Difference between Physical compatibility RDMs and Virtual compatibility RDMs (2009226)
Purpose
This article provides information on the RDM compatibility modes and helps you to choose the mode that best suits your environment requirements.
Resolution
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.
- 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.
- 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 2TB disk size for RDMs in physical compatibility mode only. The following restrictions apply:
- You cannot relocate larger than 2TB RDMs to datastores other than VMFS5.
- 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.
- To expand the size of the RDM refer Expanding the size of a Raw Device Mapping (RDM) (1007021)
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 both Cluster-in-a-box and cluster-across-boxes.
- To expand the size of the RDM refer 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.
For limitations and characteristics of RDMs during all types of migration, see: Migrating virtual machines with Raw Device Mappings (RDMs)