Virtual Machine Monitors: Current Technology and Future Trends Developed more than 30 years ago to address mainframe computing problems, virtual machine monitors have resurfaced on commodity platforms, offering novel solutions to challenges in security, reliability, and administration.
t the end of the 1960s, the virtual machine monitor (VMM) came into being as a software-abstraction layer that partitions a hardware platform into one or more virtual machines.1 Each of these virtual machines was sufﬁciently similar to the underlying physical machine to run existing software unmodiﬁed. At the time, general-purpose computing was the domain of large, expensive mainframe hardware, and users found that VMMs provided a compelling way to multiplex such a scarce resource among multiple applications. Thus, for a brief period, this technology ﬂourished both in industry and in academic research. The 1980s and 1990s, however, brought modern multitasking operating systems and a simultaneous drop in hardware cost, which eroded the value of VMMs. As mainframes gave way to minicomputers and then PCs, VMMs disappeared to the extent that computer architectures no longer provided the necessary hardware to implement them efﬁciently. By the late 1980s, neither academics nor industry practitioners viewed VMMs as much more than a historical curiosity. Fast forwarding to 2005, VMMs are again a hot topic in academia and industry: Venture capital ﬁrms are competing to fund startup companies touting their virtual-machine-based technologies. Intel,
AMD, Sun Microsystems, and IBM are developing virtualization strategies that target markets with revenues in the billions and growing. In research labs and universities, researchers are developing approaches based on virtual machines to solve mobility, security, and manageability problems. What happened between the VMM’s essential retirement and its current resurgence? In the 1990s, Stanford University researchers began to look at the potential of virtual machines to overcome difﬁculties that hardware and operating system limitations imposed: This time the problems stemmed from massively parallel processing (MPP) machines that were difﬁcult to program and could not run existing operating systems. With virtual machines, researchers found they could make these unwieldy architectures look sufﬁciently similar to existing platforms to leverage the current operating systems. From this project came the people and ideas that underpinned VMware Inc. (www.vmware.com), the original supplier of VMMs for commodity computing hardware. The implications of having a VMM for commodity platforms intrigued both researchers and entrepreneurs.
WHY THE REVIVAL?
Ironically, the capabilities of modern operating systems and the drop in hardware cost—the very 0018-9162/05/$20.00 © 2005 IEEE
Published by the IEEE Computer Society
combination that had obviated the use of VMMs during the 1980s—began to cause problems that researchers thought VMMs might solve. Less expensive hardware had led to a proliferation of machines, but these machines were often underused and incurred signiﬁcant space and management overhead. And the increased functionality that had made operating systems more capable had also made them fragile and vulnerable. To reduce the effects of system crashes and breakins, system administrators again resorted to a computing model with one application running per machine. This in turn increased hardware requirements, imposing signiﬁcant cost and management overhead. Moving applications that once ran on many physical machines into virtual machines and consolidating those virtual machines onto just a few physical platforms increased use efﬁciency and reduced space and management costs. Thus, the VMM’s ability to serve as a means of multiplexing hardware—this time in the name of server consolidation and...
References: 1. R.P. Goldberg, “Survey of Virtual Machine Research,” Computer, June 1974, pp. 34-45. 2. A. Whitaker, M. Shaw, and S. Gribble, “Scale and
Tal Garﬁnkel is a PhD candidate in computer science at Stanford University. His research interests include operating systems, distributed systems, computer architecture, and security. He received a BA in computer science from the University of California, Berkeley. Contact him at firstname.lastname@example.org.
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