VERITAS Volume Manager 3.1 Administrator's Guide

Introduction to Volume Manager
Volume Manager Conceptual Overview
Chapter 1 61
Dynamic Multipathing (DMP)
NOTE You may need an additional license to use this feature.
A multipathing condition can exist when a physical disk can be accessed
by more than one operating system device handle. Each multipath
operating system device handle permits data access and control through
alternate host-to-device pathways.
Volume Manager is configured with its own DMP system to organize
access to multipathed devices. Volume Manager detects multipath
systems by using the Universal World-Wide-Device Identifiers (WWD
IDs). The physical disk must provide unambiguous identification
through its WWD ID for DMP to access the device.
If DMP cannot identify the physical disk through its WWD ID,
identification is left to the Volume Manager device detection methods.
Device detection depends on Volume Manager recognizing on-disk
metadata identifiers.
Volume Manager DMP creates metanodes representing metadevices for
each multipath target that it has detected. Each metanode is mapped to
a set of operating system device handles and configured with an
appropriate multipathing policy. Volume Manager DMP creates
metanodes for all attached physical disks accessible through an
operating system device handle.
Volume Manager DMP manages multipath targets, such as disk arrays,
which define policies for using more than one path. Some disk arrays
permit more than one path to be concurrently active (Active / Active).
Some disk arrays permit only one path to be active, holding an alternate
path as a spare in case of failure on the existing path (Active / Passive).
Some disk arrays have more elaborate policies.
In general, Volume Manager is designed so that the VM disk is mapped
to one Volume Manager DMP metanode. To simplify VxVM logical
operations, each VM disk is mapped to a unique Volume Manager DMP
metanode. The mapping occurs whether or not the physical disk device is
connected in a multipathing configuration.
When using Volume Manager DMP, you should be aware of the layering
of device recognition: