HP UX Serviceguard Storage Management Software manual Overview of Cluster Volume Management

Cluster Volume Manager Administration

Overview of Cluster Volume Management

	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.
IMPORTANT	The cluster functionality of VxVM is supported only when used in conjunction with the
	cmvx daemon.
	Figure 4-1, “Example of a 4-Node Cluster,” 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) via FibreChannel.
	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 other
	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 (a condition
	known as network partitioning), its failure will be indistinguishable from node failure.

40 Chapter 4