What is RDM (Raw Device Mapping)?

Raw Device Mapping (RDM) is VMware’s virtualization technology that allows a virtual machine (VM) to access a logical unit number (LUN) directly. IT administrators can then format the LUN using any file system, such as New Technology File System (NTFS) for Windows, eliminating the need to use the virtual machine file system (VMFS).

RDM is beneficial in cluster configurations like VM-to-VM or physical-to-VM and storage area network (SAN) snapshots. However, it also has some limitations: It doesn’t map disk partitions and may not work on direct-attached block devices.

Definition and Characteristics of RDM

What is RDM?As the name suggests, Raw Device Mapping is a mapping file that maps a LUN directly to a VM. In other words, RDM allows VMs to bypass VMFS—VMware’s default storage management interface—and access the storage device directly. This way, an RDM acts as a proxy for a raw LUN residing in a VMFS volume.

As a proxy, RDM consists of metadata that manages and redirects the VM to the raw physical storage devices. When vSphere administrators configure the VM’s virtual disk (vDisk) to point directly to a LUN on a storage array, an RDM file gets created within the VMFS volume. With an RDM in place, any VM within the cluster can access the storage array, just like any other disk.

A VM reads the mapping file, which resides in the VMFS volume. As a proxy for the raw LUN, an RDM contains the physical storage device’s raw ID. When the VM learns of the storage array’s raw ID, it automatically sends the data it reads and writes to the storage device and no longer references the VMFS datastore.

This way, the VM perceives the RDM as a Small Computer System Interface (SCSI) device, available for the usual file operations. But in the real sense, RDM contains metadata that specifies the following:

Types of RDM

RDM has two modes: physical and virtual. The option you choose depends largely on the virtualization features you want to achieve.

Physical RDM. Physical RDM—also called pass-through RDM (pRDM)—exposes the SCSI device’s physical properties to the guest OS within the VM. Apart from the REPORT LUNs command, which gets virtualized, all other SCSI commands go directly to the guest OS. This allows the VM to take advantage of the lower-level storage features of the SCSI device.

Physical RDM is preferable in the following scenarios:

Virtual RDM. As the name suggests, a virtual RDM (vRDM) virtualizes all the SCSI devices’ underlying physical properties. As such, the device appears the same way a vDisk file would appear on a VMFS volume. Because of the SCSI device’s full virtualization, all commands get virtualized except read and write, which are passed directly to the raw LUN.

vRDM is preferable in the following scenarios:

Benefits of RDM

RDM is beneficial in various use cases, including guest OS clustering, distributed file locking, SAN snapshots, and user-friendly persistent names, among others.

Guest OS clustering. Raw device mapping provides three different ways of implementing clusters using VMs:

Distributed file locking. You can use VMFS distributed locking feature for SCSI devices with RDM. With distributed file locking, you can use a shared raw LUN without worrying about data loss.

SAN snapshots. vRDM allows the guest OS to treat the mapping file more like a vdisk file. Including RDMs in vSphere snapshots is simpler.

User-friendly persistent names. With RDM, you determine which names you would like to use for your SCSI devices. Once you create the mapping name, you don’t need to use the device’s name to refer to it.

File system operations. RDM allows the VM to leverage file system utilities with the raw LUN. As such, you can use most ordinary file operations with RDM files that operate on the SCSI device.

Limitations of RDM

Despite the RDM’s benefits, some limitations exist, such as:

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