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vMotion Compute+Storage RDM Fault Insufficient disk space

Trying to migrate from cluster A to cluster B in single vCenter, getting error while processing RDM migration. Guest VMs containing both VMDK (C: system volume) and RDM (raw disk mapped) LUN data volumes located on storage array... VMware KB (https://kb.vmware.com/s/article/1005241) instructions contain steps not available in the HTML vCenter Client.


Source Cluster A:

1. VMDK disk is on Datastore NFS storage provided by legacy NetApp spinning disk attached FCoE to cluster A running on legacy ESXi hosts.

2. RDM pointer files in VMFS Datastore on Pure storage. Datastore is accessible to all hosts in cluster A and cluster B.


Destination Cluster B:

1. VMDK disk is NFS storage provided by host in Cisco HyperFlex HCI (hyperconverged infrastructure) cluster B.

2. RDM pointer files in VMFS Datastore on the same Pure storage. Datastore is accessible to all hosts in cluster A and cluster B. Really no migration, just need to detach and re-attach the pointer!


ERROR screenshot:



vCenter 7.0 U3d

Source ESXi 6.5

Destination ESXi 7.0 U2c


The VMware KB (https://kb.vmware.com/s/article/1005241) says there should be an 'Advanced' button to inform the migration to not attempt to convert the RDM LUNs to VMDKs on the Pure Datastore. We just need to detach and re-attach the LUNs as RDMs to the guest VM when it is migrated to the new host in Cluster B. Seem like the KB 1005241 is out-dated, referring to the old Flash version of the vCenter Client? 

How do we perform a RDM migration?

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First, you must do this with the VM power off, and physical RDM vMotion is not supported.

Using the workaround:

To work around this issue:

  1. Remove the RDM from the virtual machine, and delete it from disk to ensure that the mapping file is deleted. This will not delete the data in the RDM.
  2. Add the RDM again, and put the mapping file in a different datastore than the ultimate target of the Storage vMotion.
  3. After the virtual machine reconfiguration is complete, attempt the Storage vMotion again.

    Note: To avoid downtime, you can migrate the virtual mode RDM pointer file to an alternate datastore (that is, any other datastore than the actual destination datastore), then Storage vMotion the same pointer file back using either Thick Zeroed or Thick Eager Zeroed disk format. Using the du -ah command, you can confirm if the pointer exists in the directory on the alternate datastore.

Is it when you get the error?

Since you are also doing a vStorage migration for the VM files and disks(not the RDM), you need to do this in two steps. First, the VM and disks(without the RDM) and only after you take care of the RDM.

Luciano Patrão

VCP-DCV, VCAP-DCV Design 2023, VCP-Cloud 2023
vExpert vSAN, NSX, Cloud Provider, Veeam Vanguard
Solutions Architect - Tech Lead for VMware / Virtual Backups

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Blog: https://www.provirtualzone.com | Twitter: @Luciano_PT
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