An RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. The mapping file is presented to the management software as an ordinary disk file, available for the usual file-system operations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device.
RDM has two compatibility modes:
- Physical compatibility mode
- Virtual compatibility mode
Physical compatibility mode
- Physical mode specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software.
- 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.
Note: Other VMkernel modules have the ability to intercept I/O to perform relevant operations such as NMP, Drivers, etc. For a graphical representation of the ESXi storage stack, see the VMkernel and Storage section of the vSphere Storage Guide for ESXi 5.0, ESXi 5.5 and ESXi 6.0.
- Physical mode is useful while running 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.
- Virtual Machine Snapshots are not available when the RDM is used in physical compatibility mode.
- You can use this mode for Physical-to-virtual clustering and cluster-across-boxes.
- VMFS5 supports greater than 2 TB disk size for RDMs in physical compatibility mode only. These restrictions apply:
- You cannot relocate larger than 2 TB RDMs to datastores other than VMFS5
Virtual compatibility mode
- Virtual mode specifies full virtualization of the mapped device.
- VMkernel sends only READ and WRITE to the mapped device. The mapped device appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume.
- The real hardware characteristics are hidden.
- If you are using a raw disk in virtual mode, you can realize the benefits of VMFS, such as advanced file locking for data protection and snapshots for streamlining development processes.
- Virtual mode is more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.
- You can use this mode for Cluster-in-a-box
- To expand the size of the RDM, see Expanding the size of a Raw Device Mapping (RDM) (1007021).
Note: RDM is not available for direct-attached block devices or certain RAID devices. You cannot map a disk partition as RDM. RDMs require the mapped device to be a whole LUN.