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399Administering cluster functionality
Overview of cluster volume management
membership. Each node starts up independently and has its own cluster monitor
plus its own copies of the operating system and VxVM with support for cluster
functionality. 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 you issue the appropriate command
on that node. In an HP Serviceguard cluster, a node can join the cluster
automatically at boot time.
Caution: The cluster functionality of VxVM is supported only when used in
conjunction with a cluster monitor that has been configured correctly to work
with VxVM.
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 via Fibre Channel in a Storage Area Network
(SAN).
Note: In this example, each node has two independent paths to the disks, which
are configured in one or more cluster-shareable disk groups. Multiple paths
provide resilience against failure of one of the paths, but this is not a
requirement for cluster configuration. Disks may also be connected by single
paths.
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 were used, its failure would be indistinguishable
from node failure—a condition known as network partitioning.