Document a know limitation, due to vmkfstool/NFS limitations we can't use the copyoffload fallback method to migrate NFS based VMS.

If you try to migrate a VM which resides on NFS using the copyoffload method, as NFS isn't block based but rather file based, xvmxfstool would try to use the fallback method, however when dealing with NFS datastore there is a limitation that such stores can't support RDM files which vmkfstool uses for it's cloning target. 

The populator fails with: 
I1027 11:23:41.300944 1 vsphere-xcopy-volume-populator.go:204] channel quit clone task failed with exit code 255, stderr: Failed to clone disk: Function not implemented (2490377).F1027 11:23:41.300954 1 vsphere-xcopy-volume-populator.go:206] clone task failed with exit code 255, stderr: Failed to clone disk: Function not implemented (2490377).


Version-Release number of selected component (if applicable):

MTV 2.10
OCP 4.19

Always / N%

1. Try to migrate a VM which reside on an NFS datastore, or if the VM has one of it's disks on an NFS datastore. 
2. Notice plan fails, populator log reports error mentioned above in description section. 
 

The root cause of the limitation/error "Function not implemented (2490377)." is related to NFS and RDM Pointer Files, RDM pointer files cannot be stored on an NFS datastore. An RDM is a SCSI-based feature designed to map an ESXi host's SCSI device (a LUN) directly to a virtual machine. NFS is a file-based protocol and does not have the necessary low-level SCSI commands (like the ability to lock or map a block device) required to manage an RDM pointer file. 
Thus When you run vmkfstools -i ... -d rdm(p):... it attempts to create the special RDM pointer file at a pth under NFS the function fails with the "Function not implemented" error because the NFS protocol cannot handle the required RDM operations.