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Redo Log Files

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%.


A redo log is made up of redo entries which are also called redo records. The primary function of the redo log is to record all changes made to data. Every database has a set of redo log files. The information in the redo log files are used to recover the database from a system or media failure. There are generally two or more redo log files. They are used by the database in a circular fashion. Once a redo log file is filled up, then the next redo log file is picked up for writing. Meanwhile, the filled redo log file is saved as an archived log file.

Redo Log files are stored as a group called Redo Log File groups. Each group can have one or more redo log files. Multiplexing the redo files within a group provides a higher level of resiliency.

Redo Log files are instance specific. In the RAC database architecture, each instance has its own set of redo log file groups. Even though they are specific to an individual instance, the redo log files need to be located on shared storage. This is because the redo files have to be accessed by other instances during the media or system recovery. Another important use is the hot-mining of redo log files for the use of Oracle streams where redo log files are scanned by the capture process in order to propagate the changes to other Oracle database systems.

Archive Log Files

Archive Log files are actually the saved redo log files. When a redo log file is filled up and the next redo log file is put to use, the filled redo log file is saved or archived. The saved files are known as archive log files.

Automatic archiving can be enabled on the redo log. Oracle automatically archives redo log files when the database in is in ARCHIVELOG mode.

A separate set of Archive Log files are created by each instance. Since each RAC instance has its own redo log files, the corresponding archive log files are produced when the log switch takes place. The archive log files can be written to either a local file system or to a cluster file system. Oracle does not insist upon a particular type of file system. Writing to a clustered file system has added the advantage of being available to archive all the nodes in the cluster, which becomes important in case of media recovery.

Parameter File (SPFILE)

The attributes of the instance depend on the initialization parameters used for starting up the instance. Initialization parameters control the configuration of the database system. They are the key directives to start and manage any instance in the database. While launching the database instance, parameters are specified and they remain until the instance is shutdown. Optionally, certain parameters can be modified during the instance run time by the ALTER SYSTEM SET method, provided the instance has been started with the SPFILE method.

The SPFILE feature allows the change of parameter values dynamically. It also allows them to be set either permanently or in memory only. For Oracle Real Application Clusters (RAC), one server parameter file can be used and shared among instances. The usage of a single copy of the SPFILE for the entire database provides administrative convenience and simplification. The SPFILE has to be located on a clustered file system.

 


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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