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Oracle Recovery Manager (RMAN) on RAC

Oracle RAC Cluster Tips by Burleson Consulting

Oracle RMAN provides the DBA with a comprehensive backup environment. Using RMAN, a DBA can perform a multitude of backup types. The abilities of RMAN depend entirely on whether a backup catalog is being used or not. A backup catalog is a set of tables stored in an Oracle database that track backup sets, pieces, and databases. If a catalog is not available, RMAN stores the backup data in the control file of the instance against which it is performing the backup.

RMAN also allows a block level backup, where only the changed blocks of a tablespace get backed up. For large databases, the space savings in using a block level backup can be enormous.

RMAN is also critical to Automatic Storage Management (ASM).  Since ASM is critical to GRID operations, RMAN is then critical for GRID based systems. In ASM, RMAN is responsible for tracking the ASM filenames and for performing the deletion of obsolete ASM files. ASM files cannot, however, be accessed through normal operating system interfaces; therefore, RMAN is the preferred means of copying ASM files.  It is also possible to use FTP through XDB, but generally, RMAN will be less complex. It is important to learn to use RMAN first because RMAN is the only method for performing backups of a database containing ASM files.

During conversion to ASM, RMAN is used for moving databases or files into ASM storage.

Using RMAN with the Oracle Enterprise Manager (OEM) allows for the scheduling of backup jobs with automated notification capabilities. Using OEM requires that Oracle Intelligent Agents be running on all RAC nodes. The configuration of Oracle Intelligent Agents can sometimes be complex, and the DBA needs to be careful in the setup of the OEM agents in a RAC environment. Later sections will provide a look at various RMAN scenarios in detail.

Third Party Solutions

Many providers of SAN and NAS storage also provide backup capabilities with their hardware. Complete backups of databases can be maintained with no impact on database operations with the use of shadow volumes or the Third Mirrors, EMC�s Business Continuance Volumes (BCV). Recovery using third party solutions can also be incredibly fast, as these solutions often either mirror the entire database to a second location or provide only for backup of changed blocks.

Backup of RAC Databases

The two major backup modes are cold, where the database is shutdown, and hot, where the database is open and in use. An Oracle DBA can use either of these modes to backup a RAC database. However, the main reason for utilizing Oracle RAC is to provide uninterrupted service to the customer base, so it makes little sense to use a cold backup with RAC because it requires the shutdown of all instances that are using the Oracle database.

The DBA must also ensure that all archive log files from all instances are gathered into the backup set.  If even a single archive log is missed, the integrity of the backup is placed in serious question.

Using RMAN for Backups

The DBA should seriously consider using RMAN for all RAC backups. By following a few simple requirements, a RAC instance can be backed up with little or no human intervention. If GRID and ASM storage are used for storage, RMAN must be for backups.

The RMAN process can only attach to a single RAC instance at any one point in time. The RMAN connection is called a utility connection because it does not perform any of the backup or restore operations itself.  The connection is only an information gathering tool for RMAN.

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