When you give your virtual machine direct access to a raw SAN LUN, you create a mapping file (RDM) that resides on a VMFS datastore and points to the LUN. Although the mapping file has the same.vmdk extension as a regular virtual disk file, the RDM file contains only mapping information. The actual virtual disk data is stored directly 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. When creating the RDM, you specify the LUN to be mapped and the datastore on which to put the RDM.


Launch the vSphere Client and log in to a vCenter Server system or an ESXi host.


  1. Follow all steps required to create a custom virtual machine.
  2. In the Select a Disk page, select Raw Device Mapping, and click Next.
  3. From the list of SAN disks or LUNs, select a raw LUN for your virtual machine to access directly.
  4. Select a datastore for the RDM mapping file.
    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 when NPIV is enabled.
  5. Select a compatibility mode.
    Option Description
    Physical Allows the guest operating system to access the hardware directly. Physical compatibility is useful if you are using SAN-aware applications on the virtual machine. However, powered on virtual machines that use RDMs configured for physical compatibility cannot be migrated if the migration involves copying the disk. Such virtual machines cannot be cloned or cloned to a template either.
    Virtual Allows the RDM to behave as if it were a virtual disk, so you can use such features as snapshotting, cloning, and so on.
  6. Select a virtual device node.
  7. If you select Independent mode, choose one of the following.
    Option Description
    Persistent Changes are immediately and permanently written to the disk.
    Nonpersistent Changes to the disk are discarded when you power off or revert to the snapshot.
  8. Click Next.
  9. In the Ready to Complete New Virtual Machine page, review your selections.
  10. Click Finish to complete your virtual machine.