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 Oracle Database Related and Data Files
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Database Related Files

Oracle database is a collection of physical files. These are the operating system files used by the database and database instance. Oracle RAC needs shared storage to store the files. RAC follows the shared disk model where all the cluster nodes share the same disk or storage volumes.  This is called a “shared-everything” cluster. 

 

The files included in the RAC architecture are shown in Figure 2.4. Most of them must be available simultaneously and be updateable by all the nodes/instances in the cluster. Some files can remain on the local file system.

 

Figure 2.4:  Database Related Files 

Data Files

These .dbf files are the main files of the database. The data files contain the actual data. The logical database structures like tables and indexes are physically stored in the data files. In RAC, these files are located on shared storage and are accessible by all the nodes in the cluster. One or more data files form a logical unit of database storage called the tablespace.

 

Data files can be associated with multiple instances but only one database. By locating the data files either on a clustered file system, a network file system, or a raw partition, they are made accessible by all the nodes.

Control Files

The control files contain entries that specify the physical structure of the database. This small binary file is continuously updated when a database instance is online.  The control files contain the key information such as the database name, name and location of the data files, and redo log files for the database.

 

When an instance is launched, the control files identify the data and redo log files. Control files should be multiplexed and located on the shared storage.

Redo Log Files

Oracle defines the redo log as the most crucial structure for recovery operations.  A redo log is made up of redo entries that are also called redo records. The primary function of the redo log is to record all changes made to data.  In a high update database, moving the redo logs to separate disks is advised.  Every database has a set of redo log files. The information in the redo log files is 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 and job security.

 

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 for recovery purposes.  Another important use of redo logs is hot mining of redo log files by Oracle streams where redo log files are scanned in order to propagate the changes to other Oracle database systems. 

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