6.0.1

For example, you need to use raw LUNs with RDMs in the following situations:
n
When SAN snapshot or other layered applications run in the virtual machine. The RDM beer enables
scalable backup ooading systems by using features inherent to the SAN.
n
In any MSCS clustering scenario that spans physical hosts — virtual-to-virtual clusters as well as
physical-to-virtual clusters. In this case, cluster data and quorum disks should be congured as RDMs
rather than as virtual disks on a shared VMFS.
Think of an RDM as a symbolic link from a VMFS volume to a raw LUN. The mapping makes LUNs appear
as les in a VMFS volume. The RDM, not the raw LUN, is referenced in the virtual machine conguration.
The RDM contains a reference to the raw LUN.
Using RDMs, you can:
n
Use vMotion to migrate virtual machines using raw LUNs.
n
Add raw LUNs to virtual machines using the vSphere Web Client.
n
Use le system features such as distributed le locking, permissions, and naming.
Two compatibility modes are available for RDMs:
n
Virtual compatibility mode allows an RDM to act exactly like a virtual disk le, including the use of
snapshots.
n
Physical compatibility mode allows direct access of the SCSI device for those applications that need
lower level control.
Benefits of Raw Device Mapping
An RDM provides a number of benets, but it should not be used in every situation. In general, virtual disk
les are preferable to RDMs for manageability. However, when you need raw devices, you must use the
RDM.
RDM oers several benets.
User-Friendly Persistent
Names
Provides a user-friendly name for a mapped device. When you use an RDM,
you do not need to refer to the device by its device name. You refer to it by
the name of the mapping le, for example:
/vmfs/volumes/myVolume/myVMDirectory/myRawDisk.vmdk
Dynamic Name
Resolution
Stores unique identication information for each mapped device. VMFS
associates each RDM with its current SCSI device, regardless of changes in
the physical conguration of the server because of adapter hardware
changes, path changes, device relocation, and so on.
Distributed File Locking
Makes it possible to use VMFS distributed locking for raw SCSI devices.
Distributed locking on an RDM makes it safe to use a shared raw LUN
without losing data when two virtual machines on dierent servers try to
access the same LUN.
File Permissions
Makes le permissions possible. The permissions of the mapping le are
enforced at le-open time to protect the mapped volume.
File System Operations
Makes it possible to use le system utilities to work with a mapped volume,
using the mapping le as a proxy. Most operations that are valid for an
ordinary le can be applied to the mapping le and are redirected to operate
on the mapped device.
vSphere Storage
206 VMware, Inc.