A raw device mapping (RDM) is a special file in a VMFS volume that acts as a proxy for a raw device, such as a SAN LUN. With the RDM, an entire SAN LUN can be directly allocated to a virtual machine. The RDM provides some of the advantages of a virtual disk in a VMFS datastore, while keeping some advantages of direct access to physical devices.

An RDM might be required if you use Microsoft Cluster Service (MSCS) or if you run SAN snapshot or other layered applications on the virtual machine. RDMs enable systems to use the hardware features inherent to a particular SAN device. However, virtual machines with RDMs do not display performance gains compared to virtual machines with virtual disk files stored on a VMFS datastore.

For more information on the RDM, see the ESX Configuration Guide or ESXi Configuration Guide.