Which three use cases would necissatate a RDM?

Nicolas Ni

Setup for Failover Clustering and Microsoft Cluster Service ESX ESXi vCenter Server, page 25

Cluster Physical and Virtual Machines

A standby host cluster has specific hardware and software requirements.
Use RDMs in physical compatibility mode (pass-through RDM). You cannot use virtual disks or RDMs in virtual compatibility mode (non-pass-through RDM) for shared storage. (C is correct)

Physical compatibility mode allows the guest operating system to access the hardware directly.
Physical compatibility is useful if you are using SAN-aware applications in the virtual machine.
However, a virtual machine with the physical compatibility RDM cannot be cloned, made into a
template, or migrated if the migration involves copying the disk.
Virtual compatibility allows the RDM to behave as if it were a virtual disk, so you can use such
features as snapshotting, cloning, and so on. [Therefore A is incorrect].

Recommended Detailed Material on RDM’s Physical mode for the RDM specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software. In physical mode, 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 for the owning virtual machine. Otherwise, all physical
characteristics of the underlying hardware are exposed. Physical mode is useful to run SAN management agents or other SCSI target based software in the virtual
machine.

Dave

The answer is actually C, D, and E. Distributed file locking requires RDM – http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.storage.doc_50%2FGUID-9E206B41-4B2D-48F0-85A3-B8715D78E846.html