In the early days of ESXi, VMware provided the ability to present a volume / LUN from a backend storage array to a virtual machine directly. This technology is called a Raw Device Mapping, also known as an RDM. While the introduction of RDMs provided several key benefits for end-users those topics are outside of the scope of this document. For an in-depth review of RDMs, and their benefits, it is recommended you review the Raw Device Mapping documentation from VMware.