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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 for a description of how to make newly added disk hardware known to a host system.
The multipathing policy used by DMP depends on the characteristics of the disk array:
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.
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.
A/P-C, A/PF-C and A/PG-C arrays are 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.
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.
How DMP represents multiple physical paths to a disk as one node illustrates 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
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As described in Enclosure-based naming, 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, c1t99d0 and c2t99d0, 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
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See Changing the disk-naming scheme for details of how to change the naming scheme that VxVM uses for disk devices.
See Discovering and configuring newly added disk devices for a description of how to make newly added disk hardware known to a host system.