What is Raw Device Mapping?
RDM is a mapping file in a Datastore that acts as a proxy for a raw physical storage device. The RDM allows a virtual machine to directly access and use the storage device. The RDM contains metadata for managing and redirecting disk access to the physical device.
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. But a LUN configured for physical compatibility cannot be cloned, made into a template, or migrated if the migration involves copying the disk. LUNs configured for pass-through mode can be a maximum of 64TB in size.
Virtual compatibility mode – Allows the virtual machine to use VMware snapshots. Virtual compatibility enables the LUN to behave as if it were a virtual disk. When you clone the disk, make a template out of it, or migrate it, the contents of the LUN are copied to a virtual disk (.vmdk) file.
Migrating of virtual machines with RDMs can be performed with virtual machines powered on or powered off. Raw LUNs cannot be migrated, because they are raw disks presented from the SAN. However, the RDM pointer files can be relocated if necessary.