Recently I have received a question: Is storage vMotion compatible with Raw Device Mappings (RDMs)? My first answer was: yes for virtual mode RDMs, no for physical mode RDMs.
After a little research, I had to conclude that the answer above is in some way correct, but there’s a nuance. More information can be found in the following VMware KB article:
In this article one can read: “For vSphere 4.0 and higher, Virtual Disks and Virtual Mode RDM pointer files can be relocated to the destination datastore, and can be converted to thick-provisioned or thin-provisioned disks during migration as long as the destination is not an NFS datastore. Physical Mode RDM pointer files can be relocated to the destination datastore, but cannot be converted.”
So, yes Storage vMotion is supported for virtual RDMs for both the RDM mapping file and the data disk (i.e. the contents of your RDM lun). NFS is not supported in this case. If you want to move the data on your lun to a VMDK, you have to select thin or thick disk in storage vMotion wizard. The operation is one-way: you can only move the data from RDM to VMDK.
What about physical RDMs? Yes, Storage vMotion is supported for physical RDMs, but in a very limited way: it’s only available for the mapping file. E.g., you can move the mapping file to another VMFS volume. You’re not moving the LUN data. NFS is also not supported in this process.
There’s one caveat you have to take in account; if you do a normal Storage vMotion and you are not really aware that you are using RDMs, Storage vMotion will move your RDM data disk to the selected volume. Scott Lowe has a nice article about this on his website: http://blog.scottlowe.org/2010/08/18/storage-vmotion-with-rdms/