VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Administering Dynamic Multipathing (DMP)

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 loaduniformly across the multiplepaths to the disk devices. Inthe 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, andnot atthe LUN level(as wouldbe the casefor anA/P array in

auto-trespass mode). 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.