VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Administering Dynamic Multipathing (DMP)
Introduction
Chapter 3106
Introduction
NOTE You may need an additional 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. See the VERITAS Volume Manager
Hardware Notes for information about supported disk arrays.
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.
The multipathing policy used by DMP depends on the characteristics of
the disk array:
• Active/active disk arrays (A/A arrays) permit several paths to be
used concurrently for I/O. Such arrays enable DMP to provide
greater I/O throughput by balancing the I/O load uniformly across
the multiple paths to the disk devices. In the event of a loss of one
connection to an array, DMP automatically routes I/O over the other
available connections to the array.
• Active/passive arrays in auto-trespass mode (A/P arrays) allow I/O
on the primary (active) path, and the secondary (passive) path is
used if the primary path fails. Failover occurs when I/O is received or
sent on the secondary path.
• Active/passive arrays in explicit failover mode (A/PF arrays) require
a special command to be issued to the array for failover to occur.
• Active/passive arrays with LUN group failover (A/PG arrays) treat a
group of LUNs that are connected through a controller as a single
failover entity. Failover occurs at the controller level, and not at the
LUN level (as would be the case for an A/P array in auto-trespass
mode). The primary and secondary controller are each connected to a