RDM/vRDM/Clustering Management
RDM (Raw Device Mapping)
A Raw Device Mapping (RDM) is a special file in a VMFS volume that acts as a proxy for a raw device. 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.
RDM might be required if you use Microsoft Cluster Service (MSCS) or if you run SAN Snapshot or other layered applications in the virtual machine. RDM’s better enable
Systems to use the hardware features inherent to SAN arrays Physical RDM When using Physical RDM’s the Virtual machine guest does not support snapshots. Physical RDM’s are not able to utilize the snapshots technology because this mode allows the virtual machine to manage its own snapshot or mirroring operations.
Unfortunately at this time since snapshots apply to the whole Virtual Guest, vRanger cannot backup the OS or any other drive when the VM has physical RDM attached.
***Ranger backups in regards to ESX 2.5 since you could add re-do’s to the specific disk (vmdk) that you wanted to backup. Since the adaptation of snapshots in VI3 this is not possible, and is an all or nothing snapshot scenario. Note: VMware’s Consolidated Backup does not allow for the mounting of the VMDK where a physical RDM is attached to the VM, since it also relies on snapshots. This is a limitation of VMware..
Virtual RDM
When using Virtual RDM’s the Virtual Machine DOES support snapshots and can be skipped if the OS or other data VMDK needs to be backed up with vRanger, VCB also works with vRDM at this time.
RDM vs. VMDK
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.
Physical RDM Clustering
There are two ways to create clustered VM’s.
1. Physical RDM’s
2. A single VMDK shared between two VM’s
1. If you have a physical RDM’s attached to a VM you can not create snap shots. (VMware limitation)
2. VMDK’s or RDM that are shared between two VM’s, snap shots are disabled on the entire VM
To exclude an vRDM
If you want to exclude to virtual RDM from snapshots you have to define it as an “independent” disk for the VM. To do so, you need to switch it to “virtual” mode. This is quite simple; however you need to power down the VM to change a virtual RDM to “independent” mode.
A physical mode RDM can be changed to virtual mode; this is quite simple; however you need to power down the VM to change a physical RDM to virtual. Once this is done you will lose the performance gain of having a physical RDM by changing the physical RDM it to a virtual RDM.
