Working with Raw Device Mappings
A Raw Device Mapping (RDM) is a storage device that is presented directly to a virtual machine. RDMs are available in two compatibility modes: physical and virtual. The most important difference is that the virtual compatibility mode RDMs can be a part of a VMware vSphere snapshot. Snapshots of a physical compatibility-mode RDM can only be taken on the storage array.
There are some use cases for RDMs. The most common use case is the quorum disk in a Microsoft Windows cluster. A quorum disk must be in the physical compatibility mode.
Note
For more information about using Microsoft Windows Clusters on VMware vSphere, you should read the Setup for Failover Clustering and Microsoft Cluster Service guide. You can find this guide on https://www.vmware.com/support/pubs/. VMware Knowledge Base article Microsoft Clustering on VMware vSphere: Guidelines for supported configurations (1037959) can be found at http://kb.vmware.com/kb/1037959.
To add a RDM to a virtual machine...
