Guest Connected vs RAW Device Mapping (RDM)
RAW Device Mapping (RDM) is one of oldest VMware vSphere features which introduced to resolving some limitation on virtualized environments such as virtual disks size limitation and deploying services top of fail-over clustering services.
You can use a raw device mapping (RDM) to store virtual machine data directly on a SAN LUN, instead of storing it in a virtual disk file. You can add an RDM disk to an existing virtual machine, or you can add the disk when you customize the virtual machine hardware during the virtual machine creation process.
