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Shared Storage Configuration

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%.

Setting up volumes and making sure the cluster file system is accessible by all nodes configures the shared storage. In sum, the volumes and the CFS depend on the physical storage. The cluster volume manager manages all related objects such as physical disks (LUNS), disk groups, volumes, and file systems. Oracle uses the ODM (Oracle disk manager) interface to communicate with Veritas volumes and CFS files.

Cluster Volume Manager (CVM)

CVM is basically an extension of the widely used Veritas Volume Manager. CVM extends the functionality of the VxVM to all the nodes in the cluster. Each node sees the same state of all volume resources. It follows master/slave architecture. One node usually acts as master and others as slaves. There is only one master in a given cluster. The volume manager daemon (vxconfigd) maintains the configuration of the logical volumes. Each node has the vxconfigd daemon. Changes to a volume are propagated first to the master daemon and then the master passes it on to slave daemons. These changes happen at the kernel level. CVM does not attempt to do any locking between the nodes. That is the responsibility of the application, as in the RAC database. CVM also follows the uniform shared storage model. This means that all systems must be connected to the same disk sets for a given disk group. If a node loses contact with a specific disk, it is excluded from using the disk.

Cluster File System (CFS)

Veritas CFS has evolved from the Veritas File System (VxFS). CFS allows the same file system to be simultaneously mounted on multiple nodes in the cluster.

Once again, the CFS is designed with master/slave architecture. Though any node can initiate an operation to create, delete, or resize data, the master node carries out the actual operation. CFS caches the metadata in memory, typically in the memory buffer cache or the vnode cache. A distributed locking mechanism, called GLM, is used for metadata and cache coherency among the multiple nodes.

However, with implementation of the ODM interface, Oracle RAC accesses data files stored on CFS, bypassing the file system buffer and file system locking processes. Oracle manages its own consistency mechanism. The ODM facility is automatically invoked with RAC.

VCS Framework and Service Groups

Veritas Cluster Server (VCS) is the overall framework that controls the Oracle RAC database and other required essential resources such as shared storage and listener. VCS acts as a director of operations. All activities within Oracle RAC and its infrastructure are managed with the VCS framework.

VCS manages its resources in the form of a group. A collection of resources of different types for a given application or task is called a service group. Service groups act as containers for all the necessary components and resources of an application. They are defined and managed as a single unit. A service group definition typically consists of:

* The keyword group in the configuration file.

* Name of the service group.

* The service group?s attribute values (enclosed in parentheses). VCS predefine service group attributes.

* The definition of service group resources with attributes/values.

* The dependencies between the service group resources.

In Veritas DBE/AC, two service groups are defined to support and control the RAC database. One service group deals with the volume manager resources, file system resources, listener resources, and shared Oracle Home directory. The other service group deals with the Oracle database and supports CVM and CFS resources. These two groups are specified to be parallel groups as shown in Figure 6.15. VCS does not attempt to migrate a failed service group.

Figure 6.15: Veritas DBE/AC Service Groups

DBEAC provides agents to operate and control all resources. Agents are the VCS processes that bring resources online and take them offline. Resources are brought online and offline in an orderly manner based on the resource dependency. Agents also monitor resources and report. Some of the agents are specific to advance clusters including CVMCluster, CVMVoldg and CFSMount. The Agents stop and start the resources as required. The VCS implementation for Oracle Failover clusters does not include these agents.


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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