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===== Remaining DM devices =====
==== Error description ====
LVM devices are often generated and removed by automatic processes. Examples are the creation of vhosts or the generation of snapshots while performing backups. During these processes LVM devices are sometimes removed while belonging DM devices still remain. The error becomes visible when LVM partitions cannot be created. This happens because DM devices of the same name still exist.
  root@kvm55:~# lvs
    LV              VG           Attr   LSize   Origin …
    images          vg           -wi-ao 200.00g
    lv_dbslave5501  vg           -wi-a-  20.00g
    lv_dbserver5501 vg3_dbserver -wi-ao   4.98g
    lv_nas5501      vg3_nas      -wi-ao 499.98g
  
  root@kvm55:~# dmsetup ls
  vg-images                    (252:0)
  vg-lv_dbslave5501            (252:1)
  350002ac21d632071            (252:2)
  vg3_dbserver-lv_dbserver5501 (252:3)
  350002ac21d622071            (252:4)
  vg3_nas-lv_nas5501           (252:5)
  vg3_nas-lv_nas5501_snapshot  (252:6)
In the depicted example above the command ''lvs'' displays no snapshot for the LVM partition 'lv_nas5501'. But the belonging dm device 'vg3_nas-lv_nas5501_snapshot' still exists as the output of the command ''dmsetup'' shows. In this case the creation of a snapshot with the same name will fail:
  root@kvm55:~# lvcreate -s -n lv_nas5501_snapshot -L 5G \
  /dev/vg3_nas/ lv_nas550
    Locical volume “lv_nas5501_snapshot“ already exists in volume group “ vg3_nas“
==== Failure cause ====
The failure cause is completely unknown. Because automatic processes often lead to the error, it is hard to observe the error cause.
==== Solution ====
It is possible to remove the DM device directly. Before removal check, whether the DM device still has open connections:
  root@kvm55:~# dmsetup info /dev/dm-6
  Name:              vg3_nas-lv_nas5501_snapshot
  State:             ACTIVE
  Read Ahead:        256
  Tables present:    LIVE
  Open count:        0
  Event number:      1
  Major, minor:      252, 6
  Number of targets: 1
  UUID: LVM-keMgdvei4HyiAJrdMgW8EEa8Q8vTze6cVsvVBap2GKZ5oqeO6KBNOUEZTBw8
In the example above the device has no open connections (Open count: 0) and can be removed:
  root@kvm55:~# dmsetup remove /dev/dm-6

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