Oracle Real Application clusters allows multiple instances to access a single database, the instances will be running on multiple nodes.
The table below describes the difference of a standard oracle database (single instance) an a RAC environment
The major components of a Oracle RAC system are
Shared disk system (SAN and NAS disk storage systems)
Oracle Clusterware
Cluster Interconnects
Oracle Kernel Components
Shared disk system :> Once you have you storage attached to the servers, you have three choices on how to setup the disks
Raw Volumes – normally used for performance benefits, however they are hard to manage and backup
Cluster FileSystem – used to hold all the Oracle data files can be used by windows and linux, its not used widely
AUTOMATIC STORAGE MANAGEMENT (ASM) – Oracle choice of storage management, its a portable, dedicated and optimized cluster filesystem
Oracle Clusterware :-> Oracle Clusterware software is designed to run Oracle in a cluster mode, it can support you to 64 nodes,
Clusterware software allows nodes to communicate with each other and forms the cluster that makes the nodes work as a single logical server. The software is run by the Cluster Ready Services (CRS) using the Oracle Cluster Registry (OCR)
CRS has four components
OPROCd – Process Monitor Daemon
CRSd – CRS daemon, the failure of this daemon results in a node being reboot to avoid data corruption
OCSSd – Oracle Cluster Synchronization Service Daemon (updates the registry)
EVMd – Event Volume Manager Daemon