Different WSFC cluster setups support different types of shared storage configurations. Some setups support more than one type. Select the recommended type of shared storage for best results.

Differences between Physical compatibility RDMs and Virtual compatibility RDMs

An RDM (Raw Device Mapped) 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 file-system operations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device. There are two types of compatibility modes for RDMs.
  • A Physical compatibility RDM (pRDM), specifies minimal SCSI virtualization of the mapped device.
    • Virtual Machine Snapshots are not available when the RDM is used in physical compatibility mode.
    • RDMs in physical compatibility mode are required to allow SCSI commands be directly passed to a LUN to satisfy the requirements of SCSI-3 Persistent Reservations used by WSFC.
  • A Virtual compatibility RDM (vRDM), 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 the same as a virtual disk file in a VMFS volume.
    • 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.
    • Supported for CIB configuration only.
      Note: Do not use cluster-in-a-box configuration for a production deployment.

For more information, see KB 2147661.

Table 1. Shared Storage Requirements
Storage Type Clusters on One Physical Machine

(Cluster in a Box)

Clusters Across Physical Machines

(Cluster Across Boxes)

Clusters of Physical and Virtual Machines

(Standby Host Clustering)

Clustered VMDK

No Yes No
VMDK in Virtual compatibility mode (SCSi controller sharing mode set to virtual) Yes No No


(physical compatibility mode)






(virtual compatibility mode)


(not recommended)

No No
Note: The multi-writer feature must not be used for a clustered disk resource for WSFC.

Use of in guest options for storage such as iSCSI or SMB shares within guest operating systems configured with WSFC, in any configuration supported by Microsoft, is transparent to ESXi hosts and there is no need for explicit support statements from VMware.