In general, virtual disk files are preferable to RDMs for manageability. In the physical compatibility mode, the RDM offers direct access to the SCSI device for those applications that require lower-level control.Īn RDM provides a number of benefits, but it should not be used in every situation.In the virtual compatibility mode, the RDM acts like a virtual disk file.Two compatibility modes are available for RDMs: The RDM contains a reference to the raw LUN. With RDM, VMDK files are only pointers to a VMs disk data, while on VMFS the files also. Both formats enable you to access the virtual machines disk (VMDK), but they differ in approach to storage, and VMware recommends VMFS for the vast majority of VMs. The RDM, not the raw LUN, is referenced in the virtual machine configuration. VMware virtual machines comprise a set of files in typically one of two given formats: virtual machine file system or raw device mapping. The mapping makes LUNs appear as files in a VMFS volume. Virtual Machine can directly access the storage device using RDM and RDM contains metadata which controls the disk access to the physical. RDM is basically a Mapping file acts as a proxy for a raw physical storage device placed in a VMFS volume. Think of an RDM as a symbolic link from a VMFS volume to a raw LUN. Raw device mapping (RDM) is a mapping file that provides direct access to a LUN on an iscsi or fibre channel storage system for a virtual machine. In this case, cluster data and quorum disks are configured as RDMs rather than as virtual disks on a shared VMFS. In any MSCS clustering scenario that spans physical hosts, such as virtual-to-virtual clusters and physical-to-virtual clusters.The RDM enables backup offloading systems by using features inherent to the SAN. When SAN snapshot or other layered applications run in the virtual machine.On certain occasions, you might use raw LUNs or logical disks located in a SAN.įor example, you might use raw LUNs with RDMs in the following situations: Typically, you use VMFS datastores for most virtual disk storage. As a result, it merges the VMFS manageability with the raw device access. The file gives you some of the advantages of direct access to a physical device, but keeps some advantages of a virtual disk in VMFS. The RDM contains metadata for managing and redirecting disk access to the physical device. With the RDM, a virtual machine can access and use the storage device directly. I was able to preserver the entire volume and had no issues.An RDM is a mapping file in a separate VMFS volume that acts as a proxy for a raw physical storage device. I recently moved a 1.5TB LUN attached to a physical host to a VM via RDM when the physical host died. This will be important for cloning and snapshot purposes. Save yourself some headache and make sure the datastore where the VM lives has a block size large enough to support the RDM you are attaching. This works great, remember you have a 2TB size limit on RDM's ( I think its 2TB 'officially') The same way you present a lun to your vmHOSTs on your SAN.įrom there you can add the disk to any VM as you would add a new disk to a VM in its properties, the difference is you select the RDM option instead. You will need to apply the appropriate zoning or mapping on your SAN so that the physical VMware host can access the LUN (essentially removing its acces from a physical server and moving it to the vmHOST). If you want to move that LUN away from a physical server and apply it to say a virtual machine while preserving the data on that LUN, then that's when a RDM comes into play.
Basically an RDM is a formatted LUN volume (windows, linux etc.) that already exists.
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