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Difference between Physical compatibility RDMs and Virtual compatibility RDMs (2009226)

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This article provides information on the RDM compatibility modes and helps you to select the mode that best suits your environment requirements.


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.
    • You cannot convert larger than 2 TB 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, see 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, 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.

For limitations and characteristics of RDMs during all types of migration, see Migrating virtual machines with Raw Device Mappings (RDMs) (1005241).

Additional Information

  • VMFS5 in ESXi 5.5 now supports up to 62 TB VMDK in a non-passthrough RDM.
  • VMFS5 in ESXi versions prior to 5.5 supported greater than 2 TB disk size for RDMs in physical compatibility mode only. However, for 5.5 (and 6.0), as stated in the Configuration Maximums VMware vSphere 5.5:
    • Raw Device Mapping size (virtual compatibility) - 62 TB
    • Raw Device Mapping size (physical compatibility) is 64 TB
Note: With respect to the information in Frequently Asked Questions on VMware vSphere 5.x for VMFS-5 (2003813), Increasing the size of an upgraded VMFS datastore beyond 2 TB changes the partition type from MBR to GPT. However, all other features/characteristics continue to remain same.

See Also

Request a Product Feature

To request a new product feature or to provide feedback on a VMware product, please visit the Request a Product Feature page.


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