User guide

# multipath -ll

mpath19 (360001ff0721160000000002688e10002) dm-2 DDN,S2A 6620

[size=21T][features=1 queue_if_no_path][hwhandler=0][rw]

\_ round-robin 0 [prio=50][active]

\_ 1:0:0:2 sdg 8:96 [active][ready]

\_ round-robin 0 [prio=10][enabled]

\_ 2:0:0:2 sdf 8:80 [active][ready]

The previously mentioned Paths are organized into Path Groups. Only one path group can be

active at any time. The Path Selector determines which path in the path group will be used to

handle the next IO. This IO will only go down the active path.

Below is an example of a configuration with 8 paths to the same LUN, with 2 path groups, 4

paths per path group.

[fc_host]# multipath -ll

ddn03_lun076 (360001ff010308e504c000100001d1bf1) dm-75 DDN,S2A 9900

[size=15T][features=0][hwhandler=0][rw]

\_ round-robin 0 [prio=200][active]

\_ 18:0:2:75 sduv 67:624 [active][ready]

\_ 19:0:2:75 sduw 67:640 [active][ready]

\_ 19:0:3:75 sdacf 135:624 [active][ready]

\_ 18:0:3:75 sdadj 65:848 [active][ready]

\_ round-robin 0 [prio=40][enabled]

\_ 19:0:0:75 sdev 129:112 [active][ready]

\_ 18:0:0:75 sdew 129:128 [active][ready]

\_ 19:0:1:75 sdml 69:464 [active][ready]

\_ 18:0:1:75 sdne 71:256 [active][ready]

All paths have a specific Priority. The Priority Callout function within multipath is defined in

multipath.conf and determines the priority for all paths. The group_by_prio path grouping

policy provides the path priority and is used to group paths together and determine the priority

value within the path selector.

The Path Group State displays the current status of the path to the path group. Each path

within the path group may show one of a few different status states. The “active” state means

the path is the optimal path and is capable of handling IO. The “enabled” state means the path

is capable of handling IO, but is not the optimal path to use. The “disabled” state infers that no

path is available to the active path group, and IO will go down another path group if it is in the

ready state.

The Path Group Priority is a weighted value. The multipath daemon will use the highest path

group priority value to determine the active path group.

The Path Selector is a component of multipath that chooses which path to take for the next IO.

The “round-robin” algorithm is recommended for load balancing IO across available paths, and

this is configured in the multipath.conf file using the “path_selector” directive. The “active-

passive” algorithm could also be used.