为什么我们不用虚拟RDM(RAM Device Mapping——裸磁盘映射）？
于是从结构、功能以及容量上对比物理RDM和虚拟RDM，具体如下文。
结构对比：
裸设备映射的虚拟模式是指完全虚拟化的设备映射，它的客户机操作系统文件同VMFS系统中的虚拟磁盘文件完全一样，它使得硬件特性真正的被隐藏，虚拟模式允许客户在裸磁盘映射时还能实现VMFS的好处，如先进的数据保护，简化的流程开发，使之呈现和虚拟磁盘同样的属性。
（Virtual mode for a mapping specifies full virtualization of the mapped device. It appears to theguest operating system exactly the same as a vi rtual disk file in a VMFS volume. The real hardware characteristics are hidden. Virtual mode allows customers using raw disks to realize the benefits of VMFS such as advanced file locking for data protection and redo logs for streamlining development processes. Virtual mode is also more portable across storage hardware, presenting the same behavior as a virtual disk file.）
裸设备映射的物理模式指最小SCSI虚拟化的设备映射，使得SAN管理软件达到最大的灵活性。在物理模式下，VMkernel的所有SCSI命令直接传递到设备上，但有一个例外：报告LUN命令被虚拟化，从而VMkernel可以将LUN隔离拥有的虚拟机。否则，所有的底层硬件的物理特性被暴露。物理模式是很有用的，在虚拟机中运行SAN管理代理或其他基于SCSI目标的软件。物理模式还允许虚拟到物理的群集具有成本效益的高可用性。
（Physical mode for a raw device mapping specifies minimal SCSI virtuali zation of the mapped device, allowing the greatest flexibility for SA N 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. Physical mode also allows virtual to physical clustering for cost-effective high availability）
