Tales From A Lazy Fat DBA

Difference between Image Copies and BackupSets (RMAN):

Image Copies could be of two types:
1. RMAN Image Copies
2. O.S Image Copies

RMAN Image Copies
Use the RMAN copy command to create an image copy. If the original file needs to be replaced, and if the image copy is of a datafile, then you do not need to restore the copy. Instead, Oracle provides a switch command to point the control file at the copy and update the recovery catalog to indicate that the copy has been switched. Issuing the switch command in this case is equivalent to issuing the SQL statement ALTER DATABASE RENAME DATAFILE. You can then perform media recovery to make the copy current. They have an advantage of checking for Corrupt Blocks as they are being read by RMAN.

O/S Image Copies
Oracle supports image copies created by mechanisms other than RMAN, also known as O/S copies. For example, a copy of a datafile that you make with the UNIX cp command is an O/S copy. You must catalog such O/S copies with RMAN before using them with the restore or switch commands.
You can create an O/S copy when the database is open or closed. If the database is open and the datafile is not offline normal, then you must place the tablespace in hot backup mode, that is, issue the SQL statement ALTER TABLESPACE BEGIN BACKUP before creating the copy.

BackupSets:
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. 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.

Main Differences:
•RMAN supports binary compression of backup sets, where the backup set contents are compressed before being written to disk using a compression algorithm tuned for compression of datafiles and archived log files. But with Image copy no compression is done.

•A backup set never contains empty blocks. As RMAN passes through the datafiles, it simply skips blocks that have never been used. But image copy contains empty blocks. This means that backups of datafiles as backup sets are generally smaller than image copy backups and take less time to write.

•Incremental backups can’t be performed with Image copy but incremental backups can be taken over backup sets.

•RMAN can take backup of backup sets directly to tape, if you have installed the RMAN drivers for your tape library.But RMAN can’t take backup of image copies directly to tape.