The Dynamic Multipathing (DMP) feature of Veritas Volume Manager (VxVM) provides greater  availability, reliability and performance by using path failover and load balancing. This feature is  available for multiported disk arrays from various vendors. 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.

 

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.

A/A-A or Asymmetric Active/Active arrays comply with the Asymmetric Logical Unit Access (ALUA) method specified in SCSI-3 standards. Usually an A/A-A array behaves like an A/P array rather than an A/A array. A LUN in an active-active asymmetric array can be accessed through both controllers without dramatic consequences. The only limitation is that I/O serviced through a LUN's secondary controller will experience slower performance than I/O serviced through the primary controller. During failover, an A/A-A array behaves like an A/A 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.

An A/P-F array fails over only when it receives special array model-specific SCSI commands from their hosts. Explicit failover provides the control required to achieve high performance with active-passive arrays in clusters, where multiple hosts can issue I/O requests directly to LUN.  

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 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.

Active-passive concurrent array LUNs fail over to secondary paths on alternate array controllers only when all primary paths have failed.  The DMP's ability to do load balancing over multiple primary or secondary paths to a LUN in an active passive array is fully governed by the I/O policy configured for that enclosure