Over the past several years, parallel applications using shared data access have become increasingly popular. Examples of commercially available applications include Oracle Real Application Clusters™ (RAC), Sybase Adaptive Server®, and Informatica Enterprise Cluster Edition. In addition, the semantics of Network File System (NFS), File Transfer Protocol (FTP), and Network News Transfer Protocol (NNTP) allow these workloads to be served by shared data access clusters. Finally, numerous organizations have developed internal applications that take advantage of shared data access clusters.

The cluster functionality of VxVM (CVM) works together with the cluster monitor daemon that is provided by VCS or by the host operating system. The cluster monitor informs VxVM of changes in cluster membership. Each node starts up independently and has its own cluster monitor plus its own copies of the operating system and VxVM/CVM. When a node joins a cluster, it gains access to shared disk groups and volumes. When a node leaves a cluster, it loses access to these shared objects. A node joins a cluster when you issue the appropriate command on that node.

Figure: Example of a 4-node CVM cluster shows a simple cluster arrangement consisting of four nodes with similar or identical hardware characteristics (CPUs, RAM and host adapters), and configured with identical software (including the operating system).

To the cluster monitor, all nodes are the same. VxVM objects configured within shared disk groups can potentially be accessed by all nodes that join the cluster. However, the CVM functionality of VxVM requires that one node act as the master node; all other nodes in the cluster are slave nodes. Any node is capable of being the master node, and it is responsible for coordinating certain VxVM activities.

In this example, node 0 is configured as the CVM master node and nodes 1, 2 and 3 are configured as CVM slave nodes. The nodes are fully connected by a private network and they are also separately connected to shared external storage (either disk arrays or JBODs: just a bunch of disks) via SCSI or Fibre Channel in a Storage Area Network (SAN).

In this example, each node has two independent paths to the disks, which are configured in one or more cluster-shareable disk groups. Multiple paths provide resilience against failure of one of the paths, but this is not a requirement for cluster configuration. Disks may also be connected by single paths.

The private network allows the nodes to share information about system resources and about each other's state. Using the private network, any node can recognize which other nodes are currently active, which are joining or leaving the cluster, and which have failed. The private network requires at least two communication channels to provide redundancy against one of the channels failing. If only one channel were used, its failure would be indistinguishable from node failure - a condition known as network partitioning.

You can run commands that configure or reconfigure VxVM objects on any node in the cluster. These tasks include setting up shared disk groups, creating and reconfiguring volumes, and performing snapshot operations.

The first node to join a cluster performs the function of master node. If the master node leaves a cluster, one of the slave nodes is chosen to be the new master.

See Methods to control CVM master selection.