What is RDM in Virtualization Storage | VM Storage RDM | RDM LUN | Virtualization
What is RDM (Raw Device Mapping)?
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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).
Definition and Characteristics of 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.
RDM Metadata
RDM contains metadata that specifies the following:
• Compatibility modes.
• Dynamic name resolution.
• RDM with VM clusters.
• SCSI device access modes.
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:
• Running SAN management agents and other SCSI-based software in the VM.
• Cost-effective virtual-to-physical clustering.
• Physical-to-virtual clustering.
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:
• Cluster-in-a-box and cluster-across-boxes
• VM snapshots
• VM cloning
• File locking
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:
• Cluster-in-a-box (CIB). When you have two virtual machines running on the same ESX/ESXi host, you can configure them as a cluster. This approach is useful in testing and development scenarios.
• Cluster-across-boxes (CAB). When you have VMs on different ESX/ESXi hosts, you can use vRDM to configure them as a cluster using CAB.
• Physical-to-virtual cluster. With this approach, you get the benefits of both physical and virtual clustering, even though you can’t use vRDM with this method.
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. 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:
• You cannot use RDMs with direct-attached block devices and some redundant array of independent disks (RAID) devices. RDMs use serial numbers to identify the SCSI device, which direct-attached block and RAID devices don’t support.
• You cannot use the SAN snapshot with the physical compatibility mode. The VM can only manage its own storage and other mirroring operations when using physical compatibility mode.
• You cannot redirect to a partition. RDMs need a whole LUN and not a part of it.
• You have to maintain consistent LUN IDs for mapping across all the participating hosts while migrating VMs with vMotion.
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What is RDM in Virtualization Storage | VM Storage RDM | RDM LUN | Virtualization
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