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How DMP works

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.

See "Discovering and configuring newly added disk devices" on page 82.

The multipathing policy used by DMP depends on the characteristics of the disk array.

DMP supports the following standard array types:

Active/Active (A/A) 

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. 

Asymmetric Active/Active 

(A/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. 

Active/Passive (A/P) 

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. 

Active/Passive in explicit failover mode or non-autotrespass mode (A/P-F) 

The appropriate command must be issued to the array to make the LUNs fail over to the secondary path. 

Active/Passive with LUN group failover (A/P-G) 

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. 

Concurrent Active/Passive (A/P-C) 

Concurrent Active/Passive in explicit failover mode or non-autotrespass mode (A/PF-C)  

Concurrent Active/Passive with LUN group failover (A/PG-C) 

Variants of the A/P, AP/F and A/PG array types that support 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 hub or switch between an array and a controller. Failover to the secondary (passive) path occurs only if all the active primary paths fail. 

An array support library (ASL) may define array types to DMP in addition to the standard types for the arrays that it supports.

VxVM uses DMP metanodes (DMP nodes) to access disk devices connected to the system. For each disk in a supported array, DMP maps one node to the set of paths that are connected to the disk. Additionally, DMP associates the appropriate multipathing policy for the disk array with the node. For disks in an unsupported array, DMP maps a separate node to each path that is connected to a disk. The raw and block devices for the nodes are created in the directories /dev/vx/rdmp and /dev/vx/dmp respectively.

How DMP represents multiple physical paths to a disk as one node shows how DMP sets up a node for a disk in a supported disk array.

How DMP represents multiple physical paths to a disk as one node

How DMP represents multiple physical paths to a disk as one node

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VxVM implements a disk device naming scheme that allows you to recognize to which array a disk belongs.

Example of multipathing for a disk enclosure in a SAN environment shows an example where two paths, sdf and sdm, exist to a single disk in the enclosure, but VxVM uses the single DMP node, enc0_0, to access it.

Example of multipathing for a disk enclosure in a SAN environment

Example of multipathing for a disk enclosure in a SAN environment

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See "Enclosure-based naming" on page 23.

See "Changing the disk-naming scheme" on page 118.

See "Discovering and configuring newly added disk devices" on page 82.