Q

How does the Oracle LGWR write to online redo log files?

Learn how the Oracle LGWR writes both uncommitted and committed transactions from the log buffer to the online redo log files in this tip from Oracle expert Brian Peasland.

How does the Oracle LGWR write the log buffer to the online redo log? Does it write out the entire log buffer or just the committed records? If it writes out just the committed records, would it increase 'log file parallel write' time for using a large log buffer?
LGWR does write change records in the log buffer to the online redo log files. When a user issues a COMMIT, the COMMIT is not complete until the change records have been written to the online redo logs. However, large transactions may fill the log buffer before a COMMIT is issued. For this reason, LGWR will write to the online redo logs if the log buffer becomes one-third full or after there is 1MB of new redo in the log buffer, or every 3 seconds, or before DBWn writes to the database files. As you can see, LGWR does not write just committed transactions, it may write uncommitted ones as well.
This was first published in November 2009

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