VERITAS Storage Foundation 4.1 Cluster File System HP Serviceguard Storage Management Suite Extracts, December 2005

About CVM
64 Installation and Administration Guide
About CVM
CVM allows up to 4 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 cluster monitor daemon
provided by the host operating system . The cluster monitor informs VxVM of
changes in cluster membership. Each node starts up independently and has its own
cluster monitor, plus its own copies of the operating system 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 the cluster monitor is started on that
node.
The figure “Example of a Four-Node Cluster” on page 65 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 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 were used, its failure would be
indistinguishable from node failure—a condition known as network partitioning.