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Don Burleson Blog 







Oracle 11g RAC Components and Clusterware

Oracle Database Tips by Donald Burleson

RAC Components

At a very high level, RAC architecture consists of these components:

  • Physical nodes or hosts

  • Physical interconnects and interconnect protocols

  • Oracle Clusterware

  • Oracle instances and cache fusion

  • Shared disk system

  • Clustered file system, raw devices, network file system, Automatic Storage Management

  • Network services

  • Workload Management Services - Virtual IP configuration

A complete Oracle instance consists of disk files, shared memory structures, and background processes. The shared memory area is further subdivided into numerous caches and pools which are used to transfer data, programs, and instructions from processes to and from the disks and users.

Oracle Clusterware

Each of the instances in the cluster configuration communicates with other instances by using the clusterware. Clusterware is the middleware that glues all the clustered instances and projects a single database system image. 


Cluster platforms depend on the cluster management program to track the cluster node status.  Clusterware allows clustering of servers so that they act as a single system.  Clusterware is supported and required on every operating system that is certified for Oracle RAC.

Online Install Guides for Clusterware and RAC

Oracle Clusterware must be installed before installing the Oracle database software.  Below are the URLs of two excellent click–by-click Oracle 11g RAC install guides.  Thanks to Jeff Hunter and Tim Hall for all their hard work to provide these excellent resources.  Jeff's guide prints out to be over 130 pages!


Jeff Hunter's install guide:


Tim Hall's install guide:


Oracle Clusterware was released with Oracle 10g and was known as CRS .  The product was renamed to Oracle Clusterware in version Oracle 10g R2.  Proving that the rename of CRS to Clusterware was more marketing than anything, the reference to the clusterware home directory still uses the name CRS_HOME.

Oracle Clusterware Explained

From Oracle:

"In the event of a system failure, clustering ensures high availability to users.  A redundant hardware component, such as additional nodes, interconnects, and disks, allow the cluster to provide high availability.  Such redundant hardware architectures avoid single points-of-failure and provide exceptional fault resilience.


In Real Application Cluster environments, Oracle Clusterware monitors and manages Real Application Cluster (RAC) databases.  When a node in a cluster is started, all instances, services and listeners are started automatically.  If an instance fails, the clusterware will restart the instance so the service is often restored before the administrator notices it was down.


"In this sense, Oracle Clusterware is the basis for Oracle Real Application Clusters.  Therefore, there needs to be one incarnation of the Oracle Clusterware on every node of the cluster that an Oracle RAC Database Instance is supposed to run on." (Quoted from Oracle)


Additionally, clusterware provides node monitoring for the other nodes in the system via a heartbeat signal sent over the cluster interconnect.  All nodes in a RAC cluster keep track of which nodes are available.


The shared storage provides concurrent access by all the cluster nodes to the storage array. The storage array is presented in the form of logical units (LUNS) to the cluster host or node and the file system is mounted on all nodes. Thus, when the same file system is mounted and used on all nodes in the cluster, it is called a cluster file system.


Oracle 10g and 11g provides a flexible and high performing shared storage methodology that is known as Automatic Storage Management (ASM). ASM may used in lieu of the cluster file system.

Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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