VM Migration Series – HCX & Raw Device Mappings

Just wanted to share a quick thing here.. to clarify how HCX handles Virtual Machines with Raw Device Mappings. You may encounter RDMs when working with Microsoft Clustered Servers (MSCS, WSFC).

What is a Raw Device Mapping?

From the vSphere Storage doc:

“An RDM is a mapping file in a separate VMFS volume that acts as a proxy for a raw physical storage device. With the RDM, a virtual machine can access and use the storage device directly. The RDM contains metadata for managing and redirecting disk access to the physical device.”

Effectively one virtual machine is associated with a raw LUN (typically the association happens at the ESXi host level). About RAW Device Mappings

An RDM Disk is added to the virtual machine configuration. The RDM disk is configured for Physical Compatibility or Virtual Compatibility

HCX and Raw Device Mappings

From an HCX perspective – hopefully we are in the planning phase, and considering how to migrate an application which may include the aforementioned RDMs.

This is what the HCX user manual has to say on the matter:

  • HCX User Manual – About HCX Bulk Migration – Restrictions:
    Virtual machines with Raw Device Mappings (RDM) in Physical Compatibility mode cannot be bulk migrated.
  • HCX User Manual – About HCX vMotion / Cold Migration – Limitations & Restrictions:
    VMs with Raw Disk Mapping in compatibility mode (RDM-V) can be migrated.
  • HCX User Manual – About HCX vMotion / Cold Migration – Requirements:
    VMs with Raw Disk Mapping in compatibility mode (RDM-V) can be migrated.

The statements above are not wrong, but the information is not complete, and lead to misunderstandings. Here is some additional information to clarify specifically how things work:

  • HCX cannot Bulk | vMotion | RAV migrate virtual machines with Raw Device Mappings in Physical Compatibility Mode. The migration operation fails.
  • HCX can Cold migrate Virtual Machines with Raw Device Mappings in Physical Compatibility Mode. The contents of the raw LUN mapped by the RDM are copied into a new .vmdk file at the destination,
    effectively converting a raw LUN into a virtual disk.
  • HCX can Cold | Bulk | vMotion | RAV migrate Virtual Machines with Raw Device Mappings in Virtual Compatibility Mode. The contents of the raw LUN mapped by the RDM are copied into a new .vmdk file at the destination, effectively converting a raw LUN into a virtual disk.
  • This information in this post is not applicable to the HCX OSAM migration type.
Additional Considerations for Cluster with RDMs?
  • If clustered virtual machines with RDMs MUST be migrated, an approach can be to remove the clustering elements, then migrate individual nodes, and finally rebuild the clustering services at the destination according to what is supported there.
  • As always – check with all of your vendors. Plan and test your approach before hitting go.

Hope this helps!

Related Information

vSphere Replication and Virtual Raw Device Mappings

Migrating Virtual Machines with Raw Device Mappings (VMware KB 1005241)

Clustered VMDK Support for WSFC

Microsoft WSFC with Shared Disks on vSphere 7.0 – Guidelines

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