vSphere Storage - ESXi 5.1 - Documentation - VMware

Raw Device Mapping 18
Raw device mapping (RDM) provides a mechanism for a virtual machine to have direct access to a LUN on
the physical storage subsystem (Fibre Channel or iSCSI only).
The following topics contain information about RDMs and provide instructions on how to create and manage
RDMs.
This chapter includes the following topics:
n “About Raw Device Mapping,” on page 179
n “Raw Device Mapping Characteristics,” on page 182
n “Create Virtual Machines with RDMs,” on page 184
n “Create Virtual Machines with RDMs in the vSphere Web Client,” on page 185
n “Manage Paths for a Mapped Raw LUN,” on page 186
n “Manage Paths for a Mapped LUN in the vSphere Web Client,” on page 187
About Raw Device Mapping
An RDM is a mapping file in a separate VMFS volume that acts as a proxy for a raw physical storage device.
The RDM allows a virtual machine to directly access and use the storage device. The RDM contains metadata
for managing and redirecting disk access to the physical device.
The file gives you some of the advantages of direct access to a physical device while keeping some advantages
of a virtual disk in VMFS. As a result, it merges VMFS manageability with raw device access.
RDMs can be described in terms such as mapping a raw device into a datastore, mapping a system LUN, or
mapping a disk file to a physical disk volume. All these terms refer to RDMs.
Figure 18-1. Raw Device Mapping
VMFS volume
mapping file
opens
Virtual
machine
address
resolution
reads,
writes
mapped device
VMware, Inc. 179