Dividing the Backup of a Large Datafile into Sections

If you specify the SECTION SIZE parameter on the BACKUP command, then RMAN creates a backup set in which each backup piece contains the blocks from one file section. A file section is a contiguous range of blocks in a file. This type of backup is called a multi-section backup.

Note:
You cannot specify SECTION SIZE in conjunction with MAXPIECESIZE.
The purpose of multisection backups is to enable RMAN channels to back up a single large file in parallel. RMAN divides the work among multiple channels, with each channel backing up one file section in a file. Backing up a file in separate sections can improve the performance of backups of large datafiles.

If a multisection backup completes successfully, then none of the backup sets generated during the backup contain a partial datafile. If a multisection backup is unsuccessful, then it is possible for the RMAN metadata to contain a record for a partial backup set. RMAN does not consider partial backups for restore and recovery. You must use the DELETE command to delete the partial backup set.

If you specify a section size that is larger than the size of the file, then RMAN does not use multisection backup for the file. If you specify a small section size that would produce more than 256 sections, then RMAN increases the section size to a value that results in exactly 256 sections.

To make a multisection backup:

Start RMAN and connect to a target database and recovery catalog (if used).

If necessary, configure channel parallelism so that RMAN can parallelize the backup.

Execute BACKUP with the SECTION SIZE parameter.

For example, suppose that the users tablespace contains a single datafile of 900 MB. Also assume that three SBT channels are configured, with the parallelism setting for the SBT device set to 3. You can break up the datafile in this tablespace into file sections as shown in the following example:

BACKUP
  SECTION SIZE 300M
  TABLESPACE users; 

In this example, each of the three SBT channels backs up a 300 MB file section of the users datafile.