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RMAN(Recovery MANager) Backup

RMAN backup is a block level backup(oracle block).
RMAN is a centralised backup and restore and recovery utility.
We can configure RMAN in 2 modes : catalog mode
2.catalog mode
Recommanded to configure RMAN in catalog mode. In no catalog mode, RMAN stores the configuration parameters information and backup information in the target database controlfile(reusable section).
The retention of the backup information depends on the parameter "controlfile_record_keep_time"(default value 7 days). Even if we specify a bigger value, the retention may not be guaranteed since control file grows upto Operating System limitation.

In no catalog mode, we cannot create stored RMAN scripts which can be used across the Enterprise databases.
In no catalog mode, we cannot use "keep until time" clause to retain a backup for a specific time interval.

RMAN backup retention is based on 2 factors
1.retention based on number of backups.
parameter : configure retention  policy to redundancy <value>;
2.retention based on number of days.
parameter : configure retention policy to recovery window of <x days>;

In case of RMAN, excessive redo information will not be generated.
Execution of every RMAN backup command produces backup sets in the backup destination.
A backup set is nothing but collection of backup pieces.
Separate backup pieces will be created for datafiles and archive logs.
If the instance is started with Spfile, by default RMAN takes the backup of Spfile.
For catalog mode configuration, we create a separate database called catalog database.
Catalog is a repository for all target databases and it holds the RMAN configuration parameters information and backup information.
In order to minimise the backup, restore and recovery time, recommanded to configure multiple channels.
A channel is a communication session from catalog database to target database.
Number of channels generally depends on the size of the target database as well as number of processors.
In a uni processor based system, even if we configure multiple channels, we dont gain much performance benefits.

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