Raw Device Mapping

Raw Device Mapping is a mapping file that acts as a proxy for raw disk storage allowing a virtual machine to transparently access raw disk storage. The RDM, which has a .vmdk extension contains metadata for managing and redirecting disk access to the physical device. Using a physical RDM can be logically seen as presenting a LUN to a virtual machine.

RDM Physical compatibility mode:

• Low level direct access to SCSI devices.
• VSA agent cannot backup RDM devices in physical compatibility mode. Agents must be installed within the VM to access and protect the data.
• Data on RDM storage can be protected using the IntelliSnap feature for supported hardware arrays.
• For large volumes greater than 2TB using RDM volumes can provide a performance advantage over using regular .vmdk files.

RDM Virtual compatibility mode:

• Acts like a virtual disk file which allows virtual disk snapshots to be conducted and protected using VSA.
• Only passes read / write operations to the RDM device.
• Appears to guest OS the same as .vmdk disks – hardware characteristics are masked from OS.

When the VSA agent protects VMware virtual machines, it backups all data in VMDK files and virtual RDM volumes. It does not protect any data on volumes using physical RDM. For data that is located on physical RDM volumes it is recommended to either convert the volume to a standard VMDK file or install agents in the VM to protect the data.

In certain cases, physical RDM volumes can be used as an advantage when designing solutions for protecting large databases. A VSA agent can be used to snap and backup the virtual disks as VMDK files but physical RDM volumes can be filtered from the backup. An application agent can then be installed in the VM and subclients can be configured to protect data on RDM volumes. The application agent provides communication to create consistent point-in-time backups of application data.

If the RDM volume is on a dedicated LUN, the Commvault IntelliSnap^® feature can be used to conduct hardware snapshots of the volume for point-in-time restores and for mounting the volume for proxy backup.