An RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. The mapping file is presented to the management software as an ordinary disk file, available for the usual file-system operations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device.
Key contents of the metadata in the mapping file include the location of the mapped device (name resolution), the locking state of the mapped device, permissions, and so on.
