Devil May be in Hyper-V Details, But VMware's Focus Shouldn't Be

CIO — If the devil is in the details, it stands to reason that so is his opposite number.

In the case of Microsoft's Hyper-V, the Deiological detail is the virtual port through which data flows into and out of a virtual server to be crunched.

Because they're not actual, physical machines that have actual, physical plugs as a data input/output mechanism. Instead they have emulators—drivers that look to the VM like a physical I/O port and to the host operating system like any other data interface.

In Virtual Server—Microsoft's current primary server virtualization product—the I/O mechanism is a driver that emulates a relatively generic Intel Ethernet card, according to Jeff Woolsey, senior program manager, virtualization for Microsoft.

"The Achilles Heel for virtualization is the I/O," Woolsey says. "You can virtualize memory or processors because it's relatively inexpensive and efficient. But the minute have to read a packet over wire or pull data off a disk you see the overhead and the delay. "

As an adapter it was a good choice; it supported almost everything and almost everything supported it. But it was still a straight software-emulation, an approach that "introduces a tremendous amount of overhead for I/O," Woolsey says.

That emulation overhead came on top of the inefficiency of Virtual Server itself, which had to sit on top of another host operating system, support a guest operating system (with a virtual I/O port) which, in turn, supported the application. That's a lot of software layers to go through every time the application has to pass down (waaaay down) to the microprocessor that will ultimately fulfill it.

Microsoft's solution for Hyper-V is an entirely synthetic virtual device—an "enlightened" I/O driver, in Microsoft's new terms.

The "enlightened driver" has a short and efficient path between data and virtual server, reducing the lag in every exchange of data by every virtual server running on a single physical host.

If there's any single thing—other than allowing the hypervisor to run in native mode, close to the metal—that makes Hyper-V a racer compared to Virtual Server's pedal-car, it's the efficiency of the synthetic emulated device compared to emulation of a real device.

"It's the killer success of Hyper-V, the new architecture for I/O," Woolsey says. "It shortens the path for networking, input and video; it allows us to really achieve a much higher level of performance."

VMware also has I/O emulators, and benchmarks to demonstrate how effectively they work. Microsoft is offering benchmarks from partners Intel and QLogic comparing Hyper-V's performance to Virtual Server, and showing by how badly Hyper-V "smokes" Virtual Server, in Woolsey's words.

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