Device Mapper Multipath on linux is used to detect multiple paths to devices for fail-over or performance  reasons  and  coalesces them.

If a multipath device is configured with features "1 queue_if_no_path", then any process that issues I/O will hang until one or more paths are restored. You can't even remove the device by the following regular steps.

# multipath -F    # Flush unused device off
Jun 01 12:57:23 | mpathaa: map in use
# multipath -ll   # list current paths
mpathaa (1                ) dm-17 IBM,1814      FAStT
size=1.6T features='1 queue_if_no_path' hwhandler='1 rdac' wp=rw
`-+- policy='round-robin 0' prio=0 status=enabled
  |- 4:0:0:7  sdc  8:32   failed faulty running
  `- 4:0:1:7  sdr  65:16  failed faulty running
# echo 1 > /sys/block/sdc/device/delete   # scsi hot delete single path device
# echo 1 > /sys/block/sdr/device/delete
# multipath -ll
mpathaa (1                ) dm-17 ,
size=1.6T features='1 queue_if_no_path' hwhandler='1 rdac' wp=rw
`-+- policy='round-robin 0' prio=0 status=enabled
  |- #:#:#:#  -    #:#    failed faulty running
  `- #:#:#:#  -    #:#    failed faulty running
 # multipath -F 
Jun 01 13:00:23 | mpathaa: map in use
 
To avoid this, set the no_path_retry N parameter in the /etc/multipath.conf file (where N is the number of times the system should retry a path).
Setting the no_path_retry parameter overrides the feature option features "1 queue_if_no_path", even if this features option is set in the devices section and no_path_retry is set in the defaults section of the multipath configuration file.
If you need to use the features "1 queue_if_no_path" option and you experience the issue noted here, use the dmsetup command to edit the policy at runtime for a particular LUN (that is, for which all the paths are unavailable).
For example, if you want to change the policy on the multipath device mpathc from "queue_if_no_path" to "fail_if_no_path", execute the following command.
dmsetup message mpathc 0 "fail_if_no_path"
Note that you must specify the mpathn alias rather than the path.
 
Hope it helps
 
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