vSphere Storage - ESXi 5.1 - Documentation - VMware

vSphere Storage
Although VMware recommends that you use VMFS datastores for most virtual disk storage, on certain
occasions, you might need to use raw LUNs or logical disks located in a SAN.
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 better enables
scalable backup offloading 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 physicalto-virtual
clusters. In this case, cluster data and quorum disks should be configured 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 files in a VMFS volume. The RDM, not the raw LUN, is referenced in the virtual machine configuration. 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 Client or the vSphere Web Client.
n Use file system features such as distributed file 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 file, 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 benefits, but it should not be used in every situation. In general, virtual disk
files are preferable to RDMs for manageability. However, when you need raw devices, you must use the RDM.
RDM offers several benefits.
User-Friendly Persistent
Names
Dynamic Name
Resolution
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 file, for example:
/vmfs/volumes/myVolume/myVMDirectory/myRawDisk.vmdk
Stores unique identification information for each mapped device. VMFS
associates each RDM with its current SCSI device, regardless of changes in the
physical configuration 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 different servers try to access the
same LUN.
File Permissions Makes file permissions possible. The permissions of the mapping file are
enforced at file-open time to protect the mapped volume.
File System Operations Makes it possible to use file system utilities to work with a mapped volume,
using the mapping file as a proxy. Most operations that are valid for an ordinary
file can be applied to the mapping file and are redirected to operate on the
mapped device.
180 VMware, Inc.