operating system trace file.

Figure 8-10. Shown is an operating system trace of a very busy log writer on a Linux Oracle Database 10g Release 2 system. Notice the gettimeofday calls are not tightly wrapped around each write call, reducing the gettimeofday call overhead. While not apparent in this snippet, the majority of write calls are 1MB.

The trace file snippet in Figure 8-10 is not what performance analysts like to see. The first write call took 1.3 seconds to complete, followed by 43.4 ms, 33.6 ms, and finally 116.3 ms. Obviously Oracle's IO requirements have exceeded the IO subsystem's capacity. If you see a trace file like this, you are highly likely to see the log file parallel write as your top wait event.