Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

388 Administering cluster functionality
Overview of cluster volume management
This chapter does not discuss Veritas Storage Foundation Cluster File System (SFCFS)
nor cluster management software such as Serviceguard. Such products are separately
licensed, and are not included with Veritas Volume Manager. See the documentation
provided with those products for more information about them. (For Serviceguard
documentation, go to www.docs.hp.com and click on High Availability, then on
Serviceguard.)
For additional information about using the Dynamic Multipathing (DMP) feature of
VxVM in a clustered environment, see “DMP in a clustered environment” on page 126.
For information about administering Extended Distance Clusters, see Designing Disaster
Tolerant High Availability, and see “Administering sites and remote mirrors” on
page 423.
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 Serviceguard. Serviceguard
informs VxVM of changes in cluster membership. Each node starts up independently and
has its own copies of the operating system and VxVM. When a node joins a cluster, it
gains access to shared disk groups and volumes. When a node leaves a cluster, it no longer
has access to these shared objects. A node joins a cluster when the cluster monitor is
started on that node.
Caution: The cluster functionality of VxVM is supported on HP-UX only in conjunction
with Serviceguard.
Figure 13-1 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.