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 Oracle 11g Clusterware Features
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Oracle Clusterware is required

As stated by Oracle’s documentation, “Oracle Clusterware is a requirement for using Oracle 11g RAC, and it is the only clusterware that you need for most platforms.  Although Oracle 11g RAC continues to support select third-party clusterware products on specific platforms, you MUST also install and use Oracle Clusterware.”


The Oracle Universal Installer (OUI) installs clusterware on each node.  The clusterware home is called CRS home. The CRS home is distinct from the RAC-enabled Oracle home. The CRS home can be shared by one or more nodes, or be private to each node. Vendor clusterware may be installed with Oracle Clusterware for all UNIX-based operating systems except Linux.


The Oracle Cluster Registry (OCR) contains cluster and database configuration information for RAC Cluster Ready Services (CRS), including the list of nodes in the cluster database, the CRS application, resource profiles, and the authorizations for the Event Manager  (EVM). The OCR can reside in a file on a cluster file system or on a shared raw device.  Raw devices will not be supported in future versions of Oracle (Metalink 578455.1).

Clusterware Features

The clusterware software is installed in the cluster with its own set of binaries. The CRS Home and Oracle Home are in different locations. The clusterware software installs both the Voting Disk file and the OCR file. Installation of clusterware configures the Virtual IP interface. CRS resources can also be managed by the srvctl utility.


Clusterware has many daemon processes. They are as follows:

  • CRDS – The CRS Daemon is the main background process for managing the HA operation of the service. It manages the application resources defined within the cluster. It also maintains the configuration profiles stored in the Oracle Configuration Repository.  Process will restart upon failure.

  • OCSSD – Process that manages the Cluster Synchronization Services (CSS) daemon. Manages cluster node membership and runs as oracle user. Failure of this process results in cluster restart.  Failure of OCSSD causes a node restart.

  • EVMD – This is event management logger. It monitors the message flow between the nodes and logs the relevant event information to the log files. 

  • OPROCD – Cluster process monitor.  This process only runs on platforms that do not use added third-party vendor clusterware.

Cluster Private Interconnect

The cluster private interconnect is a high bandwidth, low latency communication facility that connects each node to other nodes in the cluster and routes messages among the nodes. It is a key component in building the RAC system.


The Oracle RAC cluster interconnect is used for the following high-level functions:

  • Monitoring Health, Status, and Synchronize messages

  • Transporting lock management or resource coordination messages

  • Moving the cache buffers (data blocks) from node to node

High performance database computing involves distributing the processing across an array of cluster nodes. It requires that the cluster interconnect provide high-data rates and low-latency communication between node processes.


The interconnect technology that is employed while connecting RAC Nodes should be scalable to handle the amount of traffic generated by the cache synchronization mechanism. This is directly related to the amount of contention created by the application. The more inter-instance updates and inter-instance transfers there are, the more message traffic generated. It is advisable to implement the highest bandwidth/lowest latency interconnect that is available for a given platform.  If the server vendor and budget supports it, consider InfiniBand or 10 Gigabit Ethernet.


The volume of synchronization traffic directly impacts the bandwidth requirement. The interconnect is not something that should be under configured. 


The next section will help define the difference between a database and database instance. Understanding the difference is important in order to appreciate Oracle RAC.


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