Administrator's Guide

Cluster Volume Manager Administration
Overview of Cluster Volume Management
Chapter 4
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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.”
NOTE The default activation mode for shared disk groups is off.
Applications such as high availability and off-host backup can use disk group activation
to explicitly control volume access from different nodes in the cluster.