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Recovery in a Raw File System Environment

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%.

The use of a clustered file system is highly recommended because it is easier to manage, backup, and restore. In a RAW file system, the archive logs from each of the nodes are not available to any of the other nodes for recovery, unless some form of log copy procedure, scripted and run through a system-scheduling program, is employed, or all of the other node?s archive log locations are NFS-mounted to the node performing the recovery operation.

If the NFS mounting scheme is used and one or more nodes are not available, only an incomplete recovery can be performed, up to the first unavailable archive log. Even in a NFS scheme, it is suggested that some form of log copy script be used to allow the unavailable node?s logs to be available for recovery. The frequency of log copying should be based on the required concurrency of the database. If the DBA can only afford to lose 15 minutes worth of data, they had better have all archive logs available up to the last fifteen minute interval before the media failure.

Using RMAN, a recovery using a single tape looks identical to a single tape recovery in the OCFS environment:

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

However, this assumes all of the archived redo logs are available. If all of the logs are not available, a RMAN script similar to the following would be used to recover until the first unavailable log is encountered.  In the example log, sequence 2245 in thread two is the first unavailable log.

RUN
{
SET UNTIL LOG SEQUENCE 2245 THREAD 2'
RESTORE DATABASE;
RECOVER DATABASE;

ALTER DATABASE OPEN RESETLOGS;

The ALTER command is used to open the database. The RESETLOGS option resets the archive log sequences and renders previous archive logs unusable for most recovery scenarios.

More information on OPEN RESETLOGS is available HERE.

Parallel Recovery

Recovery with Oracle Database 10g RAC is automatically parallel for these three stages of recovery:

  • Restoration of data files.

  • Application of incremental backups.

  • Application of redo logs.

The number of channels configured for a RAC database in RMAN determines the degree of parallelism for data file restoration. In the previous example configuration, two streams could have been involved in restoring data files since two channels were configured. The degree of parallelism for restoration of incremental backups is also dependent on the number of configured channels.

Redo logs are applied by RMAN using the degree of parallelism specified in the initialization parameter recovery_parallelism.

In Oracle Database 10g, there is no server manager program, so all DBA functions are done through SQL*PLUS.  Using manual recovery methods such as SQL*PLUS, values can be specified for recovery_parallelism, since it is a dynamic parameter.  However, it cannot exceed the setting for parallel_max_servers. Using the DEGREE option for the RECOVER command, the degree of parallelism can also be controlled for other recovery operations.

Standby Databases in RAC Configuration

The Oracle Database 10g Dataguard feature has revolutionized the standby database concept. It allows Oracle10g to have both physical and logical standby databases, for either normal instances or RAC instances.

In the case of a normal instance, Oracle executes processes on the main node that handle the copying of archive logs to the standby server for physical standby. It also executes processes that read the changes from the redo logs and apply them as SQL statements, in the case of logical standby.

There are two types of standby databases supported for RAC. A standby database can be created on a single node or on a clustered node system. In Oracle10g, the DataGuard Manager does support RAC; however, if its use is desired, the Dataguard configuration must be set up manually.


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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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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