Databases Reference
In-Depth Information
In this example, the tablespace system has one data file, size 600m. The
section size
parameter (set to 150m)
breaks up the data file backup into four chunks of about 150m each.
How It Works
In a multisection backup, multiple channels back up a single data file. Each of the channels you specify will back up
a single file section, which is a contiguous set of blocks in a data file. Each of the data file sections is backed up to a
different backup piece.
■
Note
You can't specify the
section size
parameter along with the
maxpiecesize
parameter.
By default, RMAN won't let you make multisection backups for any data file smaller than 1GB, but you can override
this by simply specifying a smaller size than 1GB in the
section size
parameter of the RMAN
backup
command. You can
have up to 256 sections per data file. RMAN makes uniform-size sections, except the very last one, which may or may not
be the same size as all the other sections.
Use the
backup
command clause
section size
to perform multisection backups. If you don't specify a value for
the sections with the
section size
parameter, RMAN computes an internal default section size for that backup job.
Multisection backups offer performance benefits, since you can back up a single data file simultaneously in multiple
sections, thus parallelizing the backup. You also don't have to back up a large file all over again, if the backup fails
midway—you need to back up only those sections that weren't backed up the first time around.
You must set the initialization parameter
compatibility
to at least 11.0 when performing multisection backups,
since it's not possible to restore multisection backups with a release earlier than 11.0.
You can also use the
section size
clause with the
validate datafile
command.
The
section_size
column in both the V$BACKUP_DATAFILE and RC_BACKUP_DATAFILE views shows the
number of blocks in each section of a multisection backup. If you haven't performed any multisection backups,
the section_size column would have a zero value. The V$BACKUP_SET and RC_BACKUP_SET views tell you which
backups or multisection backups. The following example shows a query on the V$BACKUP_DATAFILE view:
SQL> select pieces, multi_section from V$BACKUP_SET;
PIECES MUL
------ --------
1 NO
2 YES
7 YES
4 NO
SQL>
The V$BACKUP_DATAFILE shows information about control files and data files in backup sets. The previous
command shows that data file 7's backup is a multisection backup.
Specifying Backup Windows
Problem
The DBA has a limited window for running the RMAN backups. The backups must complete within this specified
backup window every day.
