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Cluster Ready Services (CRS)

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Oracle10g introduces Cluster Ready Services (CRS), which provides many system management services and interacts with the vendor clusterware to coordinate cluster membership information. It provides the cluster software, sometimes called the Oracle OSD Clusterware.

The OUI installs CRS on each node on which the OUI detects that vendor clusterware is running. In addition, the CRS home is distinct from the RAC-enabled Oracle Home. The CRS home can be shared either by one or more nodes, or privately to each node, depending on the settings used when running the OUI. When vendor clusterware is present, CRS interacts with the vendor clusterware to coordinate cluster membership information.

For Oracle10g on Linux and Windows-based platforms, CRS co-exists with but does not inter-operate with vendor clusterware. Vendor clusterware may be used for all UNIX-based operating systems except for Linux. Even though, many of the UNIX platforms have their own clusterware products, use the CRS software to provide the HA support services. CRS supports services and workload management and helps to maintain the continuous availability of the services. CRS also manages resources such as virtual IP (VIP) address for the node and the global services daemon.

The Oracle Cluster Registry (OCR) contains cluster and database configuration information for Real Application Clusters 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. When installing Real Application Clusters, specify the location of the OCR.

CRS helps to package a set of application that work under CRS control and access the RAC database. The application resource profile defines the resources with which you manage the RAC. Prior to the 10g release, the cluster manager implementations on some platforms were referred to as Cluster Manager. In Oracle10g, Cluster Synchronization Services (CSS) is the cluster manager on all platforms. The Oracle Cluster Synchronization Service Daemon (OCSSD) performs this function on UNIX-based platforms.

On Windows-based platforms, the OracleCSService, OracleCRService, and OracleEVMService provides the cluster manager functionality. The installation of CRS is covered in later sections.

A Note on SKGXP, SKGXN Libraries

Oracle has defined APIs for inter-process communication and node membership. These are the APIs that facilitate integration with vendor clusterware or Oracle provided clusterware. Oracle communicates between processes on instances and does not have to know how the data is moved between systems. The Oracle API for IPC is referred to as system kernel generic interface inter-process communications (skgxp).

The OSD or clusterware normally provides a kernel module that receives communications from the skgxp library and passes it to the correct process on the correct instance on other nodes. It is the responsibility of the implementer to design the communication package in the form of a kernel module in compliance with the specification.

For example, Veritas DBE/AC provides a module to implement the skgxp functionality. This is available in the form of a library (, which is dynamically linked by Oracle at execute time. This skgxp interface utilizes the LLT multiplexer (LMX) via ioctl() calls. The LMX module is a kernel module designed to receive communications from the skgxp module and pass them to the correct process on the correct instance on other nodes. The module multiplexes communications between multiple related processes into a single multi threaded LLT port between systems.

Oracle has provided another set of APIs to provide the membership information to the RAC system. This is known as system kernel generic interface node membership (skgxn). Oracle RAC makes specific skgxn calls and gets the required membership information. The skgxn library, in turn, makes ioctl() calls to a kernel module for membership information. This kernel module is known as VCSMM (VCS membership module) and is implemented in library Oracle uses the linked skgxn library to communicate with VCSMM, which obtains membership information from GAB.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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