RAW Device Mapping

RDM can be used in two forms - Virtual Compatibility and Physical compatibility . 

Virtual compatibility specifies full compatibility of the mapped device. Where as physical mode specified minimal virtualization of iscsi virtualization of the mapped devices providing great flexibility for the SAN storage management software.

Virtual Compatibility :

1. vmkernel Sends READ - WRITE commands to the mapped devices.Mapped devices appears to guest as the exactly same as vmdk files in VMFS volume.
2. All the hardware char are hidden from the virtual machine 
3. All the VMFS features like  - Advanced file Locking , Snapshot creation .

In physical mode

1. The VMkernel passes all SCSI commands to the device, with one exception: the REPORT LUNs command is virtualized so that the VMkernel can isolate the LUN to the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed. 
2. Physical mode is useful to run SAN management agents or other SCSI target-based software in the virtual machine. Physical mode also allows virtual-to-physical clustering for cost-effective high availability.

VMFS5 supports greater than 2TB disk size for RDMs in physical compatibility mode only. The following restrictions apply:

1. You cannot relocate larger than 2TB RDMs to datastores other than VMFS5.
2. You cannot convert larger than 2TB RDMs to virtual disks, or perform other operations that involve RDM to virtual disk conversion. Such operations include cloning.

Switching between Physical and Virtual pass through

1. Power off the virtual machine.
2. Right-click the virtual machine and click Edit Settings. (Note the SCSI device that the RDM is using (for example, SCSI0:5).
3. Remove the RDM hard disk from the virtual machine, ensuring that the Delete from Disk option is selected, and then clickOK.(Note: Unlike VMDK files, the Delete from Disk option removes only the RDM pointer files. Data on the RDM remains intact.)
4. Right-click the virtual machine and click Edit Settings.
5. Add the RDM back to the virtual machine as a new disk and select virtual compatibility mode. Ensure that you use the SCSI device that you noted in step 3.
6. Power on the virtual machine.

Note: An RDM can also be changed from virtual compatibility mode to physical compatibility mode. 

1. To change from virtual to physical compatibility mode, ensure there are no snapshots and select physical compatibility mode when completing step 6 

Migrating RDM files 

It can be done in many ways :

1. Vmotion 
2. cold migration 
3. storage Vmotion