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Sunday, 9 September 2012

Introduction to Oracle 9i RAC

In an Oracle RAC environment, two 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. The main aim of Oracle RAC is to implement a clustered database to provide performance, scalability and resilience.
Oracle RAC depends on the infrastructure component Oracle Clusterware to coordinate multiple servers and their sharing of data storage
Oracle Clusterware is the cross platform cluster software required to run the Real Application Clusters (RAC) option for Oracle Database. It provides the basic clustering services at the operating system level that enable Oracle software to run in clustering mode. It enables the nodes to communicate with each other, allowing them to form the cluster of nodes which behaves as single logical server.
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.
Prior to Oracle 9i, 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. In order to resolve this issue Oracle 9i RAC uses a dedicated network connection for communications internal to the cluster.
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