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.

When you give a virtual machine direct access to an RDM disk, you create a mapping file 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 mapping file contains only mapping information. The virtual disk data is stored directly on the LUN.

During virtual machine creation, a hard disk and a SCSI or SATA controller are added to the virtual machine by default, based on the guest operating system that you select. If this disk does not meet your needs, you can remove it and add an RDM disk at the end of the creation process.



  1. Right-click a virtual machine in the inventory and select Edit Settings.
  2. On the Virtual Hardware tab, add a new RDM disk. select RDM Disk from the New device drop-down menu and click Add.
    Client Steps
    vSphere Client Click the Add New Device button and select RDM Disk from the drop-down menu.
    vSphere Web Client
    1. Select RDM Disk from the New device drop-down menu at the bottom of the wizard.
    2. Click Add.
    The Select Target LUN dialog box opens.
  3. In the Select Target LUN dialog box, select the target LUN for the raw device mapping and click OK.
    The disk appears in the virtual device list.
  4. Select the location for the mapping file.
    • To store the mapping file with the virtual machine configuration file, select Store with the virtual machine.
    • To select a location for the mapping file, select Browse and select the datastore location for the disk.
  5. Select a compatibility mode.
    Option Description

    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, a virtual machine with a physical compatibility RDM cannot be cloned, made into a template, or migrated if the migration involves copying the disk.


    Allows the RDM to behave as if it were a virtual disk, so that you can use such features as taking snapshots, cloning, and so on. When you clone the disk or make a template out of it, the contents of the LUN are copied into a .vmdk virtual disk file. When you migrate a virtual compatibility mode RDM, you can migrate the mapping file or copy the contents of the LUN into a virtual disk.

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

    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 from an LSI Logic controller and share a data disk with another virtual machine using a BusLogic controller with bus sharing turned on.

  7. (Optional) If you selected virtual compatibility mode, select a disk mode to change the way that disks are affected by snapshots.
    Disk modes are not available for RDM disks using physical compatibility mode.
    Option Description
    Dependent Dependent disks are included in snapshots.
    Independent - Persistent

    Disks in persistent mode behave like conventional disks on your physical computer. All data written to a disk in persistent mode are written permanently to the disk.

    Independent - Nonpersistent

    Changes to disks in nonpersistent mode are discarded when you power off or reset the virtual machine. With nonpersistent mode, you can restart the virtual machine with a virtual disk in the same state every time. Changes to the disk are written to and read from a redo log file that is deleted when you power off or reset.

  8. Click OK.