As Figure 8-6 clearly shows, even an inactive log writer background process will wake up every 3 seconds, even if it's just to flush the general redo log buffer. However, in Oracle9i Release 2, which introduced redo strands, experiments demonstrate the entire redo log buffer is not flushed every 3 seconds; only the general redo log buffer is flushed.

The output shown in Figure 8-6, which contains no write statements over a 9-second interval, occurred while updates were being issued about once each second! This demonstrates the log writer background process does not flush the enter redo log buffer every 3 seconds. Thankfully, as soon as a commit was issued, the log writer immediately issued a write call to the operating system.

As with the database writer background process, the log writer background process sleeps by issuing a semaphore call. (As you learned in Chapter 3, when a server process sleeps, in contrast to both the database and log writer background processes, it issues a select call.) This allows another process to wake up the log writer, triggering it to flush the general redo log buffer. For example, if the database writer background process wants to write a dirty buffer to disk, it must ensure any redo associated with the buffer has already been successfully written into an online redo log group. So, the database writer background process will trigger the log writer background process to write and post the log file sync wait event. When the log writer background process is finished writing, the database writer background process can issue its write statement to the operating system.