Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage
 

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 in the 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 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.