Overview Of Oracle DBA RAC

In database computing, Oracle Real Application Clusters (RAC) - an option for the Oracle Database software produced by Oracle Corporation and introduced in 2001 with Oracle9i - provides software for clustering and high availability in Oracle database environments. Oracle Corporation includes RAC with the Standard Edition, provided the nodes are clustered using Oracle Clusterware.

DBA RAC Overview

Oracle RAC allows multiple computers to run Oracle RDBMS software simultaneously while accessing a single database, thus providing clustering.

In a non-RAC Oracle database, a single instance accesses a single database. The database consists of a collection of data files, control files, and redo logs located on disk. The instance comprises the collection of Oracle-related memory and operating system processes that run on a computer system.

In an Oracle RAC environment, 2 or more computers (each with an Oracle RDBMS instance) concurrently access a single database. This allows an application or user to connect to either computer and have access to a single coordinated set of data.


Oracle RAC depends on the infrastructure component Oracle Clusterware to coordinate multiple servers and their sharing of data storage. The FAN (Fast Application Notification) technology detects down-states. RAC administrators can use the srvctl tool to manage RAC configurations,

Cache Fusion

Prior to Oracle 9, network-clustered Oracle databases used a storage device as the data-transfer medium (meaning that one node would write a data block to disk and another node would read that data from the same disk), which had the inherent disadvantage of lackluster performance. Oracle 9i addressed this issue: RAC uses a dedicated network connection for communications internal to the cluster.

Since all computers/instances in a RAC access the same database, the overall system must guarantee the coordination of data changes on different computers such that whenever a computer queries data, it receives the current version - even if another computer recently modified that data. Oracle RAC refers to this functionality as Cache Fusion. Cache Fusion involves the ability of Oracle RAC to "fuse" the in-memory data cached physically separately on each computer into a single, global cache.
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