Through Oracle's wait interface, Oracle reports the log writer background process write time as the wait event, log file parallel write. As with other parallel wait events, this simply means a multiblock write. The redo log buffer is structured in relatively small operating system-size blocks (typically 512 bytes), not Oracle-size blocks, so it is normal for log writer activity to be performed with multiple blocks. When Oracle's redo generation requirements overpower the IO subsystem's capacity to quickly process the sequential multiblock writes, the log file parallel write wait event will be one of the top wait events (likely the top one).

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.