Difference between Physical compatibility RDMs and Virtual compatibility RDMs

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.
   
• 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

 To expand the size of the RDM, see Expanding the size of a Raw Device Mapping (RDM).

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).

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).

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:
  • 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, 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)
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