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

Chapter
3
Administering dynamic
multipathing (DMP)
Note: You need a full license to use this feature.
The dynamic multipathing (DMP) feature of Veritas Volume Manager (VxVM) provides
greater reliability and performance by using path failover and load balancing. This feature
is available for multiported disk arrays from various vendors.
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 “Discovering and configuring newly added disk devices” on page 81 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:
■ An Active/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.
For 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