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Lesson 4Multiplexing and backup sets
ObjectiveDiscuss how and why Recovery Manager multiplexes backup sets

Multiplexing | Backup sets (Recovery Manager)

Question: Does Oracle Recovery Manager still multiplex backup sets?
As you are aware by now, Recovery Manager controls the creation and use of backup sets. Part of this control includes the potential combining of information from different data files or archive logs together in a single backup file. To understand how this works and how you can affect it, you have to understand the concept of a backup piece and the filesperset command.

Backup Pieces

Normally, each backup_spec in a backup command will create a single backup file, which is referred to as a backup piece. If you specify multiple data files or archive logs in a backup_spec, the data blocks from these files will be multiplexed together. Multiplexing means that the blocks from the separate data files will be intermingled in a single file. This intermingling makes sense to RMAN, but you might want to keep these blocks separate to have the granularity of your backup pieces match the granularity of your data files.

The filesperset Option

The filesperset option lets you specify the number of files that will be in each backup piece. If you specify three files per set and have three data files for a particular backup_spec, RMAN will use each piece for a separate data file. If you specify three files per set and have more than three data files, RMAN will allocate the data files to the three files, multiplexing where necessary.
The filesperset option can be indicated for the entire backup or for an individual backup_spec.
The following Slide Show illustrates how the filesperset option works in practice:

Multiplexing and filesperset

1) Files per Set 1
1) Without a FILESPERSET option, the data blocks from the three datafiles in the backup spec are multiplexed.

2) Files per Set 2
2) If the number of files in the FILESPERSET match the number of datafiles, Recovery Manager will assign each one to a separate file, avoiding multiplexing.

3) Files per Set 3
3) If there are more files that FILESPERSET, Recovery Mangager arbirtrarily multiples some of the files.

In the next lesson, you will learn how to create backups in parallel.

Backup Sets Properties - Quiz

Click the Quiz link below to test your knowledge of backup sets.
Backup Sets Properties - Quiz