vsphere-esxi-vcenter-server-55-storage-guide

vSphere StorageFor example, you need to use raw LUNs with RDMs in the following situations:• When SAN snapshot or other layered applications run in the virtual machine. The RDM better enablesscalable backup offloading systems by using features inherent to the SAN.• In any MSCS clustering scenario that spans physical hosts — virtual-to-virtual clusters as well asphysical-to-virtual clusters. In this case, cluster data and quorum disks should be configured as RDMsrather than as virtual disks on a shared VMFS.Think of an RDM as a symbolic link from a VMFS volume to a raw LUN. The mapping makes LUNs appearas files in a VMFS volume. The RDM, not the raw LUN, is referenced in the virtual machine configuration.The RDM contains a reference to the raw LUN.Using RDMs, you can:• Use vMotion to migrate virtual machines using raw LUNs.• Add raw LUNs to virtual machines using the vSphere Web Client.• Use file system features such as distributed file locking, permissions, and naming.Two compatibility modes are available for RDMs:• Virtual compatibility mode allows an RDM to act exactly like a virtual disk file, including the use ofsnapshots.• Physical compatibility mode allows direct access of the SCSI device for those applications that needlower level control.Benefits of Raw Device MappingAn RDM provides a number of benefits, but it should not be used in every situation. In general, virtual diskfiles are preferable to RDMs for manageability. However, when you need raw devices, you must use theRDM.RDM offers several benefits.User-Friendly PersistentNamesProvides 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 refer to it bythe name of the mapping file, for example:/vmfs/volumes/myVolume/myVMDirectory/myRawDisk.vmdkDynamic NameResolutionDistributed File LockingFile PermissionsFile System OperationsStores unique identification information for each mapped device. VMFSassociates each RDM with its current SCSI device, regardless of changes inthe physical configuration of the server because of adapter hardwarechanges, path changes, device relocation, and so on.Makes it possible to use VMFS distributed locking for raw SCSI devices.Distributed locking on an RDM makes it safe to use a shared raw LUNwithout losing data when two virtual machines on different servers try toaccess the same LUN.Makes file permissions possible. The permissions of the mapping file areenforced at file-open time to protect the mapped volume.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 anordinary file can be applied to the mapping file and are redirected to operateon the mapped device.156 VMware, Inc.