Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 3, Administering Dynamic Multipathing (DMP)
How DMP Works
105
Note Both paths of an Active/Passive array are not considered to be on different
controllers when mirroring across controllers (for example, when creating a volume
using vxassist make specified with the mirror=ctlr attribute).
For A/P-C, A/PF-C and A/PG-C arrays, load balancing is performed across all the
currently active paths as is done for Active/Active arrays.
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 121.
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 130, 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.