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

Chapter 9, CVM Administration
Overview of Cluster Volume Management
101
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 of 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. Regardless of which
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 a distributed lock manager, for example).
Activation Modes of 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
the table “Allowed and Conflicting Activation Modes” on page 102.
Note The default activation mode for shared disk groups is off (inactive).
Applications such as high availability (HA) and off-host backup can use disk group
activation to explicitly control volume access from different nodes in the cluster.
The activation mode of a disk group controls volume I/O from different nodes in the
cluster. It is not possible to activate a disk group on a given node if it is activated in a
conflicting mode on another node in the cluster.