I’ve been getting a number of questions from the field on whether StorageLink provides the capabilities of VMware RDM for XenServer and Hyper-V. A little detail is warranted, mainly because the answer is a bit different for XenServer and Hyper-V.
XenServer: The LUN per virtual disk model in effect provides the capabilities of raw device mapping. Prior to XenServer 5.5, customers got their LUN per virtual disk via the custom storage adapters for Netapp and Equallogic. XenServer 5.5 expands this to a larger set of storage arrays, and thats where the latest evolution of StorageLink comes in. StorageLink allows a user to assign/map a set of LUNs, and expose them as raw devices to your VM, in effect, the capability that RDM provides. This applies to existing and formatted LUNs too.
(The caveat here is that the same LUN cannot be attached to two VMs at the same time. This means that this virtual disk cannot be used as a quorum disk in an MSCS cluster of Windows VMs. But then again, Windows 2008 Failover Clustering has Node Majority, and a quorum device is no longer needed)
Hyper-V: StorageLink allows you to assign/map a set of LUNs (existing or new) and expose them as pass-through devices to Hyper-V. No caveats. These LUNs can be shared between VMs. Same as RDM.
One point worth mentioning is that StorageLink does not come in the data path. Once storage volumes are set up and mapped to virtual machines, IO is done using the existing hypervisor mechanism.