Raw Device Mapping is a mapping file that provides direct access to LUN on a iSCSI or fiber channel storage system. RDM is basically a mapping file acts as a proxy for a raw physical storage device placed in a VMFS volume.
Virtual machine can directly access the storage device using RDM and RDM contains metadata which controls the disk access to the physical device. RAW Device Mapping (RDM) gives you some of the advantages of direct access to a physical device while keeping some of the advantages of a virtual disk in VMFS.
Types of RDM Compatibility mode:
- Virtual compatibility mode: Allows an RDM to act exactly like a virtual disk file including the use of snapshots.
- Physical compatiblity mode: Allows direct access of the SCSI device for those applications that need lower level control.
Use cases for RDM:
- For Microsoft cluster configuration in VM
- For running SAN management software inside a VM
- For any application runing a VM that needs to access a device using hardware specific SCSI comands.
- RDM is useful in physical-to-virtual conversion operations by avoiding migration of a large data LUN to VMDK.
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