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

vSphere Storage• Replication softwareSuch software uses a physical compatibility mode for RDMs so that the software can access SCSI devicesdirectly.Various management products are best run centrally (not on the ESXi machine), while others run well onthe virtual machines. VMware does not certify these applications or provide a compatibility matrix. To findout whether a SAN management application is supported in an ESXi environment, contact the SANmanagement software provider.RDM Considerations and LimitationsCertain considerations and limitations exist when you use RDMs.• The RDM is not available for direct-attached block devices or certain RAID devices. The RDM uses aSCSI serial number to identify the mapped device. Because block devices and some direct-attach RAIDdevices do not export serial numbers, they cannot be used with RDMs.• If you are using the RDM in physical compatibility mode, you cannot use a snapshot with the disk.Physical compatibility mode allows the virtual machine to manage its own, storage-based, snapshot ormirroring operations.Virtual machine snapshots are available for RDMs with virtual compatibility mode.• You cannot map to a disk partition. RDMs require the mapped device to be a whole LUN.• If you use vMotion to migrate virtual machines with RDMs, make sure to maintain consistent LUN IDsfor RDMs across all participating ESXi hosts.• Flash Read Cache does not support RDMs in physical compatibility. Virtual compatibility RDMs aresupported with Flash Read Cache.Raw Device Mapping CharacteristicsAn RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. Themapping file is presented to the management software as an ordinary disk file, available for the usual filesystemoperations. To the virtual machine, the storage virtualization layer presents the mapped device as avirtual SCSI device.Key contents of the metadata in the mapping file include the location of the mapped device (nameresolution), the locking state of the mapped device, permissions, and so on.RDM Virtual and Physical Compatibility ModesYou can use RDMs in virtual compatibility or physical compatibility modes. Virtual mode specifies fullvirtualization of the mapped device. Physical mode specifies minimal SCSI virtualization of the mappeddevice, allowing the greatest flexibility for SAN management software.In virtual mode, the VMkernel sends only READ and WRITE to the mapped device. The mapped deviceappears to the guest operating system exactly the same as a virtual disk file in a VMFS volume. The realhardware characteristics are hidden. If you are using a raw disk in virtual mode, you can realize the benefitsof VMFS such as advanced file locking for data protection and snapshots for streamlining developmentprocesses. Virtual mode is also more portable across storage hardware than physical mode, presenting thesame behavior as a virtual disk file.158 VMware, Inc.