Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

391Administering cluster functionality
Overview of cluster volume management
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 cooperation 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 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 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 detail in the table “Activation modes for shared disk groups.”
Note: The default activation mode for shared disk groups is off (inactive).
Special uses of clusters, such as high availability (HA) applications and off-host backup,
can use disk group activation to explicitly control volume access from different nodes in
the cluster.
Table 13-1 Activation modes for shared disk groups
Activation mode Description
exclusivewrite (ew) The node has exclusive write access to the disk group. No other
node can activate the disk group for write access.