Just as Oracle sessions must inevitably wait for db file I/O, they must also wait for log file I/O. These waits occur when a COMMIT is issued. A COMMIT causes the Redo log writer session to flush the contents of the redo log to the redo file. The user session must wait for this write operation to complete before the COMMIT statement returns control.
The session issuing the commit waits on the log file sync event. When the Log Writer process issues the I/O request, it waits on the log file parallel write event.
Both wait events are inevitable and usually account for between 10-20% of total non-idle wait times in a healthy database.
The average wait time for a log file parallel write is an important measure. It indicates how quickly the log writer process can flush the redo buffer. It is a good indicator of the efficiency of the redo log device. Values of 0.2 hundredths of a second are good, and values of up to five hundredths of a second are not unusual. Values above this range may indicate contention for the redo log device. Improve Redo Log Writer Performance
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