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

394 Administering cluster functionality
Overview of cluster volume management
The local detach policy is intended for use with shared mirrored volumes in a cluster. This
policy prevents I/O failure on a single slave node from causing a plex to be detached. This
would require the plex to be resynchronized when it is subsequently reattached. The local
detach policy is available for disk groups that have a version number of 70 or greater.
Note: For small mirrored volumes, non-mirrored volumes, volumes that use hardware
mirrors, and volumes in private disk groups, there is no benefit in configuring the local
detach policy. In most cases, it is recommended that you use the default global detach
policy.
The detach policies have no effect if the master node loses access to all copies of the
configuration database and logs in a disk group. If this happened in releases prior to 4.1,
the master node always disabled the disk group. Release 4.1 introduces the disk group
failure policy, which allows you to change this behavior for critical disk groups. This
policy is only available for disk groups that have a version number of 120 or greater.
The following sections describe the detach and failure policies in greater detail.
Global detach policy
Caution: The global detach policy must be selected when Dynamic MultiPathing (DMP) is
used to manage multipathing on Active/Passive arrays, This ensures that all nodes
correctly coordinate their use of the active path.
The global detach policy is the traditional and default policy for all nodes on the
configuration. If there is a read or write I/O failure on a slave node, the master node
performs the usual I/O recovery operations to repair the failure, and the plex is detached
cluster-wide. All nodes remain in the cluster and continue to perform I/O, but the
redundancy of the mirrors is reduced. When the problem that caused the I/O failure has
been corrected, the mirrors that were detached must be recovered before the redundancy
of the data can be restored.
Local detach policy
The local detach policy is designed to support failover applications in large clusters where
the redundancy of the volume is more important than the number of nodes that can access
the volume. If there is a write failure on a slave node, the master node performs the usual
I/O recovery operations to repair the failure, and additionally contacts all the nodes to see
if the disk is still acceptable to them. If the write failure is not seen by all the nodes, I/O is
stopped for the node that first saw the failure, and the application using the volume is also
notified about the failure.
If required, configure the cluster management software to move the application to a
different node, and/or remove the node that saw the failure from the cluster. The volume