VERITAS Volume Manager 4.1 Administrator's Guide

Administering Cluster Functionality
Overview of Cluster Volume Management
Chapter 10352
Overview of Cluster Volume Management
In recent years, tightly coupled cluster systems have become increasingly
popular in the realm of enterprise-scale mission-critical data processing.
The primary advantage of clusters is protection against hardware
failure. Should the primary node fail or otherwise become unavailable,
applications can continue to run by transferring their execution to
standby nodes in the cluster. This ability to provide continuous
availability of service by switching to redundant hardware is commonly
termed failover.
Another major advantage of clustered systems is their ability to reduce
contention for system resources caused by activities such as backup,
decision support and report generation. Businesses can derive enhanced
value from their investment in cluster systems by performing such
operations on lightly loaded nodes in the cluster rather than on the
heavily loaded nodes that answer requests for service. This ability to
perform some operations on the lightly loaded nodes is commonly termed
load balancing.
The cluster functionality of VxVM works together with the cluster
monitor daemon that is provided 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 with support for cluster
functionality. When a node joins a cluster, it gains access to shared disks.
When a node leaves a cluster, it no longer has access to shared disks. A
node joins a cluster when the cluster monitor is started on that node.
Figure 10-1, “Example of a 4-Node Cluster,” illustrates 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 Fibre Channel. Each node has two independent paths
to these disks, which are configured in one or more cluster-shareable disk
groups.
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