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Create/add a RAW Device to a virtual Machine in vSphere 6


I recently had the case to map two RAW devices to a virtual machine which run as a Node of a Microsoft SQL cluster.
The questions that went through my mind and i should clarify; Does it make sense to attach RDMs as quorum disk? Which compatibility mode is suitable for this scenario and what are the limitations of the respective mode?

What are RAW Device Disks:

The LUN is directly provisioned from the SAN to the virtual machine bypassing the virtualization level. Thus, the disk formatting will be done on the operating system and not in the VMKernel.
The RDM Mapping File provides direct access to storage and allow the use of SAN inherent features (for example, SAN-level snapshots)


When is the use of RDM recommended?

You should use RDM instead of VMDK when configuring Microsoft Cluster systems, whether it’s P2P, P2V or V2V. The data and quorum disks are mapped in this case as RDM.
Previously, the size of the VMDK was also decisive which type od disks we use, VMDK or RDM, now no longer. Theoretically we can now also attach VMDK with 62 TB to a VM. Does it make sense? may be 😉

Compatibility mode:

Physical Mode:

  • Minimal SCSI virtualization and great flexibility for the SAN software
  • All SCSI commands except the REPORT LUNs command are passed to the device, thus the VMKernel isolates the LUN to the VM
  • It allows a V2P cluster configuration for Low Cost HA
  • Snasphot, cloning or convert to template are not possible with this mode

Virtual Mode

  • The virtual mode sets the full virtualization of the mapped device. The VMkernel only sends READ and WRITE to the assigned device. The associated device appears to the guest operating system just like a virtual disk file in a VMFS volume.
  • Take full advantage of VMFS: snapshot, clones, templates
  • Flash Read Cache support only RDMs in virtual mode.

The size of the disc is no longer decisive which mode you should take
Raw Device Mapping size (virtual compatibility) – 62TB
Raw Device Mapping size (physical compatibility) is 64TB

How to configure raw device mapping

in my case i will use RDM in physical Mode


first check if your SAN provisionned LUNs are available in your ESXi Host


Edit the VM Settings




Click on Add new device and choose RDM Disk




choose the appropriate LUN


The new Disk should be attached to a different SCSI adapter


if you browse the datastore you can see the new RDM File



the last step is on the OS. Initialize and format your new Disk.



Repeat the same steps on the second Cluster Node VM and don´t forget to check the type of the SCSI adpater.

That´s it – VURIT 😉