What Is RDM (Raw Device Mapping)?

what is rdm

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Raw Device Mapping (RDM) allows the virtual machine to access the repository instead of going directly through VMFS. RDM is configured after a physical storage device, i.e., a disk or LUN, is assigned to one or more virtual machines.

In other words, it is impossible to install part of a physical storage device on a virtual machine. Different types of storage, such as local SCSI disks, iSCSI disks, and Fiber Channel disks, can be used with Raw Device Mapping.

There are specific circumstances when a storage device must be presented directly to the guest operating system in the virtual machine. Virtual mode RDM for physical to virtual clustering of the virtual disk storage can separate VMFS volume.

In VMware vSphere virtual volumes, this functionality is provided by Raw Device Mapping (RDM), a VMFS volume stored file that acts as a proxy for the Raw Device Mapping. Virtual machine data is stored literally on the network device in the storage area instead of being stored in a VMDK file on a VMFS data warehouse.

An RDM file is created in a VMFS repository and points to the raw device. The file has a .vmdk extension and contains only the mapping information, not the actual data.

Storing data in this way is helpful if you are using applications in your virtual machines that are aware of SAN and must know the physical features of your SAN storage device. RDMs are also used with Microsoft Cluster Service (MSCS).

The breakdown of Raw Device Mapping RDM

In addition to Raw Device Mapping RDM devices, VMware servers can also use a virtual machine file system (VMFS) to access storage. RDM is a mapping file located in a separate volume of the VMFS that maps the LUN directly to the virtual machine.

The RDM file is completed when vSphere administrators configure the VM virtual drive to point directly to the LUN of a storage array. With RDM, any virtual machine in the VM cluster can access the storage array just like any other disk.

It serves as a proxy and consists of metadata for managing and redirecting virtual machines to raw physical storage devices. These metadata specify:

RDM with VM clusters

It is convenient for VM clusters that need to access the same SCSI device in failover.

Compatibility modes

A physical model for agents aware of SAN in VM and virtual mode for SAN recordings and advanced file locking operations.

Dynamic name resolution

Identification data for each raw LUN, which is then resolved by VMFS based on the current SCSI

SCSI device access modes

Three options are provided:

  1. vDisk file of VMFS data store
  2. physical mode RDM
  3. RDM virtual mode

Types of Raw Device Mapping

  • virtual compatibility mode
  • physical compatibility mode

Virtual Compatibility Mode

Virtual compatibility mode determines the complete virtualization of the mapped device. The guest operating system looks like a VMFS-volume virtual disk file.

The basic hardware features are secreted. Virtual compatibility mode lets clients using raw disks understand the benefits of VMFS, such as advanced data locking and repetition logs to streamline development processes.

Virtual compatibility mode is also more portable through storage hardware, displaying the same behavior as a virtual disk file.

Physical Compatibility Mode

The physical properties of the primary storage logical unit number or LUN are exposed to the guest operating system in the virtual machine in the physical compatibility mode.

This physical compatibility mode is suitable for agents aware of SAN in the VM and provides minimal SCSI virtualization of the raw LUN. Physical compatibility mode is also labeled as pass-through RDM or pRDM.

All SCSI commands are transmitted directly to the guest operating system, so the VM can effectively use several lower-level storage functions.

When to use Raw Device Mapping

Raw Device Mapping is generally most useful in complex or layered storage situations. Some administrators prefer Raw Device Mapping when performing SAN recordings on a virtual machine, and SAN handles backups.

Raw Device Mapping also supports vMotion for virtual machine migration using LUN instead of virtual disks. And Raw Device Mapping is better for grouping through physical host systems. Cluster data and quorum drives should use RDM instead of shared VMFS virtual disks.

But some limitations need to be considered with RDM. For example, RDM must be mapped to a full LUN; it does not support partial storage entities, such as disk partitions. Additionally, RDM does not work on all devices.

For instance, RDM uses the serial numbers of storage devices to provide proper identification of storage media.

Yet, some storage devices, such as directly connected block storage and RAID devices, do not display a serial number. Raw Device Mapping is not available for those storage devices without a serial number.

Benefits of Raw Device Mapping

At first, you need helpful software and LUNs greater than 2TB. Other benefits include:

  • User-friendly persistent names;
  • vMotion;
  • Snapshots – virtual compatibility mode only;
  • NPIV.

Drawbacks of Raw Device Mappings

  • RDM cannot be created on directly attached block devices;
  • Virtual machine recordings can not be used for RDMs in physical compatibility mode;
  • NFS is not supported as an RDM migration destination


Now, let’s conclude … Raw Device Mapping is a mapping file that operates as a proxy for a raw physical storage device housed in a VMFS volume.

Virtual machines can directly access the storage using Raw Device Mapping, and Raw Device Mapping contains metadata that controls disk access performance to the physical device.

Raw Device Mapping is one of two significant strategies VMware servers can utilize to access storage. Raw Device Mapping is suitable for disk I/O dependent operations, such as high-performance computing.

Under Raw Device Mapping, the logical unit manager can be attached directly to a virtual machine. The other method is a virtual machine file system (VMFS). VMware recommends VMFS for most applications.

Connecting direct-attached block devices can improve performance, mainly when grouped with virtual machines. Virtual disk files

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We are meticulously looking forward to creating more concerning Raw Device Mapping and serving you with helpful information about the virtualization domain.