408Administering cluster functionality

Overview of cluster volume management

Guidelines for choosing detach and failure policies

In most cases it is recommended that you use the global detach policy, and particularly if any of the following conditions apply:

■If you are using the VCS agents that monitor the cluster functionality of Veritas Volume Manager, and which are provided with Veritas Storage FoundationTM for Cluster File System HA and Veritas Storage Foundation for databases HA. These agents do not notify VCS about local failures.

■When an array is seen by DMP as Active/Passive. The local detach policy causes unpredictable behavior for Active/Passive arrays.

■For clusters with four or fewer nodes. With a small number of nodes in a cluster, it is preferable to keep all nodes actively using the volumes, and to keep the applications running on all the nodes, rather than keep the redundancy of the volume at the same level.

■If only non-mirrored, small mirrored, or hardware mirrored volumes are configured. This avoids the system overhead of the extra messaging that is required by the local detach policy.

The local detach policy may be suitable in the following cases:

■When large mirrored volumes are configured. Resynchronizing a reattached plex can degrade system performance. The local detach policy can avoid the need to detach the plex at all. (Alternatively, the dirty region logging (DRL) feature can be used to reduce the amount of resynchronization that is required.)

■For clusters with more than four nodes. Keeping an application running on a particular node is less critical when there are many nodes in a cluster. It may be possible to configure the cluster management software to move an application to a node that has access to the volumes. In addition, load balancing may be able to move applications to a different volume from the one that experienced the I/O problem. This preserves data redundancy, and other nodes may still be able to perform I/O from/to the volumes on the disk.

If you have a critical disk group that you do not want to become disabled in the case that the master node loses access to the copies of the logs, set the disk group failure policy to leave. This prevents I/O failure on the master node disabling the disk group. However, critical applications running on the master node fail if they lose access to the other shared disk groups. In such a case, it may be preferable to set the policy to dgdisable, and to allow the disk group to be disabled.