Veritas Storage Foundation 5.0.1 Cluster File System Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

4 Cluster Volume Manager Administration
A cluster consists of a number of hosts or nodes that share a set of disks. The main benefits of
cluster configurations are:
• Availability—If one node fails, the other nodes can still access the shared disks. When
configured with suitable software, mission-critical applications can continue running by
transferring their execution to a standby node in the cluster. This ability to provide continuous
uninterrupted service by switching to redundant hardware is commonly termed failover.
Failover is transparent to users and high-level applications for database and file-sharing.
You must configure cluster management software, for example Serviceguard, to monitor
systems and services, and to restart applications on another node in the event of either
hardware or software failure. Serviceguard also allows you to perform general administrative
tasks such as joining or removing nodes from a cluster.
• Off-host processing—Clusters can reduce contention for system resources by performing
activities such as backup, decision support and report generation on the more lightly loaded
nodes of the cluster. This allows businesses to derive enhanced value from their investment
in cluster systems.
With CVM, a set of VxVM disks can be accessed simultaneously by multiple, or all, cluster
nodes. The same logical view of the disk configuration (and any changes to this configuration)
is available on all the nodes. When VxVM cluster functionality is enabled, all of the nodes
in a cluster can share VxVM objects.
This chapter contains the following topics:
• “Overview of Cluster Volume Management” (page 29)
• “Private and Shared Disk Groups” (page 31)
• “Activation Modes for Shared Disk Groups” (page 31)
• “Connectivity Policy of Shared Disk Groups” (page 33)
• “Disk Group Failure Policy” (page 33)
• “Limitations of Shared Disk Groups” (page 34)
• “Recovery in a CVM Environment” (page 34)
Overview of Cluster Volume Management
Tightly coupled cluster systems have become increasingly popular in enterprise-scale
mission-critical data processing. The main advantage clusters offer is protection against hardware
failure. If the master node fails or otherwise becomes 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 clustered systems offer is their ability to reduce contention for system
resources caused by activities such as backup, decision support and report generation. Enhanced
value can be derived from cluster systems by performing such operations on lightly loaded nodes
in the cluster instead of 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.
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. A node joins a cluster
when the cluster monitor is started on that node. When a node joins a cluster, it gains access to
shared disks. When a node leaves a cluster, it no longer has access to those shared disks.
Overview of Cluster Volume Management 29