Veritas Storage Foundation 5.0 Cluster File System Administration Guide Extracts for HP Serviceguard Storage Management Suite, Second Edition, May 2008

Cluster File System Architecture
About Veritas Cluster Volume Manager Functionality
Chapter 2
23
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 described in
Table 2-2, “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.
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 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 in order for the volumes in the disk
group to become accessible for application I/O from that node. The ability of applications
to read from, or to write to, volumes is dictated by the activation mode of a shared disk
group. Valid activation modes for a shared disk group are exclusivewrite, readonly,
sharedread, sharedwrite, and off (inactive). These activation modes are described in
Table 2-2, “Activation Modes for Shared Disk Groups.”
NOTE The default activation mode for shared disk groups is off (inactive).
Special use clusters, such as high availability (HA) applications and off-host backup, can
employ disk group activation to explicitly control volume access from different nodes in
the cluster.
Table 2-2 Activation Modes for Shared Disk Groups
Activation Mode Description
exclusivewrite The node has exclusive write access to the disk group. No other
node can activate the disk group for write access.
readonly The node has read access to the disk group and denies write
access for all other nodes in the cluster. The node has no write
access to the disk group. Attempts to activate a disk group for
either of the write modes on other nodes fail.
sharedread The node has read access to the disk group. The node has no
write access to the disk group, however other nodes can obtain
write access.
sharedwrite The node has write access to the disk group.
off The node has neither read nor write access to the disk group.
Query operations on the disk group are permitted.