VERITAS Volume Manager 4.1 Administrator's Guide

Administering Dynamic Multipathing (DMP)

DMP in a Clustered Environment

Chapter 3116

DMP in a Clustered Environment

NOTE You need an additional license to use the cluster feature of VxVM.

In a clustered environment where Active/Passive type disk arrays are shared by multiple

hosts, all nodes in the cluster must access the disk via the same physical path. Accessing a

disk via multiple paths simultaneously can severely degrade I/O performance (sometimes

referred to as the ping-pong effect). Path failover on a single cluster node is also coordinated

across the cluster so that all the nodes continue to share the same physical path.

Prior to release 4.1 of VxVM, the clustering and DMP features could not handle automatic

failback in A/P arrays when a path was restored, and did not support failback for explicit

failover mode arrays. Failback could only be implemented manually by running the vxdctl

enable command on each cluster node after the path failure had been corrected. In release

4.1, failback is now an automatic cluster-wide operation that is coordinated by the master

node. Automatic failback in explicit failover mode arrays is also handled by issuing the

appropriate low-level command. If required, this feature can be disabled by selecting the “no

failback” option that is deﬁned in the array policy module (APM) for an array.

NOTE Support for automatic failback of an A/P array requires that an appropriate

ASL (and APM, if required) is available for the array, and has been installed on

the system. See “Administering the Device Discovery Layer” on page 66,

“Conﬁguring Array Policy Modules” on page 133, and the VERITAS Volume

Manager Hardware Notes for more information.

For Active/Active type disk arrays, any disk can be simultaneously accessed through all

available physical paths to it. In a clustered environment, the nodes do not all need to access a

disk via the same physical path.

Enabling or Disabling Controllers with Shared Disk Groups

VxVM does not allow enabling or disabling of controllers connected to a disk that is part of a

shared VERITAS Volume Manager disk group.

For example, consider a disk array, containing all or part of a shared disk group, that is

connected through a controller on each of the cluster nodes. In such a situation, the vxdmpadm

enable and disable operations fail when applied to the controller on any of the nodes, and the

following error message is displayed: