I just want to raise a warning flag for DBAs using RMAN and flash recovery area in Oracle 10g.

An RMAN script is usually something like:

backup database plus archivelog

However, the behavior of plus archivelog depends on whether optimization is enabled or disabled in RMAN. The RMAN reference document says that when the plus archivelog command is part of a backup script, RMAN:

1. runs an ALTER SYSTEM ARCHIVE LOG CURRENT statement.
2. runs the BACKUP ARCHIVELOG ALL command. Note that if backup optimization is enabled, RMAN only backs up logs that have not yet been backed up.
3. backs up the files specified in the BACKUP command.
4. runs an ALTER SYSTEM ARCHIVE LOG CURRENT statement.
5. backs up any remaining archived redo logs

As the result, if backup optimization is off (which it is by default in RMAN), plus archivelogs will backup all archivelogs. If the archivelog destination is a flash recovery area, your database may keep very old archivelogs (assuming space is not an issue). Thus, the backup size will gradually grow and fill up the flash recovery area, since each backup includes all archivelogs in the flash recovery area.

The lesson is, to avoid backing up archivelogs that have already been backed up when using plus archivelog in a backup script, make sure you enable RMAN optimization.

You can also find more details at http://oradbatips.blogspot.com/. Hope this helps.

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