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Raw device mapping

Raw device mapping

(also RDM)

Raw device mapping definition

Raw device mapping (RDM) is a virtualization technology developed by VMware. It permits a virtual machine (VM) to directly access a logical unit number (LUN), bypassing the standard virtual machine file system (VMFS). By doing so, IT administrators gain the flexibility to format the LUN using any desired file system.

This direct access capability makes RDM particularly advantageous in certain scenarios, such as cluster configurations. Examples of these configurations include VM-to-VM clustering and physical-to-VM clustering. Additionally, RDM supports using storage area network (SAN) snapshots effectively.

See also: virtual machine migration, host virtual machine, server virtualization

Examples of raw device mapping

  • Microsoft Cluster Service. For applications running in a Windows Failover Clustering environment (previously known as Microsoft Cluster Service or MSCS), shared disks are needed between the VM nodes. RDM is often used for the shared quorum and data disks.
  • SAN management. SAN management tools or advanced features (like remote replication) require direct LUN access, provided by RDM.
  • Database systems. Large-scale databases might be set up on RDMs to achieve better disk I/O performance or to use specific features of SAN storage solutions.
  • Backup. Backup software might need direct access to the block device for efficient disk-level backup operations.
  • Legacy applications. Older applications may expect direct access to the storage layer and may not function correctly with virtualized storage. RDM can bridge this gap.
  • Physical-to-virtual (P2V) or Virtual-to-physical (V2P) migrations. If an organization anticipates frequent migrations between physical servers and VMs, RDM helps ensure that physical servers and VMs can access the disk.
  • Testing and development. Developers and testers can simulate real-world scenarios by having VMs interact directly with LUNs on the SAN, as in a physical server setup.

Further reading

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