Understand use cases for Raw Device Mapping

July 18, 2010 Leave a Comment

Raw device mapping (RDM) is a method for a VM to have direct access to a LUN on a Fibre Channel or iSCSI system.  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.

RDM offers several benefits:

  • User-Friendly Persistent Names

  • Dynamic Name Resolution

  • Distributed File Locking

  • File Permissions

  • File System Operations

  • Snapshots

  • vMotion

  • SAN Management Agents

  • N-Port ID Virtualization

Certain limitations exist when you use RDMs:

  • Not available for block devices or certain RAID devices

  • Available with VMFS-2 and VMFS-3 volumes only

  • No snapshots in physical compatibility mode

  • No partition mapping
You need to use raw LUNs with RDMs in the following situations:
  • When SAN snapshot or other layered applications are run in the virtual machine.  The RDM better enables scalable backup offloading systems by using features inherent to the SAN.

  • 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 configured as RDMs rather than as files on a shared VMFS.

Information for this article was gathered from the ESX Configuration Guide.

 

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