Donald K. Burleson    Oracle9i RAC Tips

Shared Storage Configuration

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.

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 9i RAC.

Oracle Disk Manager

ODM is a standard API created by Oracle for doing database I/O. The ODM facility bypasses the file system cache and provides equal or better performance than the raw partitions. ODM features are available in both the file system files and for raw partition volumes. ODM is also cluster-aware. We have covered more details about the ODM in Chapter 5, ‘RAC Server and Disk Technology’.

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