The online redo logs do not post data into the tablespaces. That is not their job. The online redo logs are used for database recovery. These structures are used for "rolling forward" any changes made to the database in a recovery situation.
Furthermore, a COMMIT is not complete until the transaction has been written to the online redo logs. Notice that a COMMIT does not have to force data to be written to any tablespace's datafiles. The datafiles can be updated at a later time by the DBWR process.
When you take a backup while the database is up and running, then any changes made to the database are still logged in the online redo log files. This assists us in recovery should a failure take place.
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This was first published in February 2002