An Oracle Real Application Clusters (Oracle RAC) database has multiple database Instances hosted on a RAC Cluster. These instances access a single set of database files on shared storage. Typically, the shared storage is provided by an Oracle ASM instance local to the node hosting the Oracle RAC database instance. Oracle ASM instances can run on a separate physical server from the database servers and a server can access Oracle ASM from another server in the cluster. Every server does not necessarily have a local Oracle ASM instance.
Oracle RAC nodes and instances:
- Nodes: Individual servers participating in the cluster and have its own local storage and operating system.
- Instances: Separate copies of the database running on each node. Every database instance access and manage the same Oracle RAC database files stored on the shared storage.
Shared storage is a high-performance storage system that is accessible by all cluster nodes. It stores Oracle RAC database files, including datafiles, redo logs, and control files, ensuring all instances have access to the most recent data.