Raw Device Mapping (RDM)

There are certain situations when a storage device must be presented directly to the guest operating system inside a VM. In VMware vSphere, this functionality is provided via a raw device mapping (RDM), which is a file stored on a VMFS volume that acts as a proxy for the raw physical device.

Virtual machine data is stored directly on the storage area network device, as opposed to storing it in a VMDK file on a VMFS datastore. An RDM file is created on a VMFS datastore and points to the raw device. The file has a .vmdk extension and contains only the mapping information and not the actual data. Storing the data this way is useful if you are running applications in your virtual machines that are SAN-aware and must know the physical characteristics of the SAN storage device. RDMs are also used with Microsoft Cluster Service (MSCS).

You can use RDMs in two different modes:

• Physical – the VMkernel passes through all SCSI commands with the exception of the REPORT LUNs command. This RDM mode is used when running SAN-aware applications in a virtual machine. Note that physical mode RDMs can’t be included in a vSphere snapshot.
• Virtual – allows the guest OS to treat the RDM more like a virtual disk. Virtual mode RDMs can be included in a vSphere snapshot.