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

Overview of Cluster Volume Management
100 Installation and Administration Guide
Private and Shared Disk Groups
Two types of disk groups are defined:
◆ Private disk groups—belong 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 root disk group
(rootdg) is always a private disk group.
◆ Shared disk groups—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.
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. 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. You can also import or deport a shared disk group
at any time; the operation takes places 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
and joins the cluster. When a node leaves the cluster, it deports all its imported shared
disk groups, but they remain imported on the surviving nodes.
Reconfiguring a shared disk group is performed with the co-operation of all nodes.
Configuration changes to the disk group happen simultaneously on all nodes and the
changes are identical. Such changes are atomic in nature, which means that they either
occur simultaneously on all nodes or not at all.