Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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. DMP can also differentiate between different enclosures
of a supported array that are connected to the same host system.
See “Discovering and configuring newly added disk devices” on page 83.
The multi-pathing policy that is used by DMP depends on the characteristics of
the disk array.
DMP supports the following standard array types:
Allows 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.
Active/Active (A/A)
A/A-A or Asymmetric Active/Active arrays can
be accessed through secondary storage paths with
little performance degradation. Usually an A/A-A
array behaves like an A/P array rather than an
A/A array. However, during failover, an A/A-A
array behaves like an A/A array.
An ALUA array behaves like an A/A-A array.
Asymmetric Active/Active (A/A-A)
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.
This policy supports 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 switch between an array and a controller.
Failover to the secondary (passive) path occurs
only if all the active primary paths fail.
Active/Passive (A/P)
Administering Dynamic Multi-Pathing
How DMP works
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