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Raw device mapping gives VMs direct storage access, which is helpful in VMware environments with layered or complicated storage systems. But IT administrators should be aware that raw device mapping, or RDM, doesn't support disk partitions. And it comes with other drawbacks, too.
VMware virtual machines typically access a virtual disk such as a Virtual Machine File System (VMFS) volume, but raw device mapping allows VMs to directly access physical storage entities, such as a logical unit number (LUN). RDM provides a map that relates the virtual disk to the physical LUN, which makes the LUN appear as if it were a file in a VMFS volume.
Admins typically choose to connect a virtual machine to a conventional virtual hard disk or an RDM disk when they first create the VM, but it is possible to add an RDM to an existing VM. When creating a new RDM, an administrator selects which LUN to map and the desired data store that holds the RDM. From that point, the RDM remains on the VMFS data store where it points to the physical LUN.
Raw device mapping can operate in either virtual or physical compatibility modes. When a virtual machine uses RDM in virtual compatibility mode, the VM thinks it interacts with a virtual disk file, but it actually communicates with a physical LUN -- the VM doesn't know the difference. The physical compatibility mode offers an alternative mapping that allows direct access to SCSI devices, which can be useful for applications that otherwise expect low-level disk access. Storage management software tools -- such as storage area network and storage resource management, snapshot, and replication software -- often employ physical compatibility mode.
When to use raw device mapping
IT administrators can use RDM with iSCSI initiators to deliver raw device mapping benefits, but this approach can eliminate access to some popular virtualization features.
RDM is generally most useful in complex or layered storage situations. For example, some admins prefer RDM when they run SAN snapshots in a VM and the SAN handles the backups. Raw device mapping also supports vMotion to migrate VMs using LUNs, rather than virtual disks. And RDM is better for clustering across physical host systems. The cluster data and quorum disks should employ RDM, rather than virtual disks on a shared VMFS.
But there are some limitations to consider with RDM. For example, RDM must map to a complete LUN; it does not support partial storage entities such as disk partitions. Plus, RDM doesn't work on all devices. As one example, RDM uses serial numbers of storage devices to ensure proper storage asset identification, but some storage devices -- such as some direct-attached block storage and RAID devices -- do not expose a serial number. Without a serial number, RDM is not available for those storage assets.
Finally, the choice of using raw device mapping in physical or virtual compatibility mode can affect some storage activities. For example, a disk does not support snapshots with RDM in physical compatibility mode; the VM must manage its own snapshot or mirroring tasks. If admins require VM snapshots under RDM, the RDM must run in virtual compatibility mode. As another example, flash read cache expects to use RDMs in virtual compatibility mode and does not support RDMs in physical compatibility mode.
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