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Raw device mapping (RDM) is a method of disk virtualization in VMware that allows virtual machines to use a storage logical unit number (LUN) device to be directly connected to a virtual machine in a storage area network (SAN). This direct connection can boost disk access performance in I/O-intensive operations.
Raw device mapping is one of two schemes that VMware servers can use to access storage. The other method is Virtual Machine File System (VMFS). VMware recommends VMFS for most applications.
Under RDM, a logical unit manager can be directly attached to a virtual machine. Connecting a device directly can improve performance, especially when virtual machines are clustered. RDM is suited to operations that depend on disk I/O, such as high performance computing.