Recovery Manager: 07 Creating compressed backups

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Untill Oracle 9i version you could reduce the size of backups backing up only used blocks and skipping unused blocks: in this way you would reduce only the backup sizes of datafiles that were oversized or had significant free space.

Oracle 10g RMAN has introduced the capability to compress backups: indeed it is possible now to compress backups regardless of the contents of the datafiles, using the new BACKUP AS COMPRESSED command.
Compressed backups work only with backup sets (database, tablespace, and datafile backup sets), so you cannot compress image copies.

The following command will backup an entire database using a compressed backup
set:

RMAN> backup as compressed backupset database;

To configure the default backup type for a compressed backup set use the
following command:

RMAN> configure device type disk backup type to compressed backupset;

Compressed database backup sets are compressed at approximately a 5-to-1
ratio, or 20 percent of the size of a standard backup set.

Recovery Manager: 06 Creating image copies

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An image copy in RMAN is equivalent to an operating system copy command such as cp or dd in Unix, or COPY in Windows. Image copies can be stored only on disk, so you cannot use tape channels for image copies, but if multiple files are to be copied, then you can consider parallelism.
This means that many of the great features of backup sets (such as incremental backup, compression, writing directly to tape, or controlling the size of the output pieces) cannot be used.
But it means that a restore can be very fast, because there is no need to extract the file from a backup set.

To configure the default backup type for an image copy use the
following command. This parameter or setting configures the type of backup to be an image
copy:

RMAN>configure device type disk backup type to copy;

The following command shows an example of using the RMAN COPY command to create an image copy of two database files (the SYSTEM tablespace and a control file) to the /u01/app/oracle/flash_recovery_area/ directory:

RMAN> run
2> {
3> allocate channel ch1 type disk;
4> copy datafile 1 to '/u01/app/oracle/flash_recovery_area/SYSTEM01.DBF' ,
5> current controlfile to '/u01/app/oracle/flash_recovery_area/CONTROL01.ctl';
6> }

In the above image copy example, the location of the SYSTEM01.DBF file must be known before you do the copy or you have to know that the datafile number 1 is your SYSTEM datafile. Anyway you can understand this requires a lot of extra work.
Likely in Oracle 10g, there is the new BACKUP AS COPY command: now you can perform image copies of an entire database, multiple tablespaces, datafiles, and archive logs without having to specify all of the individual files.
The following command shows an example of using the new RMAN BACKUP AS COPY command to create an image copy of an entire database:

RMAN> backup as copy database;

If you have not changed the default configuration the above command launchs one disk channel
and copies all the datafiles and the controlfile to the flash recovery area.
The following command instead will backup all the archive log files to the flash recovery area, deleting all the input file after the command is successful completed.

RMAN> backup as copy archivelog all delete all input;