Once migrated into a vmdk file you cannot migrate back to a RDM (unless you create a new RDM and copy the data from within the guest.). With RDMs in virtual compatibility mode, the raw disk will be migrated to either a thin provisioned or thick vmdk. With RDMs in physical compatibility mode, only the mapping file will be migrated. There are a few things to be aware of though. vmfs/volumes/myVolume/myVMDirectory/myRawDisk.vmdkĪn RDM can be included when you carry out a svMotion or a cold migration. Instead you will be able to refer to the device using the name of the mapping file. Once the RDM has been created you will not need to refer to the device by it’s device name.
User Friendly Persistent Names provide a friendly name for RDM device.
RAW DEVICE MAPPING VMWARE ESXI 5 SOFTWARE
Benefits and Use Cases of Raw Device MappingĪlong with the benefits mentioned above around using SAN software and LUNs greater than 2TB, other benefits include User Friendly Persistent Names, vMotion, Snapshots (virtual compatibility mode only), and NPIV. However, you cannot convert these larger RDMs to virtual disks as it would exceed the 2TB maximum vmdk size. With VMFS5, you can have RDMs larger than 2TB, but only when the RDM is in physical compatibility mode. Though it is important to note, if a VM with a RDM in virtual mode is cloned, the contents of the RDM lun is copied into a vmdk (as the raw lun itself cannot be cloned by a host). Because of this, virtual compatibility mode enables you to use features such as snapshots and cloning. As far as the guest operating system is concerned, it operates in exactly the same way as a vmdk on a VMFS volume. Virtual Compatibility Mode virtualises all SCSI commands between the virtual machine and the storage device. This includes VM snapshots, cloning and storage vMotion. The downside is that you cannot perform certain VM management functions on disks configured in physical mode. A physical mode RDM also allows for Physical to Virtual clustering. An example of a SAN application that requires a physical mode RDM is NetApp’s SnapManager suite. This can be useful when running SAN management software inside a virtual machine, as SAN management agents tend to need direct access to the device, without having the SCSI commands intercepted by the host/virtualised. The host passes all SCSI commands, with the exception of the REPORT LUNS command, directly to the storage device. Physical Compatibility Mode means that there will be minimal SCSI virtualisation of the mapped device. There are two modes in which you can configure an RDM Virtual Compatibility Mode and Physical Compatibility Mode, seen here in the virtual machine settings when adding an RDM: A virtual machine reads the mapping file, then subsequently sends it’s read and write requests directly to the raw LUN. This mapping file acts as a proxy for the physical device and contains metadata used for managing and redirecting access to the raw disk. A mapping file is created, and is referenced in the VM’s configuration file, which points to the raw LUN. You could think of an RDM as a symbolic link from a VMFS volume to a raw LUN.
RAW DEVICE MAPPING VMWARE ESXI 5 WINDOWS
This LUN can then be formatted with any file system, such as NTFS for Windows virtual machines, which removes the need for creating a VMFS datastore and storing a virtual disk (vmdk) on it. An RDM, or Raw Device Mapping, is a mechanism for giving a virtual machine direct access to a LUN.
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