Oracle RAC 10g Overview

Oracle RAC, introduced with Oracle9i, is the successor to Oracle Parallel Server (OPS). RAC allows multiple instances to access the same database (storage) simultaneously. It provides fault tolerance, load balancing, and performance benefits by allowing the system to scale out, and at the same time—because all nodes access the same database—the failure of one instance will not cause the loss of access to the database.

At the heart of Oracle RAC is a shared disk subsystem. All nodes in the cluster must be able to access all of the data, redo log files, control files and parameter files for all nodes in the cluster. The data disks must be globally available to allow all nodes to access the database. Each node has its own redo log and control files but the other nodes must be able to access them in order to recover that node in the event of a system failure.

One of the bigger differences between Oracle RAC and OPS is the presence of Cache Fusion technology. In OPS, a request for data between nodes required the data to be written to disk first, and then the requesting node could read that data. With cache fusion, data is passed along a high-speed interconnect using a sophisticated locking algorithm.

Not all clustering solutions use shared storage. Some vendors use an approach known as a federated cluster, in which data is spread across several machines rather than shared by all. With Oracle RAC 10g, however, multiple nodes use the same set of disks for storing data. With Oracle RAC, the data files, redo log files, control files, and archived log files reside on shared storage on raw-disk devices, a NAS, a SAN, ASM, or on a clustered file system. Oracle's approach to clustering leverages the collective processing power of all the nodes in the cluster and at the same time provides failover security.

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