You can store virtual machine data directly on a SAN LUN instead of storing it in a virtual disk file. This ability is useful if you are running applications in your virtual machines that must detect the physical characteristics of the storage device. Mapping a SAN LUN allows you to use existing SAN commands to manage storage for the disk.
When you map a LUN to a VMFS volume, vCenter Server creates a Raw Device Mapping (RDM) file that points to the raw LUN. Encapsulating disk information in a file allows vCenter Server to lock the LUN so that only one virtual machine can write to it at a time.
The RDM file has a .vmdk extension, but the file contains only disk information that describes the mapping to the LUN on the ESXi host. The actual data is stored on the LUN. You can create the RDM as an initial disk for a new virtual machine or add it to an existing virtual machine.
1. On the Select a Disk page of the New Virtual Machine wizard, select Raw Device Mapping and click Next.
2. From the list of SAN disks or LUNs, select a LUN for your virtual machine to access directly and click Next.
3. Select a datastore for the LUN mapping file and click Next.
You can place the RDM file on the same datastore where your virtual machine configuration file resides, or select a different datastore.
NOTE To use vMotion for virtual machines with enabled NPIV, make sure that the RDM files of the virtual machines are located on the same datastore. You cannot perform Storage vMotion or vMotion between datastores when NPIV is enabled.
4. Select a compatibility mode and click Next.
5 Accept the default or select a different virtual device node.
6 (Optional) To change the way disks are affected by snapshots, click Independent and select an option.you won’t see the option if you select “Physical” as compatibility mode. In my example I’ve selected physical mode, so this option is not presented to us.
7 Click Next.
Your changes are recorded and the Ready to Complete page opens.
Go to Virtual Machine property to confirm RDM is created as could be seen below.