VERITAS Volume Manager 4.1 Administrator's Guide

Administering Cluster Functionality

Overview of Cluster Volume Management

Chapter 10354

NOTE You must run commands that conﬁgure or reconﬁgure VxVM objects on

the master node. Tasks that must be initiated from the master node

include setting up shared disk groups, creating and reconﬁguring

volumes, and performing snapshot operations.

VxVM determines that 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. In Figure 10-1, “Example of a

4-Node Cluster,” node 0 is the master node and nodes 1, 2 and 3 are slave

nodes.

Private and Shared Disk Groups

Two types of disk groups are deﬁned:

• 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 access is

restricted to one system only. The root disk group (rootdg) is always a

private disk group.

• Shared disk groups—shared by all nodes. A shared (or

cluster-shareable) disk group is imported by all cluster nodes.

Disks in a shared disk group must be physically accessible from all

systems that may join the cluster.

In a cluster, most disk groups are shared. 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

restrictions.

You can use the vxdg command to designate a disk group as

cluster-shareable as described in “Importing Disk Groups as Shared” on

page 374. When a disk group is imported as cluster-shareable for one

node, each disk header is marked with the cluster ID. As each node

subsequently joins the cluster, it recognizes the disk group as being

cluster-shareable and imports it. As system administrator, you can also

import or deport a shared disk group at any time; the operation takes

places in a distributed fashion on all nodes.