If you are using a storage subsystem that is automatically detected (see Section 5.3, “Supported Storage Subsystems”), no further configuration of the /etc/multipath.conf file is required.

Otherwise, create the /etc/multipath.conf file and add an appropriate device entry for your storage subsystem. See /usr/share/doc/packages/multipath-tools/multipath.conf.annotated for a template with extensive comments.

After having set up the configuration, you can perform a “dry-run” with multipath -v2 -d, which scans the devices, then displays what the setup would look like. The output is similar to the following:

3600601607cf30e00184589a37a31d911 
[size=127 GB] 
[features="0"] 
[hwhandler="1
    emc"] 
\_ round-robin 0 [first] 
  \_ 1:0:1:2 sdav 66:240  [ready ]
  \_ 0:0:1:2 sdr  65:16   [ready ]
\_ round-robin 0 
  \_ 1:0:0:2 sdag 66:0    [ready ]
  \_ 0:0:0:2 sdc  8:32    [ready ]    
   

Paths are grouped into priority groups. Only one priority group is ever in active use. To model an active/active configuration, all paths end up in the same group. To model active/passive configuration, the paths that should not be active in parallel are placed in several distinct priority groups. This normally happens completely automatically on device discovery.

The output shows the order, the scheduling policy used to balance I/O within the group, and the paths for each priority group. For each path, its physical address (host:bus:target:lun), device node name, major:minor number, and state is shown.