A standard RMAN backup set contains one or more backup pieces, and each of these pieces consists of the data blocks for a particular datafile stored in a special compressed format. When a datafile needs to be restored, therefore, the entire datafile essentially needs to be recreated from the blocks present in the backup piece.
An image copy of a datafile, on the other hand, is much faster to restore because the physical structure of the datafile already exists. Oracle 10g now permit image copies to be created at the database, tablespace, or datafile level through the new RMAN directive BACKUP AS COPY. For example, here is a command script to create image copies for all datafiles in the entire database: