Databases Reference
In-Depth Information
How It Works
The size of a backup set made using RMAN equals the sum of the bytes in each of the backup pieces that are part of
that backup set. By default, the maximum size of a backup set is unlimited, as you can see by issuing the following
command:
RMAN> show maxsetsize;
RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE MAXSETSIZE TO 1 G;
RMAN>
The
configure maxsetsize
command applies to both disk and tape backups.
■
Note
You can't specify the number of backup pieces in a backup set.
Since your backup will fail if a large file in the database being backed up is larger than the value of the
maxsetsize
parameter, make sure the value of this parameter is at least as large as the largest data file being backed up by
RMAN. If you're backing up to tape, you run the risk of losing all your data even if just one of the tapes fails. Using the
maxsetsize
parameter, you can force RMAN to back up each backup set to a separate tape, thus limiting the damage
to the contents of only the single failed tape.
Finally, know that the
maxsetsize
parameter doesn't give you absolute control over the size of the backup set
that RMAN will create. The
maxsetsize
parameter is only one of the factors determining the size of backup sets. In
addition to the setting of the
maxsetsize
parameter, RMAN takes into account the following factors when determining
the sizing of RMAN backup sets:
backup
command.
•
Number of input files specified in each
•
Number of channels you allocate. Each allocated channel that's not idle will produce at least
one backup set. RMAN also aims to divide work so all allocated channels have roughly an
equal amount of work to do.
•
Default number of files in each backup set.
•
Default number of files that a single channel reads simultaneously (eight).
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You can't limit the size of image copies. By definition, an image copy must be identical to the original data file,
so you really don't have a choice here regarding the size of a copy—it'll simply be the same size as the original data file.
Note
In Chapter 8, you'll learn about RMAN's restartable backups feature. Following a backup failure, RMAN will back
up only the data that wasn't backed up before. That is, once it's backed up, the same data won't be backed up again
if a backup fails midway. Using the
maxsetsize
parameter of your
backup
command, you can make smart use of this
restartable backup feature. For example, if you set
maxsetsize
to 10MB for a backup, RMAN produces a new backup
set after every 10MB worth of backup output. Let's say your backup failed after backing up 12 backup sets. Following a
restart of the backup after a backup failure, RMAN won't have to back up the data already backed up to the 12 backup
sets before the backup failure.
