Backup Sets and Image Copies:

The backup command lets you make two types of RMAN backups: backup sets and image

copies. By default, all RMAN backups are in the form of backup sets. Each backup set contains one or more backup pieces, which are files in an RMAN-specific format. Backup sets are the default backup type for both disk- and tape-based backups.

A backup set is a logical structure that consists of a minimum of one backup piece, which is a physical, RMAN-specific format file that actually contains the backed-up data. A backupset can contain data from one or more datafiles, archived redo log files, or control files.

By default, a backup set contains just one backup piece. However, you can limit the size of a backup piece by using the maxpiecesize parameter. If you do this and the backup set size is larger than the backup piece size specified by the maxpiecesize parameter, there’ll be multiple backup pieces within that backup set.

 RMAN determines the number of backup sets for a backup according to an internal algorithm. However, you can limit the size of a backup set by specifying the maxsetsize parameter. You can also indirectly control the number of backup sets made by RMAN for each backup by specifying the filesperset parameter, which limits the number of input files (datafiles, archived redo log files, and so on) that can be backed up into a single backup set.

The key difference between an image copy and a backup set is that:

• RMAN can write blocks from many files into the same backup set (known as multiplexing) but can’t do so in the case of an image copy—an image copy is identical, byte by byte, to the original datafile, control file, or archived redo log file.
•  An RMAN image copy and a copy you make with an operating system copy command such as dd (which makes image copies) are identical.
• Since RMAN image copies are identical to copies made with operating system copy commands, you may use user-made image copies for an RMAN restore and recovery operation after first making the copies “known” to RMAN by using the catalog command.
• You can make image copies only on disk but not on a tape device. "backup as copy database;" Therefore, you can use the backup as copy option only for disk backups, and the backup as backupset option is the only option you have for making tape backups.

After this point, there’s no difference between those image copies made by you and those made by RMAN. During a restore operation, if you have both image copies and backup sets from the same time period, RMAN prefers to use an image copy over a backup set.

• This is because there is more overhead involved in sorting through a backup set to get the files to restore.
• In addition image copies offer yet another benefit during a restore and recovery operation. If you need to restore a current datafile and happen to have an image copy of that datafile available, you can use the switch command to simply point the database to the replacement file instead of the original datafile. This eliminates the need to restore the datafile, thus speeding up database recovery considerably.

Deleting data doesn't reduce the size of the backup

You sometimes may have a fairly large database that is taking many hours to backup. You are doing full image backups. You or your users may be tempted to delete data to "reduce the size of the backup" and/or to "reduce the time it takes to backup" and/or to "reduce the duration that datafiles are in backup mode".
Unfortunately, merely deleting rows [even 10% or 99%] in table[s] doesn't really reduce the size or duration of most image backups. The delete operation only adds to the total amount of undo and a very large increase in redo and archivelog generation for which you may have to scramble for disk space.
An image backup [whether by "cp" or "cpio" or "tar"] of the Datafiles will still see the Datafiles as having the same size because the delete doesn't shrink the datafiles.
An RMAN backup will still have to backup as many datablocks as before the delete. This is true _even_ if you are successful in deleting rows in such a manner that some or many blocks are 100% empty. Once a datablock has got formatted it will always be backed-up by RMAN, whether there are rows or no rows in it subsequently.