Veritas Storage Foundation 5.0 Cluster File System Administration Guide Extracts for HP Serviceguard Storage Management Suite, Second Edition, May 2008

Cluster File System Architecture

About Veritas Cluster Volume Manager Functionality

Chapter 2

About Veritas Cluster Volume Manager Functionality

CVM supports up to 8 nodes in a cluster to simultaneously access and manage a set of

disks under VxVM control (VM disks). The same logical view of the disk configuration

and any changes are available on each node. When the cluster functionality is enabled,

all cluster nodes can share VxVM objects. Features provided by the base volume

manager, such as mirroring, fast mirror resync, and dirty region logging are also

supported in the cluster environment.

NOTE RAID-5 volumes are not supported on a shared disk group.

To implement cluster functionality, VxVM works together with the cmvx daemon

provided by HP. The cmdx daemon informs VxVM of changes in cluster membership.

Each node starts up independently and has its own copies of HP-UX, Serviceguard, and

CVM. 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 Serviceguard

is started on that node.

Figure 2-1 illustrates a simple cluster 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) via 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 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 is used, its failure will be

indistinguishable from node failure—a condition known as network partitioning.

Figure 2-1 Example of a Four-Node Cluster

Redundant

Fibre Channel

Connectivity

Cluster-Shareable

Disks

Redundant Private Network

Node 0

Master

Node 1

Slave

Node 2

Slave

Node 3

Slave

Cluster-Shareable

Disk Groups