I cannot clean Dangling Device Mappers.
OnApp 3.x, 4.x, 5.x
If the disk belongs to an online virtual machine, then rebooting the virtual machine should fix the issue. You can find which virtual machine uses that disk by following this article.
If the VM is offline, then the entries are on one of the HVs. They can be found with
dmsetup ls --tree | grep <<disk_id>> and removed using
[firstname.lastname@example.org ~]# dmsetup ls --tree | grep asdfasdfasdf12
[email@example.com ~]# dmsetup remove asdfasdfasdf12
[firstname.lastname@example.org ~]# dmsetup remove 0_asdfasdfasdf12
[email@example.com ~]# dmsetup remove 1_asdfasdfasdf12
[firstname.lastname@example.org ~]# dmsetup remove 2_asdfasdfasdf12
[email@example.com ~]# dmsetup remove 3_asdfasdfasdf12
'Dangling device mapper' indicates there is some orphaned connections on the HV for a VM disk - these would not usually cause data loss, but they can interfere with transactions and backups as the storage may have issues activating or deactivating a given disk.