Veritas Volume Manager 5.0 Administrator's Guide (September 2006)
122 Administering dynamic multipathing (DMP)
How DMP works
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.
â– 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 3-1 illustrates how DMP sets up a node for a disk in a supported disk array.