VERITAS Volume Manager 4.1 Administrator's Guide

Administering Dynamic Multipathing (DMP)
How DMP Works
Chapter 3 111
How DMP Works
Multiported disk arrays can be connected to host systems through multiple paths. To detect
the various paths to a disk, DMP uses a mechanism that is specific to each supported array
type. DMP can also differentiate between different enclosures of a supported array type that
are connected to the same host system.
See “Configuring Newly Added Disk Devices” on page 63 for a description of how to make
newly added disk hardware known to a host system.
The multipathing policy used by DMP depends on the characteristics of the disk array:
•AnActive/Passive array (A/P array) allows access to its LUNs (logical units; real disks or
virtual disks created using hardware) via the primary (active) path on a single controller
(also known as an access port or a storage processor) during normal operation.
In implicit failover mode (or autotrespass mode), an A/P array automatically fails over by
scheduling I/O to the secondary (passive) path on a separate controller if the primary path
fails. This passive port is not used for I/O until the active port fails. In A/P arrays, path
failover can occur for a single LUN if I/O fails on the primary path.
In Active/Passive arrays with LUN group failover (A/PG arrays), a group of LUNs that are
connected through a controller is treated as a single failover entity. Unlike A/P arrays,
failover occurs at the controller level, and not for individual LUNs. The primary and
secondary controller are each connected to a separate group of LUNs. If a single LUN in
the primary controller’s LUN group fails, all LUNs in that group fail over to the secondary
controller’s passive LUN group.
Active/Passive arrays in explicit failover mode (or non-autotrespass mode) are termed
A/PF arrays. DMP issues the appropriate low-level command to make the LUNs fail over
to the secondary path.
A/P-C, A/PF-C and A/PG-C arrays are variants of the A/P, AP/F and A/PG array types that
support concurrent I/O and load balancing by having multiple primary paths into a
controller. This functionality is provided by a controller with multiple ports, or by the
insertion of a SAN hub or switch between an array and a controller.
• An Active/Active disk array (A/A arrays) permits several paths to be used concurrently for
I/O. Such arrays allow DMP to provide greater I/O throughput by balancing the I/O load
uniformly across the multiple paths to the LUNs. In the event that one path fails, DMP
automatically routes I/O over the other available paths.
VxVM uses DMP metanodes (DMP nodes) to access disk devices connected to the system. For
each disk in a supported array, DMP maps one metanode to the set of paths that are
connected to the disk. Additionally, DMP associates the appropriate multipathing policy for