Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

You can run commands that configure or reconfigure VxVM objects on any node
in the cluster. These tasks include setting up shared disk groups, creating and
reconfiguring volumes, and performing snapshot operations.
The first node to join a cluster performs the function of master node. If the master
node leaves a cluster, one of the slave nodes is chosen to be the new master.
Private and shared disk groups
The following types of disk groups are defined:
Belongs to only one node. A private disk group can only be imported
by one system. LUNs in a private disk group may be physically
accessible from one or more systems, but access is restricted to only
one system at a time.
The boot disk group (usually aliased by the reserved disk group name
bootdg) is always a private disk group.
Private disk group
Can be shared by all nodes. A shared (or cluster-shareable) disk group
is imported by all cluster nodes. LUNs in a shared disk group must be
physically accessible from all systems that may join the cluster.
Shared disk group
In a CVM cluster, most disk groups are shared. LUNs in a shared disk group are
accessible from all nodes in a cluster, allowing applications on multiple cluster
nodes to simultaneously access the same LUN. A volume in a shared disk group
can be simultaneously accessed by more than one node in the cluster, subject to
license key and disk group activation mode restrictions.
You can use the vxdg command to designate a disk group as cluster-shareable.
See “Importing disk groups as shared” on page 455.
When a disk group is imported as cluster-shareable for one node, each disk header
is marked with the cluster ID. As each node subsequently joins the cluster, it
recognizes the disk group as being cluster-shareable and imports it. In contrast,
a private disk group's disk headers are marked with the individual node's host
name. As system administrator, you can import or deport a shared disk group at
any time; the operation takes place in a distributed fashion on all nodes.
Each LUN is marked with a unique disk ID. When cluster functionality for VxVM
starts on the master, it imports all shared disk groups (except for any that do not
have the autoimport attribute set). When a slave tries to join a cluster, the master
sends it a list of the disk IDs that it has imported, and the slave checks to see if it
can access them all. If the slave cannot access one of the listed disks, it abandons
its attempt to join the cluster. If it can access all of the listed disks, it joins the
cluster and imports the same shared disk groups as the master. When a node
Administering cluster functionality (CVM)
Overview of clustering
428