Oracle Real Application Cluster (RAC) Architecture, Components, Storage Options and Management

Oracle Real Application Cluster (RAC) is Oracle's parallel cluster system that consists of two main parts: Oracle Clusterware (the cluster‑ready software) and the RAC database instances that run on each node.

In a RAC environment each database instance runs on a separate operating system but all instances access the same Oracle database files stored on shared storage (RAW, ASM, OCFS2, NFS, etc.) via a private network that monitors node health and synchronizes data.

Oracle Clusterware provides the core clustering services. Its key components are Cluster Ready Services (CRS) for resource and node management, a voting disk for quorum and health information, and the Oracle Cluster Registry (OCR) that stores configuration data for the cluster and database.

RAC supports multiple shared‑storage methods: raw devices for high I/O performance, Automatic Storage Management (ASM) for simplified shared‑disk handling, Oracle Cluster File System (OCFS/OCFS2) for cross‑platform file sharing, and third‑party file systems such as GFS or NFS. Each method has distinct performance and maintenance trade‑offs.

The main advantages of RAC include automatic load balancing across nodes, continuous high‑availability service through CRS‑driven failover, horizontal scalability that increases concurrent connections, and parallel execution that improves transaction response times.

However, RAC also introduces higher complexity in administration, requires skilled personnel, can suffer performance degradation if the underlying design is poor, and incurs additional hardware and Oracle Enterprise Edition licensing costs.

RAC process management involves several background daemons: CSS (Cluster Synchronization Services) and its CSSD process, CRS and its CRSD process, OPROCD for I/O fencing, ONS for Fast Application Notification, EVM for event management, and the Global Cache Service (GCS) together with the Global Enqueue Service (GES) which use Cache Fusion to keep SGA buffers consistent across nodes.

When planning a RAC installation, disk space must be allocated for voting disks, OCR, Clusterware binaries, and the database files themselves; the article includes topology diagrams and storage‑size tables (shown in the embedded images).