What is RDM (Raw Device Mapping)?
About Raw Device Mapping An RDM is a mapping file in a separate VMFS volume that acts as a proxy for a raw physical storage device. With the RDM, a virtual machine can access and use the storage device directly. The RDM contains metadata for managing and redirecting disk access to . Nov 16, · 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.
The RDM allows a virtual machine to directly access and use the storage device. LUN is a unique alphanumeric number of the storage unit. We can say a Virtual Machine in VMware virtualization has access and use storage in two ways. The file gives you some of the advantages of direct access to a physical device while keeping some advantages of a virtual disk in VMFS. It increases the how to make the perfect cuppa of files.
The RDM disk file has the same. We can understand RDM functionality with below picture. Virtual machine can easily access it, perform reads and writes operations. The mapping file is presented to the management software as a normal disk file. RDMs have following benefits:. To configure an RDM for a virtual machine is a super easy task for the server administrators.
This scenario comes in the picture if you want that your virtual machine should use the same LUN and storage unit to read and write the purpose for multiple VMs. Nice blog right here!
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There are two compatibility modes available for RDMs:
Mar 29, · Raw device mapping (RDM) is a method of disk virtualization in VMware that allows virtual machines to use a storage logical unit number (LUN) device to be directly connected to a virtual machine in a storage area network (SAN). This direct connection can boost disk access performance in I/O-intensive operations. May 05, · In the early days of ESXi, VMware provided the ability to present a volume / LUN from a backend storage array to a virtual machine directly. This technology is called a Raw Device Mapping, also known as an RDM. While the introduction of RDMs provided several key benefits for end-users those topics are outside of the scope of this document. Mar 10, · Raw device mapping (RDM) is an option in the vSphere environment that enables a storage LUN to be directly presented to a virtual machine from the storage array.
Raw device mapping RDM is an option in the vSphere environment that enables a storage LUN to be directly presented to a virtual machine from the storage array. Also or where SAN-aware applications are running inside a virtual machine. Virtual machine can access the RDM directly from storage device. The RDM contains metadata for managing and redirecting disk access to the physical device. The mapped file will give some of the advantages of direct access to a physical device, but keeps some advantages of a virtual disk in VMFS.
As a result, it merges the VMFS manageability with the raw device access. RDM can be used in the following situations. Clustering scenario that spans physical hosts, such as virtual-to-virtual clusters and physical-to-virtual clusters.
Two compatibility modes with RDM. Virtual compatibility mode — RDM acts like a virtual disk file. The RDM can use snapshots. Cannot use snapshot feature with RDM in physical compatibility.
Physical compatibility mode allows the virtual machine to manage its own, storage-based, snapshot or mirroring operations. Snapshots feature is available for RDMs with virtual compatibility mode. You cannot map to a disk partition. RDM cannot be used in every situation. Provides a user-friendly name for a mapped device. When you use an RDM, you do not need to refer to the device by its device name ,you can refer fit by the name of the mapping file.
Dynamic Name Resolution. Stores unique identification information for each mapped device. VMFS associates each RDM with its current SCSI device, regardless of changes in the physical configuration of the server because of adapter hardware changes, path changes, device relocation, and so on. File Permissions. Makes file permissions possible. The permissions of the mapping file are enforced at file-open time to protect the mapped volume. File System Operations.
Makes it possible to use file system utilities to work with a mapped volume, using the mapping file as a proxy. Most operations that are valid for an ordinary file can be applied to the mapping file and are redirected to operate on the mapped device.
Makes it possible to use virtual machine snapshots on a mapped volume. Snapshots are not available when the RDM is used in physical compatibility mode.
The mapping file acts as a proxy to allow vCenter Server to migrate the virtual machine by using the same mechanism that exists for migrating virtual disk files. SAN Management Agents. Similarly, any software that needs to access a device by using hardware-specific SCSI commands can be run in a virtual machine. This kind of software is called SCSI target-based software. This ability makes the HBA port appear as multiple virtual ports, each having its own ID and virtual port name.
Virtual machines can then claim each of these virtual ports and use them for all RDM traffic. VMware works with vendors of storage management software to ensure that their software functions correctly in environments that include ESXi.
Find below few applications. Note:Various management products are best run centrally not on the ESXi machine , while others run well on the virtual machines. VMware does not certify these applications or provide a compatibility matrix. Posted By Rajesh Radhakrishnan March 10 Benefits RDM cannot be used in every situation. File Permissions Makes file permissions possible. File System Operations Makes it possible to use file system utilities to work with a mapped volume, using the mapping file as a proxy.
Snapshots Makes it possible to use virtual machine snapshots on a mapped volume. Thank you for reading the post. Share the knowledge if you feel worth sharing it.