Virtual Disk ‘X’ is a mapped direct access LUN that is not accessible

After some LUNs are removed and re-presented to an ESX cluster you are unable to power on VMs or migrate them to other hosts in a cluster and see the following error:

Virtual Disk 'X' is a mapped direct access LUN that is not 
accessible

You also receive the following error when doing trying to view the properties of those RDMs when clicking Manage Paths in the Virtual Machine Properties:

There is no multipath configuration for this LUN

The cause of this issue is what is known as a VML mismtach. This occurs when the VML identifier for an RDM on two or more ESX hosts are not same – they are mis-matched. This issue is highlighted in VMware KB article 1016210.

The first thing to do is to ensure that the LUN IDs are the same across all the ESX hosts. If not, they need to be removed and re-presented so that the LUN IDs match. Secondly, you need to remove all the affected RDMs and then re-add them per the steps below:

  1. Power down the virtual machine.
  2. Right-click the virtual machine and click Edit Settings.
  3. Make note of the raw device mapped disks which are attached to the virtual machine.
  4. Remove the raw device mapped disks from the virtual machine.
  5. Click OK.
  6. Right-click the virtual machine and click Edit Settings.
  7. Click Add to re-add the previously removed raw device(s).
  8. Click OK.
  9. Power on the virtual machine and re-attempt the operation which previously failed.

I personally found that the steps above still did not resolve the issue. I then removed the RDMs and cold migrated the faulting VM to the other ESX host in the cluster and was able to add the RDMs. This identified that the cause was the previous ESX server. I power-cycled the faulty ESX server (Windows Admin style) and, migrated the VM back to it and was then able to add the RDMs successfully:

  • http://www.sysads.co.uk Isaac

    Great article