RDM (Raw Device Mapping)是VMware 服务器虚拟化环境中，用以允许一个虚拟机直接访问SAN（Storage Area Networking）中的一个存储LUN（Logical Unit Number）。
从VMware ESX 2.5开始，RDM（Raw Device Mapping）允许VMFS文件卷中一个特殊的文件当作一个Raw Device (裸设备)的代理。
RDM也可以叫做裸设备映射，虚拟机直接使用存储中的LUN，而不经过虚拟化层。
（直译的太差，求大神代为翻译，详见正文）
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RDM简介
What Is Raw Device Mapping?
Introduced with ESX Server 2.5, raw device mapping allows a special file in a VMFS volume to act
as a proxy for a raw device. The mapping file contains metadata used to manage and redirect
disk accesses to the physical device. The mapping file gives you some of the advantages of a
virtual disk in the VMFS file system, while keeping some advantages of direct access to physical
device characteristics. In effect, it merges VMFS manageability with raw device access.
In prior releases, ESX Server has supported using raw disks as storage for virtual machines in
addition to virtual disk files stored in a VMFS volume. While VMFS is recommended for most
virtual disk storage, there is sometimes a need for raw disks. The most common use is as data
drives for Microsoft Cluster Services (MSCS) configurations using clusters between physical
machines, or between physical and virtual machines
With ESX Server 2.5, VMware is encouraging the use of raw device mapping in the following
situations:
· When SAN snapshot or other layered applications are run in the virtual machine. Raw
device mapping better enables scalable backup offloading systems using the features
inherent to the SAN.
· In any MSCS clustering scenario — virtual to virtual clusters as well as physical to virtual
clusters. VMware now recommends that cluster data and quorum disks be configured as
