Administering cluster functionality 399

Overview of cluster volume management

membership. Each node starts up independently and has its own cluster monitor plus its own copies of the operating system and VxVM with support for cluster functionality. When a node joins a cluster, it gains access to shared disk groups and volumes. When a node leaves a cluster, it no longer has access to these shared objects. A node joins a cluster when you issue the appropriate command on that node. In an HP Serviceguard cluster, a node can join the cluster automatically at boot time.

Caution: The cluster functionality of VxVM is supported only when used in conjunction with a cluster monitor that has been configured correctly to work with VxVM.

Figure 13-1illustrates 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). 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 via Fibre Channel in a Storage Area Network (SAN).

Note: 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.