vRanger and RDMs (4044997)

vRanger and RDMs

Information on vRanger and RDMs

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Physical RDM

A Raw Device Mapping (RDM) is a special file in a VMFS volume that acts as a proxy for a raw device. 

RDM might be required if you use Microsoft Cluster Services (MSCS) or if you run SAN Snapshot or other layered applications in the virtual machine. RDM’s better enable

When using a Physical RDM, the VM does not support snapshots.

To backup a physical RDM

If you have a physical RDM’s attached to a VM, the ESX host will not be able to create snapshots on the VM. (VMware limitation)

Deploy the vRanger Physical Agent on the VM.  Backup the VM (and the RDM) using the vRanger Physical Agent (backup as a Physical).  It is recommended that the SAN vendor VSS writers should be installed on the VM.

Virtual RDM

When using Virtual RDMs, the VM does support snapshots and can be backed up with vRanger using network based backup.  vRDMs are not supported by Hot-Add.

NOTE:  any VMDKs or vRDM that are shared between two VMs, snapshots are disabled on the VM and cannot be backed up by vRanger

A physical RDM can be changed to virtual RDM using the vSphere client; however you need to power down the VM to change from physical RDM to virtual RDM.

To exclude an vRDM from backup

In a single VM backup job, de-select the vRDM disk

If the VM is part of a group of VMs to be backed up by a single backup job, to exclude to virtual RDM define it as an "independent" disk for the VM.

RDM vs. VMDK The RDM provides some of the advantages of a virtual disk in the VMFS file system while keeping some advantages of direct access to physical devices. An RDM is used when performance is an issue, in the need of clustering, or if you have a volume that is already formatted with NTFS or other file system. Virtual RDM performance is the same as a VMDK since they are both virtual containers for data. There is no performance gain with a vRDM. With a Physical RDM there is more of a performance gain in regards to Disk I/O, but once again snap shots are disabled to the entire VM when a RDM is attached.