vSphere Storage Migrations of VMs with Raw Device Mappings (RDM)

I was running into a sea of confusion over the expected behaviour of performing Storage Migrations on VMs which had Raw Device Mappings (both Physical & Virtual compatibility modes).

After a bit of Googleing I came across a VMware Blog which covers the scenarios quite nicely.

To quote:

This is what I observed, testing with both pRDMs and vRDMs.

VM with Physical (Pass-Thru) RDMs (Powered On – Storage vMotion):

  • If I try to change the format to thin or thick, then no Storage vMotion allowed.
  • If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN.

VM with Virtual (non Pass-Thru) RDMs (Power On – Storage vMotion):

  • On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM)

VM with Physical (Pass-Thru) RDMs (Powered Off – Cold Migration):

  • On a migrate, if I chose to change the format (via the advanced view), the pRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN

VM with Virtual (non Pass-Thru) RDMs (Power Off – Cold Migration):

  • On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM).

As you can see, there are 3 occasions when an RDM could be converted to a VMDK. Perhaps the most surprising is the fact that a pRDM could be converted to a VMDK, when a cold migration of the VM is attempted, and the format is changed.

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vSphere Best Practices with IBM SVC/Storwize family

I often get asked “What is the recommended multipathing configuration with vSphere and SVC?” and “Are there recommended LUN sizes for VMFS datastores?”.

Luckily there is a white paper that covers a lot of Best Practices when using vSphere with SAN Volume Controller and/or Storwize products.

Similarly, for larger SAN design considerations there is a Redbook that covers a large number of settings specifically regarding vSphere.