VERITAS Volume Manager 3.1 Administrator's Guide

Cluster Functionality
Cluster Functionality Overview
Chapter 7 297
Cluster Functionality Overview
The cluster functionality in the Volume Manager allowsmultiple hosts to
simultaneously access and manage a given set of disks under Volume
Manager control (
VM disks
). A cluster is a set of hosts sharing a set of
disks; each host is referred to as a node in the cluster. The nodes are
connected across a network. If one node fails, the other node(s) can still
access the disks. The Volume Manager cluster feature presents the same
logical view of the disk configurations (including changes) on all nodes.
NOTE With cluster support enabled, the Volume Manager supports up to four
nodes per cluster.
The sections that follow provide more information on the cluster
functionality provided by the Volume Manager.
Shared Volume Manager Objects
When the cluster feature is enabled, Volume Manager objects can be
shared by all of the nodes in a given cluster.
The Volume Manager cluster feature allows for two types of disk groups:
Private disk groups—belong to only one node. A private disk group is
only imported by one system. Disks in a private disk group may be
physically accessible from one or more systems, but actual access is
restricted to one system only.
Cluster-shareable disk groups—shared by all nodes. A
cluster-shareable (or
shared
) disk group is imported by all cluster
nodes. Disks in a cluster-shareable disk group must be physically
accessible from all systems that may join the cluster.
In a Volume Manager cluster, most disk groups are shared. However, the
root disk group (rootdg) is always a private disk group.
Disks in a shared disk group are accessible from all nodes in a cluster,
allowing applications on multiple cluster nodes to simultaneously access
the same disk. A volume in a shared disk group can be simultaneously
accessed by more than one node in the cluster, subject to licensing and
disk group activation mode descriptions.