Recently I did some testing of Storage QoS in Server 2012 R2 with IOMeter. I was quite impressed that IO can quickly be reduced on an IO hungry VM.
For those who haven't heard of this great new functionality.
Storage Quality of Service (QoS)
Provides the ability to specify maximum and minimum I/O loads in terms of operations per second (IOPS) for each virtual disk on the VM.
Prevents VM’s from consuming all of the available I/O bandwidth to the underlying physical resource.
Supports Dynamic, Fixed and Differencing Virtual Disks.
Here is a demo of this in action. At first I maxed out the IOPs with IOMeter, I then went on to configure Storage QoS and straight away the IOP's are reduced.