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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.

The Oracle Universal Installer (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 either be shared by one or more nodes, or private to each node, depending on the settings when the OUI is run. 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 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. When Real Application Clusters is installed, the location of the OCR is specified.

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 RAC is managed.

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 provide the cluster manager functionality.

CRS Features

CRS is required for installing the Oracle 10g RAC system. CRS can be run on top of vendor provided Cluster Software. The vendor supplied clusterware is however optional. 

The CRS software is installed in the cluster with its own set of binaries. The CRS Home and Oracle Home are in different locations. CRS software installation uses the two shared disk locations or files which are the Voting Disk and OCR file. Installation of CRS configures the Virtual IP interface. Virtual IP is associated with defined Workload Service. CRS resources can also be managed by the srvctl utility.

CRS 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. Basically it manages the application resources defined within the cluster. It also maintains the configuration profiles stored in the Oracle Configuration Repository.

OCSSD ? This process is associated with the Automatic Storage Management (ASM) instance. This daemon is spawned to manage shared access of the disk devices to the clustered nodes. It manages the basic cluster locking and understands the nodes and its membership status.

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

Cluster Private Interconnect

The cluster 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.

In case of RAC database, the 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.

Innterconnect technology that is employed 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, the more message traffic it generates. It is advisable to implement the highest bandwidth, lowest latency interconnect that is available for a given platform.

The volume of synchronization traffic directly impacts the bandwidth requirement, and messaging delays are highly dependant on the IPC protocol. The interconnect is not something that should be under configured, assuming scalability is a key objective.

Oracle recommends that for Linux environments with interconnect bandwidth is 1GB Ethernet, use the UDP as IPC protocol in preference to TCP. 10g has extended support for emerging technologies like Infiniband which will greatly improve interconnect scalability and standardization for large numbers of nodes, as well as provide a choice of interconnects under Linux.


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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