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. 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

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 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