process (vmwp.exe) in the parent partition (which gives poor performance), and
wherever possible, to avoid using it. Now I’m saying every single Hyper-V virtual
machine has to boot from a hard disk attached to the emulated IDE controller.
Doesn’t that mean every virtual machine will have terrible disk performance for the
operating system disk? Yes, a little, but mostly no, because the architects of Hyper-V
did something very clever with the IDE controller.
The IDE controller had to emulate a common IDE controller to provide compatibility
with all operating systems in which the components needed to use synthetic, VMBus-
enabled devices that would not natively be available. Once an operating system is
installed in a Hyper-V virtual machine, one of the first steps is to install Hyper-V
Integration Services, which enlightens the operating system to its virtualized state and
allows it to leverage the synthetic devices available via the VMBus. Integration
Services also enables tighter integration between Hyper-V and the operating system,
such as time synchronization, data exchange, backup services, shutdown, and more.
Once the integration services have been installed and loaded, the IDE controller
switches under the covers from being an emulated IDE device to being a synthetic
device that uses the VMBus and the VSC/VSP model via a component in the guest
called the fast pass filter (storflt). It therefore matches the performance of the
synthetic SCSI controllers that are also available. This means that provided Hyper-V
Integration Services is installed, there is no performance difference between using the
IDE or SCSI controller in a virtual machine after the operating system has booted. The
SCSI controller does offer additional functionality, which is why its use is still
preferred for assets such as data disks.
SCSI CONTROLLER
By default, a generation 1 virtual machine does not have a SCSI controller, but up to
four SCSI controllers can be added to a virtual machine by using the Add Hardware
area of the virtual machine’s property page, as shown in Figure 2.5. Once a virtual
machine has four SCSI controllers, the option to add SCSI controllers will be grayed
out. The SCSI controller is a pure synthetic device fully leveraging the kernel, in-
memory VMBus, which gives essentially the highest, bare-metal storage performance.
The term bare-metal refers to a system that does not use virtualization. When
something is compared to bare metal, the comparison is to a nonvirtualized
environment. In this case, studies have shown that there is no performance loss from
using storage attached to the SCSI controller as compared to the raw performance
capabilities of the underlying storage.