Oracle in a Nutshell

Archived redo logs

While reading the previous explanation, you might have wondered how to avoid losing the critical information in the redo log when Oracle cycles over a previously used redo log.

There are actually two ways to address this issue. The first is quite simple: you don’t avoid losing the information, and you suffer the consequences in the event of a failure. You will lose the history stored in the redo file when it’s overwritten. If a failure occurs that damages the datafiles, you must restore the entire database to the point in time when the last backup occurred. No redo log history exists to reproduce the changes made since the time the last backup occurred, so you will be out of luck. Very few Oracle shops make this choice, because the inability to recover to the point of failure is unacceptable—it results in lost data.

The second and more practical way to address the issue caused by recycling redo logs is to archive the redo logs as they fill. To understand archiving redo logs, you must first understand that there are actually two types of redo logs for Oracle:

Online redo logs

The operating system files that Oracle cycles through to log the changes made to the database

Archived redo logs

Copies of the filled online redo logs made to avoid losing redo data as the online redo logs are overwritten

An Oracle database can run in one of two modes with respect to archiving redo logs:

NOARCHIVELOG

As the name implies, no redo logs are archived. As Oracle cycles around the logs, ...