Difference between Physical compatibility RDMs and Virtual compatibility RDMs
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Difference between Physical compatibility RDMs and Virtual compatibility RDMs

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Article ID: 323064

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Updated On:

Products

VMware vCenter Server VMware vSphere ESXi

Issue/Introduction

This article provides information on the RDM compatibility modes and helps you to select 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.
  • 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.
 
For limitations and characteristics of RDMs during all types of migration, see Migrating virtual machines with Raw Device Mappings (RDMs) (1005241).


Additional Information

  • VMFS5 from ESXi 5.5 support 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, from ESXi 5.5, as stated in the Configuration Maximums VMware vSphere 5.5:
    • Raw Device Mapping size (virtual compatibility) - 62 TB
    • Raw Device Mapping size (physical compatibility) - 64 TB
 
  • As per VMware Configuration Maximums , VMFS6 supports: 
    • Raw Device Mapping size (virtual compatibility) - 62 TB
    • Raw Device Mapping size (physical compatibility) - 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.
Migrating virtual machines with Raw Device Mappings (RDMs)
Extending a RDM with snapshots can result in corrupted data
Switching a raw data mapping between physical and virtual compatibility modes in ESX/ESXi
Expanding the size of a Raw Device Mapping (RDM)
Raw Device Mapping for local storage
Frequently Asked Questions on VMware vSphere 5.x for VMFS-5
物理兼容 RDM 与虚拟兼容 RDM 之间的差异
物理互換の RDM と仮想互換の RDM の違い