Purpose of Online Redo Log Files

Redo log files provide a means and a mechanism for allowing Oracle to redo transactions in the event of a database failure. Every transaction that occurs that alters data in any way on the database (other than transactions such as direct loads, direct reads, and loads that are done with NOLOGGING option) is written synchronously to the redo log files to provide a recovery mechanism in the case of media or other system failure. This includes even transactions that have not been committed, undo segment information, and any schema and object management statements for objects in the database.

Redo log files are used in situations where instances fail. They are used so that you can recover data that had already been committed but that had never made it as far as being written out to the data files. The redo log files are used only for recovery purposes.

The following section looks at the structure of the redo log files and the redo log groups.