RMAN Backup Formats:Backup Sets

RMAN can also store backup information a logical structure called a backup set. A
backup set contains the data from one or more datafiles, archived redo logs, or control
files or SPFILE. (Datafiles and archivelogs cannot be mixed together in the same
backup set.) You can also back up existing backup sets into another backup set.

A backup set consists of one or more files in an RMAN-specific format, known as
backup pieces. By default, a backup set consists of one backup piece. For example,
you can back up ten datafiles into a single backup set consisting of a single backup
piece (that is, one backup piece will be produced as output, the backup set consists of
the single file containing the backup piece, and the backup piece and the backup set
that contains it will be recorded in the RMAN repository). A file cannot be split across
backup sets.

When backing up datafiles to backup sets, RMAN is able to skip some datafile blocks
that do not currently contain data, reducing the size of backup sets and the time
required to create them. This behavior, known as unused block compression, means
that backups of datafiles as backup sets are generally smaller than image copy backups
and take less time to write. This behavior is fundamental to how RMAN writes datafiles
into backup pieces, and cannot be disabled.

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