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Recovery Catalog Sizing Information

Because sizing a recovery catalog is not an exact science, the following table, included in some Oracle documentation, should be used with caution. If a separate database is created for the recovery catalog, use these guidelines for one year's space requirements for one production database.
The following table contains legacy information which applied to Oracle 8, but does not apply to later versions of Oracle because the computing capabilities of the succeeding servers are able to accomodate more
  1. disk space
  2. ram and
  3. CPU (processing power) .

Table Space requirements
System 50MB
Temp 5 MB
Rollback 5MB
Recovery catalog 10MB
Online logs* 1MB each
Recommendation is three groups with two members per group.
As you can see from this table, the space requirements for a recovery catalog are fairly small, between 75MB and 100MB.

RMAN Architecture

You can start performing backups with RMAN without installing or configuring any components. Simply invoke the RMAN client by using the RMAN executable (named rman) from the $ORACLE_HOME/bin directory, and you are ready to initiate backup. Just specify the target database you want to work with at the command line.
You can perform backup and recovery actions with RMAN through the RMAN client or through the Enterprise Manager GUI. In addition to the RMAN client, you may use additional optional components to make your backup and recovery strategy robust and easy:

Recovery catalog: The target database control file will always store the RMAN repository, which is the set of RMAN-related backup and recovery information. This data is also referred to as RMAN's metadata. However, it is smarter to use a dedicated database to store the RMAN repository. You can then create a special schema called the recovery catalog in this dedicated database and have RMAN store its repository in it, thus avoiding the risk of the critical metadata being overwritten when the control file runs out of space. As you will see, using a recovery catalog has several other advantages.
fast recovery area: This is a location on disk where the database will store the backup- and recovery–related files. This is also optional but highly recommended.
Media management layer: As mentioned earlier, RMAN can directly interact only with disk drives. If you want to use tape drives to store your backups, you will need a media management layer in addition to RMAN, since RMAN cannot directly interact with the tape drives. You can use any of several Oracle-certified third-party media management layers. Oracle also provides Oracle Secure Backup, which it claims is the "most well-integrated media management layer for RMAN backups. Together, RMAN and Oracle Secure Backup provide a complete end-to-end backup solution for all Oracle environments. An RMAN session in Unix/Linux systems consists of the following processes:
  1. The RMAN client process.
  2. A default channel, which is the connection to the target database.
  3. Additional channels you allocate and the corresponding target connection to each of the target databases.
  4. If you’re using the recovery catalog, there will be a catalog connection to the recovery catalog database.
  5. During database duplication or TSPITR operations, there will be an auxiliary connection to the auxiliary instance.
  6. By default, RMAN makes one polling connection to each of the target databases to help monitor the execution of RMAN commands on the different allocated channels.

Overview of RMAN Architecture consisting of 1) Oracle server processes, 2) target database, 3) Fast Recovery Area, 4) default disk location