VMFS datastore fails to unmount with 'filesystem is busy' due to stale VM world reference

Products

• VMware vSphere ESXi

Issue/Introduction

• A VMFS datastore to unmount from a single host with warning:
  
  The resource 'Datastore Name: <datastoreName> uuid: <VMFS_UUID>' is in use. Cannot unmount volume 'Datastore Name: <datastoreName> VMFS uuid: <VMFS_UUID>' because 'file system is busy'. Correct the problem and retry the operation.
  
  
• There are no VMs or VM Templates registered on the datastore. This is confirmed from the vSphere Client.
  
  
  

Environment

VMware vSphere ESXi (all versions)

Cause

In a rare instance, where a VM storage is migrated from the datastore to be unmounted, and the VM continues to run on the same host, the VM world remains associated with the datastore's backing LUN:

Verification:
On the LUN backing the datastore, where the VM currently runs:

esxcli storage core device world list -d naa.#############################
Device                                                        World ID    Open Count   World Name
---------                                                        -----------     ----------------   -----------------
... 
naa.#############################  521994                       1   vmm0:<VmName>
...

On the LUN backing the datastore to be unmounted, the same VM world ID remains referenced:

esxcli storage core device world list -d naa.#############################
Device                                                        World ID    Open Count   World Name
---------                                                        -----------     ----------------   -----------------
... 
naa.#############################  521994                       1   vmm0:<VmName>
...

(The world ID is an example and will differ for any given VM.)

The VM is confirmed to be running from the new datastore:
esxcli vm process list 
...
<VmName>
World ID: 521994
Process ID: 0
VMX Cartel ID: 521977
UUID: ## ## ## ## ## ## ## ##-## ## ## ## ## ## ## ##
Display Name:<VmName>
Config File: /vmfs/volumes/<VMFS UUID>/<VmName>/<VmName>.vmx

Resolution

• Migrate the VM to another ESXi host.
  
  • This causes the VM world to be terminated on the host, removing the reference to the world on the LUN with datastore to be unmounted.
    
    
• Unmount the datastore.
  
  
• The VM may be migrated back to the original host if required.

Additional Information

There are several possible reasons why a datastore may report "in use/fileystem is busy" when unmount is attempted. For a list of possible causes see:

Detach a LUN device from ESXi hosts