VMware Storage : RDM: Raw Device Mappings

Hi Friends,

Today i want to post lot of things, as i am not getting enough time on week days. In this post i want to let you know the details about RDM.

What is RDM?

 imageRDM is nothing but Raw Device Mapping to the Virtual Machine, usually to give storage to the virtual machine we will take some space from the physical hard disk and add it as .vmdk file. This is a standard process. I have a question here, like is it possible to add the LUN directly to the virtual machine instead of adding to the ESX server ? yes it is possible using this technology. When you try to add the HDD to the existing virtual machine you will get some options and RDM is one of them. When you add a LUN using RDM what happens is a file is created on the respective Datastore with the virtual machine files which give access to the actual physical LUN which is allocated for this VM. That means the file acts as a proxy between the VM and actual physical storage. You can access the physical LUN directly from the VM with all other VMDK disks.

You can configure RDM in two ways:

Virtual compatibility mode—this mode fully virtualizes the mapped device, which appears to the guest operating system as a virtual disk file on a VMFS volume. Virtual mode provides such benefits of VMFS as advanced file locking for data protection and use of snapshots.image

Physical compatibility mode—this mode provides access to most hardware characteristics of the mapped device. VMkernel passes all SCSI commands to the device, with one exception, thereby exposing all the physical characteristics of the underlying hardware. In this mode, the mapping is done as follows, when we create a mapping, the configuration stored in a file and that file is stored with the vm files in datastore. This file points to the raw device and makes it accessible to the vm.

RDM limitations
There are two types of RDMs: virtual compatibility mode RDMs and physical compatibility mode RDMs. Physical mode RDMs, in particular, have some fairly significant limitations:

  • No VMware snapshots
  • No VCB support, because VCB requires VMware snapshots
  • No cloning VMs that use physical mode RDMs
  • No converting VMs that use physical mode RDMs into templates
  • No migrating VMs with physical mode RDMs if the migration involves copying the disk
  • No VMotion with physical mode RDMs

Virtual mode RDMs address some of these issues, allowing raw LUNs to be treated very much like virtual disks and enabling functionality like VMotion, snapshotting, and cloning. Virtual mode RDMs are acceptable in most cases where RDMs are required. For example, virtual mode RDMs can be used in virtual-to-virtual cluster across physical hosts. Note that physical-to-virtual clusters across boxes, though, require physical mode RDMs.

While virtual disks will work for the large majority of applications and workloads in a VI environment, the use of RDMs--either virtual mode RDMs or physical mode RDMs--can help eliminate potential compatibility issues or allow applications to run virtualized without any loss of functionality.

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