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VMware, Citrix Shed Light On The Importance Of Hypervisors

The software has the potential to break a stranglehold that Microsoft and other big system vendors have had on the exclusive tie between the application and the chip.

What's so great about hypervisors? Seldom has a piece of software gained the aura that VMware's ESX Server or the open source Xen, both hypervisors, carry with them at the moment.

It doesn't hurt that VMware just staged a successful IPO Tuesday, raising $957 million on Wall Street. On the following day, Citrix said it would pay $500 million to acquire XenSource, the little company behind Xen, the other major hypervisor on the market. If nothing else, we know hypervisors are hot.

Does the money lining up behind them mean hypervisors are about to replace the venerable operating system, as VMware CTO Mendel Rosenblum appeared to say at LinuxWorld last week. Or is the hypervisor just another marcher in the long column of faceless software soldiers inside the data center?

Hypervisors are different. They are smart enough to translate application logic into machine code that the x86 instruction set inside a chip can execute. But they are not operating system replacements. They are merely operating system neutral and don't care which operating systems sit above them.

In that sense, their importance lies in the fact that they break a stranglehold that Microsoft and other big system vendors have had on the exclusive tie between the application and the chip. They open the door to new ways of handling processing tasks for an application, reducing the exclusive role of the operating system. Hypervisors might even allow replacing the operating system someday through a new intermediary, like the Java Virtual machine. But such changes are many years away, and operating systems as of today remain a key component inside the virtual machine itself.

Consider what Rosenblum said before his remark on operating system vulnerability. His main point was hypervisors displace -- not replace -- operating systems as the software that talks to the hardware.

Operating systems have held the "privileged" position of passing instructions to the hardware since the advent of general purpose computing. Think Sun Microsystems' Sparc chips and you automatically think of Solaris. Think Intel x86 instruction set chips and you often think of Windows, their dominant operating system.

Hypervisors offer an opportunity to break this exclusive link, Rosenblum said. By talking directly to the hardware, the hypervisor interposes itself between the operating system and the hardware, "lifting" the operating system up a notch and opening the door to new ways of handling operating system tasks, such as memory management.

Putting on his hat as a Stanford University professor of computer science, he said operating system replacement is possible under such circumstances. But he acknowledged he knew of only one example commercially available, BEA's WebLogic Server-Virtual Edition, which became available in July and still lacks a management component. WebLogic can perform operating system tasks, such as thread management, for a Java application by using the Java virtual machine, a runtime environment that can send instructions through the hypervisor to hardware.

So in the future, displacement may lead to operating system replacement. But his example is Java specific and not universally applicable. Nor is it yet a common real life occurrence.

"A Java virtual machine is almost like an operating system itself. The JVM can communicate between [Java application] threads and get something [the application] to run at a lot higher level of efficiency..."

"Virtualization's impact," he concluded, "is to change the role of the operating system."

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