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Media Recovery in RAC Instances

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


Media recovery is only required if a physical insult occurs to the database files, preventing proper access to Oracle data or transaction files. The presentation of RMAN in the backup section of this chapter has been intended to instruct the DBA to copy Oracle files, making recovery possible if a physical insult occurs.

If the Oracle database uses self-created scripts for backup, self-generating recovery scripts may be beneficial as well. An example is available on the Rampant website, using the username and password provided with this book. Generating a recovery script and storing it with the backup set assures an easily performed recovery.

Generally, it is wisest to use RMAN or a third party tool to perform backups in complex environments. Any backup and recovery process must be thoroughly tested. An untested backup and recovery plan is no plan at all. The steps for media recovery depend on what files are lost. However, RMAN simplifies this, since it will recover files up to a complete database, if need be.

Once a failed file is detected, RMAN, a third party program, or the DBA?s own scripts and procedures are used to bring the affected file or files back from the backup media. From ARCHIVELOG mode, apply the archived logs to bring the database to a fully recovered state. If archive logs are not used, the loss of any datafile means the database must be fully recovered to the point in time of the last backup.

The node that performs recovery must be able to access all files that need to be recovered. This means that the recovery node must be able to access the online redo logs, datafiles, rollback tablespaces, and all archived redo logs from all database instances. This requirement to see all archive logs may require that the NFS mount strategy be implemented.

Using RMAN to Recover a RAC Environment

As in backup, there are two environments that must be considered when performing a RAC recovery, these are:

* Recovery in an OCFS environment

* Recovery in a RAW filesystem environment

In an OCFS environment, all nodes can see all files. This ability of OCFS greatly simplifies the recovery process. The recovery process using OCFS does not require NFS-mounting or elaborate archive log copying schemes. For example, in the sample environment using RAC nodes AULTLINUX1 and AULTLINUX2, which support instances AULT1 and AULT2, the commands for an OCFS-based recovery could be as simple as:

CONFIGURE DEVICE TYPE sbt PARALLELISM 1;
CONFIGURE DEFAULT DEVICE TYPE TO sbt;
RESTORE DATABASE;
RECOVER DATABASE;

If more than one tape device is available, the parallelism of the recovery can be increased, thus reducing recovery time. For example:

CONFIGURE DEVICE TYPE sbt PARALLELISM 2;
CONFIGURE DEFAULT DEVICE TYPE TO sbt;
CONFIGURE CHANNEL 1 DEVICE TYPE sbt CONNECT 'SYS/kr87m@ault1';
CONFIGRE CHANNEL 2 DEVICE TYPE sbt CONNECT 'SYS/kr87m@ault2';
RESTORE DATABASE;
RECOVER DATABASE;

Since Oracle RMAN uses autolocation, the channel connected to each node restores the files backed up by that node. Remember, the configure commands only have to be issued once.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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