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

390 Administering cluster functionality
Overview of cluster volume management
capable of being the master node, and it is responsible for coordinating certain VxVM
activities.
Note: You must run commands that configure or reconfigure VxVM objects on the master
node. Tasks that must be initiated from the master node include setting up shared disk
groups, creating and reconfiguring volumes, and performing snapshot operations.
VxVM determines that 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. In “Example of a 4-node cluster,” node 0 is the master node and nodes 1, 2 and 3
are slave nodes.
Private and shared disk groups
Two types of disk groups are defined:
In a cluster, most disk groups are shared. Disks in a shared disk group are accessible from
all nodes in a cluster, allowing applications on multiple cluster nodes to simultaneously
access the same disk. A volume in a shared disk group can be simultaneously accessed by
more than one node in the cluster, subject to licensing and disk group activation mode
restrictions.
You can use the
vxdg command to designate a disk group as cluster-shareable as
described in “Importing disk groups as shared” on page 410. 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. As system administrator, you can also import or deport a shared
disk group at any time; the operation takes place in a distributed fashion on all nodes.
Each physical disk 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 have the
noautoimport 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 imports the same shared disk groups as the master
Private disk group Belongs to only one node. A private disk group is only imported by one
system. Disks in a private disk group may be physically accessible from
one or more systems, but access is restricted to one system only. The
boot disk group (usually aliased by the reserved disk group name
bootdg) is always a private disk group.
Shared disk group Can be shared by all nodes. A shared (or cluster-shareable) disk group is
imported by all cluster nodes. Disks in a shared disk group must be
physically accessible from all systems that may join the cluster.