126Administering dynamic multipathing (DMP)

How DMP works

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 arrays in explicit failover mode (or non-autotrespass mode) are termed A/PF arrays. DMP issues the appropriate low-level command to make the LUNs fail over to the secondary path.

The paths of an Active/Passive array are not considered to be on different controllers when mirroring across controllers (for example, when creating a volume using vxassist make specified with the mirror=ctlr attribute). A/P-C, A/PF-C and A/PG-C arrays are variants of the A/P, A/PF 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 Active/Active disk array (A/A arrays) permits 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 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.

Note: An array support library (ASL) may define additional array 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.

Figure 3-1illustrates how DMP sets up a node for a disk in a supported disk array.