Log file wait events

Just as Oracle sessions must wait for database file I/O, they must also wait for log file I/O. Such waits occur whenever a COMMIT statement causes a write to the redo log. The session issuing the COMMIT waits on the log file sync event. When the log writer issues the I/O request, it waits on the log file parallel write event.

Both these wait events are inevitable, and often 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 below one-hundredth 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.

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