vsphere-esxi-vcenter-server-55-storage-guide

Raw Device Mapping 17Raw device mapping (RDM) provides a mechanism for a virtual machine to have direct access to a LUN onthe physical storage subsystem (Fibre Channel or iSCSI only).The following topics contain information about RDMs and provide instructions on how to create andmanage RDMs.This chapter includes the following topics:• “About Raw Device Mapping,” on page 155• “Raw Device Mapping Characteristics,” on page 158• “Create Virtual Machines with RDMs in the vSphere Web Client,” on page 160• “Manage Paths for a Mapped LUN in the vSphere Web Client,” on page 161About Raw Device MappingAn 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 containsmetadata 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 someadvantages 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, ormapping a disk file to a physical disk volume. All these terms refer to RDMs.Figure 17‐1. Raw Device MappingVirtualmachineopensreads,writesVMFS volumemapping fileaddressresolutionmapped deviceAlthough VMware recommends that you use VMFS datastores for most virtual disk storage, on certainoccasions, you might need to use raw LUNs or logical disks located in a SAN.VMware, Inc. 155