RDM Raw Device Mapping Provides a mechanism for the virtual machine (VM) to be directly connected to LUN on physical storage. The RDM contains metadata for managing and redirecting disk access to the physical device. RDM permit the use of SAN commands. Furthermore, RDM can be used for Clustering Services between virtual machines or physical and virtual machines.

RDM can be configured in either virtual or physical compatibility mode:

  • Virtual compatibility mode allows an RDM to act exactly like a virtual disk file, including the use of snapshots.
  • Physical compatibility mode allows direct access of the SCSI device for those applications that need lower level control.

References: VMware

Hope this helps.