For virtual machines running on an ESX/ESXi host, instead of storing virtual machine data in a virtual disk file, you can store the data directly on a SAN LUN. This is useful if you are running applications in your virtual machines that must know the physical characteristics of the storage device. Additionally, 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. For details about RDM, see the ESX Configuration Guide or ESXi Configuration Guide.

The RDM file has a .vmdk extension, but the file contains only disk information that describes the mapping to the LUN on the ESX/ESXi host. The actual data is stored on the LUN.

In most cases, you can accept the default device node. For a hard disk, a nondefault device node is useful to control the boot order or to have different SCSI controller types. For example, you might want to boot off an LSI Logic controller and share a data disk with another virtual machine using a Buslogic controller with bus sharing turned on.


You cannot deploy a virtual machine from a template and store its data on a LUN. You can only store its data in a virtual disk file.


Select a target LUN and click Next.


Select the datastore on which to store the LUN mapping file.

Store with the virtual machine files

Specify datastore


Click Next.

The Select Compatibility Mode page opens.


Select a compatibility mode and click Next.




Allows the guest operating system to access the hardware directly. Taking a snapshot of this virtual machine will not include this disk.


Allows the virtual machine to use VMware snapshot and other advanced functionality.


Accept the default virtual device node or select a different node.


(Optional) Select the virtual disk Independent mode and select an option.




The disk operates normally except that changes to the disk are permanent even if the virtual machine is reverted to a snapshot.


The disk appears to operate normally, but whenever the virtual machine is powered off or reverted to a snapshot, the content of the disk returns to its original state. All later changes are discarded.

Independent disks are not affected by snapshots.


Click Next.

Your changes are recorded and the Ready to Complete page opens.

Review the virtual machine configuration.