Raw device mapping (RDM) is method to provide direct access to a LUN on a iscsi or fibre channel storage system for a virtual machine. 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.

Use cases for Raw Device Mapping

1.For Microsoft cluster configuration in Virtual machine (Virtual-to-virtual or physical-to-virtual)

2.For configuring a virtual machine to use N_Port ID Virtualization (NPIV)

3.For running SAN management software (Storage resource management software, storage array snapshot software, replication software,etc) inside a virtual machine

4.For any application running in a virtual machine that needs to access a device using hardware-specific SCSI commands

5.RDM is useful in physical-to-virtual conversion operations by avoiding migration of a large data LUN to a VMDK.

Types of Raw Device Mapping

1. virtual compatibility mode

2. physical compatibility mode

Virtual compatibility mode preserves the ability to perform virtual machine snapshots.

Limitations of physical compatibility mode 

1.VMware snapshots are not allowed

2.VCB (VMware consolidated backup) support is not possible, because VCB requires VMware snapshots

3.Cloning of VMs that use physical mode RDMs is not possible

4.Converting VMs into templates that use physical mode RDMs is not possible

5.Migration of VMs with physical mode RDMs if the migration involves copying the disk is not possible.

6.No VMotion with physical mode RDMs

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