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. All these terms refer to RDMs.

Although VMware recommends that you use VMFS datastores for most virtual disk storage, on certain occasions, you might need to use raw LUNs or logical disks located in a SAN.

When you give your virtual machine direct access to a raw SAN LUN, you create an RDM disk that resides on a VMFS datastore and points to the LUN. You can create the RDM as an initial disk for a new virtual machine or add it to an existing virtual machine. When creating the RDM, you specify the LUN to be mapped and the datastore on which to put the RDM.
Although the RDM disk file has the same.vmdk extension as a regular virtual disk file, the RDM contains only mapping information. The actual virtual disk data is stored directly on the LUN.

Compatibility Modes

Two compatibility modes are available for RDMs:

§  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.