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

126 Administering dynamic multipathing (DMP)
How DMP works
You can use the vxdmpadm command to change the I/O policy for the paths to an
enclosure or disk array as described in “Specifying the I/O policy” on page 141.
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 85 and “Configuring array policy
modules” on page 154.
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
Prior to release 5.0, VxVM did not allow enabling or disabling of paths or controllers
connected to a disk that is part of a shared Veritas Volume Manager disk group. From
VxVM 5.0 onward, such operations are supported on shared DMP nodes in a cluster.