Veritas Storage Foundation 5.0.1 Cluster File System Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

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 boot disk group (usually aliased by
the reserved disk group name bootdg) 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 a master node, it imports all shared disk groups (except for any that have the noautoimport
attribute set). When a secondary node tries to join a cluster, the master node sends it a list of the
disk IDs that it has imported, then the secondary node checks to see if it can access all of them.
If the secondary node cannot access one of the listed disks, it abandons its attempt to join the
cluster. If the secondary node can access all of the listed disks, it imports the same shared disk
groups as the master node and joins the cluster. When a node leaves a cluster, it deports all of
its imported shared disk groups, but they remain on the nodes that are still members of the
cluster.
Reconfiguration of 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.
Whether all members of the cluster have simultaneous read and write access to a cluster-shareable
disk group depends on its activation mode setting as discussed in “Activation Modes for Shared
Disk Groups”. The data contained in a cluster-shareable disk group is available as long as at least
one node is active in the cluster. The failure of a cluster node does not affect access by the
remaining active nodes in the cluster. Regardless of which cluster node accesses a cluster-shareable
disk group, the configuration of the disk group looks the same.
NOTE: Applications running on each node can access the data on the VM disks simultaneously.
VxVM does not protect against simultaneous writes to shared volumes by more than one node.
It is assumed that applications control consistency (by using Veritas Storage Foundation Cluster
File System or a distributed lock manager, for example).
Activation Modes for Shared Disk Groups
A shared disk group must be activated on a node for the volumes in the disk group to become
accessible for I/O from that node. The ability of applications to read from or to write to volumes
is determined by the activation mode of a shared disk group. Valid activation modes for a shared
disk group are exclusive-write, read-only, shared-read, shared-write, and off (inactive). Activation
modes are described in Table 4-1: “Activation Modes for Shared Disk Groups”.
Overview of Cluster Volume Management 31