Raw Device Mapping

ESX Server Configuration Guide page 135.
RDM is a mapping that acts as a proxy for a raw physical storage device and resides as file in a seperate VMFS volume. RDM allows the virtual machine to directly access and use the storage device. RDM file contains metadata for managinf and redirecting disk access to the physical device. (FC Channel and iSCSI only)

RDM provides some of the advantage of direct access to a physical device while keeping some advantages of a virtual disks in VMFS. As a result, it merges VMFS manageability with raw device access.

Mapping a raw device into a datastore, mapping a system LUN, or mapping a disk file to a physical desk volume can be used as terms to refer to RDMs.

Situations considering to use raw LUNs with RDMs.

• When SAN snapshot or other layered applications are run in the virtual machine. The RDM better enables scalable backup offloading systems by using features inherent to the SAN
• In any MSCS clustering scenario that spans physical hosts - virtual-to-virtual clusters as well as physical-to-virtual clusters. In this case, cluster data and quorum disks should be configured as RDMs rather than as files on a shared VMFS.

Think of a RDM as a symbolic link from a VMFS volume to a raw LUN. The mapping amkes LUNs appear as files in a VMFS volume. The RDM, not the raw LUN, is referenced in the virtual machine configuration. The RDM contains a reference to the raw LUN.
Using RDM, you can

• Use vMotion to migrate virtual machines using raw LUNs.
• Add raw LUNs to virtual machines using the vSphere Client.
• Use file system features such as distributed file locking, permissions,and naming

When you can not see LUNs provisioned to the host, you might need to set "config.vpxd.filter.rdmFilter" advanced vCenter option to "False".
Refer KB at http://kb.vmware.com/kb/1010513
Two compatibility modes are available for RDMs

• Virtual compatibility mode: Allows a 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.

Benefits of Raw Device Mapping

• User-Friendly Persistent Names
• Dynamic Name Resolution
• Distributed File Locking
• File Permissions
• File System Operations
• Snapshots
• vMotion
• SAN Management Agents
• N-Port ID Virtualization(NPIV)

Limitations of Raw Device Mapping

• Not available for block devices or certain RAID devices: RDM uses a SCSI serial number to identify the mapped device. Because block devices and some direct-attach RAID devices do not export serial numbers, they can not be used with RDMs.
• Available with VMFS-2 and VMFS-3 volumes only.
• No snapshots in physical compatibility mode.
• No partition mapping: RDM requires the mapped device to be a whole LUN.