Virtual disk compatibility modes provide flexibility in how Raw Device Mappings (RDM) function.

Virtual mode for an RDM specifies full virtualization of the mapped device. It appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume. The real hardware characteristics are hidden. Virtual mode enables you to use VMFS features such as advanced file locking and snapshots. Virtual mode is also more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file. When you clone the disk, make a template out of it, or migrate it (if the migration involves copying the disk), the contents of the LUN are copied into a virtual disk (.vmdk) file.

Physical mode for the RDM specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software. In physical mode, 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 for the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed. 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. LUNs attached to powered-on virtual machines and configured for physical compatibility cannot be migrated if the migration involves copying the disk. Such LUNs cannot be cloned or cloned to a template either.