Understanding multipath Utility to Configure DM-Multipath

The multipath utility is the device mapper target auto-configurator, which is used to detect and configure multiple paths to devices. Use the following command to display usage:

# multipath -h
multipath-tools v0.4.9 (05/33, 2016)
Usage:
  multipath [-a|-A|-c|-w|-W] [-d] [-T tm:val] [-r] [-i] [-v lvl] [-p pol] [-b fil] [-q] [dev]
  multipath -l|-ll|-f [-v lvl] [-b fil] [dev]
  multipath -F [-v lvl]
  multipath -t
  multipath -h

Where:
  -h      print this usage text
  -l      show multipath topology (sysfs and DM info)
  -ll     show multipath topology (maximum info)
  -f      flush a multipath device map
  -F      flush all multipath device maps
  -a      add a device wwid to the wwids file
  -A      add devices from kernel command line mpath.wwids
          parameters to wwids file
  -c      check if a device should be a path in a multipath device
  -T tm:val
          check if tm matches the multipathd timestamp. If so val is
          whether or not the device is a path in a multipath device
  -q      allow queue_if_no_path when multipathd is not running
  -d      dry run, do not create or update devmaps
  -t      dump internal hardware table
  -r      force devmap reload
  -i      ignore wwids file
  -B      treat the bindings file as read only
  -b fil  bindings file location
  -w      remove a device from the wwids file
  -W      reset the wwids file include only the current devices
  -p pol  force all maps to specified path grouping policy :
          . failover            one path per priority group
          . multibus            all paths in one priority group
          . group_by_serial     one priority group per serial
          . group_by_prio       one priority group per priority lvl
          . group_by_node_name  one priority group per target node
  -v lvl  verbosity level
          . 0 no output
          . 1 print created devmap names only
          . 2 default verbosity
          . 3 print debug information
  dev     action limited to:
          . multipath named 'dev' (ex: mpath0) or
          . multipath whose wwid is 'dev' (ex: 60051..)
          . multipath including the path named 'dev' (ex: /dev/sda)
          . multipath including the path with maj:min 'dev' (ex: 8:0) 

Some of the available options are described as follows:

• -v [verbosity] – Specify the verbosity level when displaying paths and multipaths.
• -l – List the multipath topology.
• -ll – List the maximum multipath topology information.
• -f – Flush a multipath device map. Use –F to flush all multipath device maps.
• -c – Check if a device should be a path in a multipath device.
• -p failover | multibus | group_by_serial | group_by_prio |group_by_node_name – Force all maps to the specified path grouping policy.
• -r – Force device map reload.

You can optionally specify a device name to update only the device map that contains the specified device. Use the /dev/sd# format, the major:minor format, the multipath map name (for example, mpathN), or the WWID to specify a device.

A sample output of the multipath –ll command is as follows:

# multipath -ll
...
mpathb(36001405346939038cc9480caf0dd9a9d) dm-3 LIO-ORG ,IBLOCK size=10g features=‘0’ hwhandler=‘0’ wp=rw
‘-+- policy=‘service-time 0’ prio=1 status=active
‘- 2:0:0:2 sdb 8:16 active ready running 

mpatha(36001405a7c28190541f4d61880050090) dm-2 LIO-ORG ,IBLOCK size=10g features=‘0’ hwhandler=‘0’ wp=rw
‘-+- policy=‘service-time 0’ prio=1 status=active
      ‘- 2:0:0:1 sda 8:0     active ready running
... 

The output for mpathb is described as follows:

• mpathb – User-friendly device name
• 36001405346939038cc9480caf0dd9a9d – Unique WWID
• dm-3 – sysfs file name
• LIO-ORG – Vendor name
• IBLOCK – Linux BLOCK device
• size=10g – Size of the DM device
• features=‘0’ – DM features supported
• hwhandler=‘0’ – Hardware handler
• wp=rw – Write permission, set to read-write
• policy=‘service-time 0’ – Path selector algorithm
• prio=1 – Path group priority
• status=active – Path group state
• 2:0:0:2 – SCSI information: host, channel, scsi_id, and LUN
• sdb – Linux device name
• 8:16 – Major and minor numbers
• active ready running – DM path and physical path state