Figure 8-18. Shown are the log writer (LGWR) and archive (ARCH) background processes, three online redo log groups, and a disk containing all the archived redo logs. The log writer background process is currently writing to online redo log group RLOG 100, and the archive background process is reading online redo log group RLOG 300 and writing it to the archived redo log disk.

To complete a database checkpoint, the database writer must write every associated dirty buffer to disk, and then every database file and control must be updated with the latest checkpoint number. Keep in mind that regardless of the archive mode, Oracle promises that committed transaction data will reside in an Oracle database file, either in .dbf files or redo log files.

Using Figure 8-18 as a reference, suppose the log writer (LGWR) background process just finished writing to redo log group RLOG 100, and is ready switch to redo log group RLOG 200 and begin writing. However, the database writer background process (not shown) is still finishing the checkpoint related to the redo entries in redo log group RLOG 200. If the log writer background process were to switch and begin writing into redo log group RLOG 200, it would overwrite committed transaction details that at that time may only reside in the buffer cache. If the instance crashed, the committed transactions, which reside only in the buffer cache, would be lost forever! To prevent this, in this example, the log writer background process will wait to switch and wait to begin writing into redo log group RLOG 200 until the database writer background process has finished writing all the dirty buffers related to the checkpoint. While the log writer background process is waiting (effectively stopped writing) it will post a log file switch (checkpoint incomplete) wait event.