Oracle data
files can be up to 128 TB in size. Normally, the smallest unit of an RMAN
backup is an entire file. This is not practical with such large files. RMAN can
optionally break up large files into sections and back up and restore these
sections independently. You do this by creating multisection backups, which
break up the files generated for the backup set into separate files. This can
be done only with backup sets, not image copies.
Each file
section is a contiguous range of blocks of a file. Each file section can be
processed independently, either serially or in parallel. Backing up a file into
separate sections can improve the performance of the backup operation, and it
also allows large file backups to be restarted.
A
multisection backup job produces a multipiece backup set. Each piece contains
one section of the file. All sections of a multisection backup, except perhaps
for the last section, are of the same size. There are a maximum of 256 sections
per file.
Note: You should not apply large values
of parallelism to back up a large file that resides on a small number of disks,
as that would defeat the purpose of the parallel operation; multiple
simultaneous accesses to the same disk device would be competing with each
other.
This
feature is built into RMAN. No installation is required beyond the normal
installation of Oracle Database 11g. COMPATIBLE must be set to at least
11.0, because earlier releases cannot restore multisection backups.
Creating
RMAN Multisection Backups
The BACKUP
and VALIDATE DATAFILE commands accept the following option:
SECTION SIZE
<integer> [M | K | G]
Use this to
specify your planned size for each backup section. The option is both a backup
command– and backup spec–level option, so that you can apply different section
sizes to different files in the same backup job.
Viewing
Metadata About Your Multisection Backup
•
The
V$BACKUP_SET and RC_BACKUP_SET views have a MULTI_SECTION column, which
indicates whether this is a multisection backup or not.
•
The
V$BACKUP_DATAFILE and RC_BACKUP_DATAFILE views have a SECTION_SIZE column,
which specifies the number of blocks in each section of a multisection backup.
Zero means a whole-file backup.
