VMware - RDM

About Raw Device Mapping

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

The file gives you some of the advantages of direct access to a physical device while keeping some advantages of a virtual disk in VMFS. As a result, it merges VMFS manageability with raw device access.

RDMs can be described in terms such as mapping a raw device into a datastore, mapping a system LUN, or mapping a disk file to a physical disk volume. 

Physical Compatibility Mode

In physical mode the SCSI commands are passed via the VMkernel, as all the hardware characteristics are exposed to the VM.
VMFS-5 supports up to 64TB disks size.

• Can not convert a 2TB+ RDM disk to a virtual disk or clone the VM for that fact, it is not supported.
• No snapshot can be taken with PCM disks.
• MS Clustering is supported in PCM.

Virtual Compatibility Mode

• In virtual compatibility mode the maximum VMDK file can only be 2TB in size.
• Virtual machine snapshots are available for RDMs with virtual compatibility mode
• You can storage vMotion VCM disk to other datastores.
• MS Clustering is not supported in VCM.