Backing Up Oracle - Computing at Cornell

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The second archivelog backup example is more complex — but also more
safe. Assume that a very important database is configured in the following
way:
Oracle uses multiple log groups
Each group contains multiple log members
At least two log archive destinations are in use
Because the backup archivelog all command backs up exactly one copy of
each distinct log sequence number, RMAN does not put two copies of the
same log sequence number into the same backup set. Furthermore, if you
specify the delete input option, RMAN only deletes the specific copy of the
archived redo log that it backs up.
The easiest solution in this case is to back up both copies of each archived
redo log and then delete both copies. Use the like pathname parameter to
indicate which destination to use. The like parameter allows you to match
filenames in both archive destinations.
84 Backing Up Oracle using Tivoli Storage Management