If you've got Oracle 10g, this is a really cool feature that allows for synthetic full backups inside Oracle.
If you're backing up to disk with Oracle (actual disk, not a VTL), you can select COPY as the backup type instead of BACKUPSET. A backupset is the "normal" backup, which is basically a blob of data that contains one or more data files (think tar file). A COPY, on the other hand, is simply that. It's a copy of the oracle data file, which is why this backup type exists only on disk. (It's called an image copy.)
If you have an image copy on disk, you can perform an incremental backup and have that incremental backup update the image copy, resulting in a new full backup without having to actually take another full backup (i.e. a synthetic full). That could help a lot when backing up large databases — as long as you've got enough room to store an entire copy of your database.
One bummer is that there doesn't appear to be a way to move that image copy to tape inside RMAN. You could back up the files with regular backup software, but I don't think you'd be able to use RMAN to restore with them, as RMAN will only know about the current, updated version of the backup set. If you wanted to restore a previous version, it wouldn't know what to do with that.
I wonder if you could use these image copies to restore outside of RMAN, as in a user-managed backup. Then I wonder if you could apply redo logs to those restored datafiles. Wish I had some time to play with Oracle to find out.
Anyone know the answers to my questions?