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Introduction
Of the many approaches to x86 systems virtualization
available in the market today, the hypervisor architecture—in
which virtual machines are managed by a software layer that is
installed on bare metal—has gained the greatest market
acceptance. This fact has translated into rapid growth and a
large and expanding customer base for VMware, which
pioneered x86 hypervisors in 2001 with the launch of VMware®
ESX Server. It is no wonder, then, that the hypervisor market
has attracted attention recently from Microsoft
1
and the usual
assortment of venture-funded startups, including the recently
created XenSource.
Competitive marketing notwithstanding, the facts in this market
are as follows:
• The VMware product architecture is rooted in its
experience in solving real-world customer problems. The
choices VMware has made in its hypervisor-based ESX Server
reflect the practical focus on offering the highest levels of
performance, reliability and compatibility. In contrast, both
XenSource and Microsoft have chosen architectural paths that
allow them to get products to market more quickly. These
products may satisfy a limited set of use cases, but have yet
to grapple with the architectural issues of building an enter-
prise-class hypervisor. As they attempt to broaden their appli-
cability, they will encounter the same real-world issues that
VMware did when it first entered the market. The difference,
of course, is that VMware solved these problems long ago.
• VMware offers a wide range of production-tested
solutions, and provides a comprehensive set of innovative
technologies to augment the basic partitioning functions
of its hypervisor. While an architectural comparison is of
interest to those trying to predict the long-term direction of
virtualization technology, what ultimately matters to users are
the solutions that they deploy based on virtualization. Today
VMware offers products that customers are actively using in
production deployments to meet their business demands.
Virtualization: Architectural Considerations
And Other Evaluation Criteria
• VMware is the only enterprise-ready hypervisor available.
Product features aside, vendors must answer questions such
as: How well will the products work with what the customer
already has? How well supported is it? And how manageable
is it? Users rightfully demand a certain level of enterprise
readiness before they broadly deploy a technology in
production. As with solutions, enterprise readiness is a
function of product maturity. VMware has customer refer-
ences that attest to the maturity of VMware based solutions.
• This paper examines these issues—architecture, solution
support and enterprise readiness—in greater detail.
Comparisons focus primarily on XenSource. Microsoft has not
released their hypervisor product and therefore can not be
compared at this time.
Architectural Considerations
The x86 architecture was never designed for virtualization.
Consequently, high-performance virtualization is difficult
to achieve. There are three ways to address the problem of
virtualizing the x86 architecture:
• Transparent virtualization allows operating systems, or
particular components of the operating system, to run inside
virtual machines without modification.
• Paravirtualization requires the operating system to be
modified before it can run inside the virtual machine.
Depending on the part of the operating system being
changed, the modification may be expected and supported
by the operating system vendor (e.g., new drivers) or not (e.g.,
changes to the kernel).
• Hardware can provide explicit support for virtualization. To
date such support is comparatively rare, but as virtualization
has become a standard layer inside enterprise data centers,
hardware vendors have responded with roadmaps promising
such support.
These methods are not mutually exclusive. In fact, it is a
combination of all three, applied to the basic elements of the
hypervisor—CPU, memory management and I/O—that will
ultimately provide the greatest performance, reliability and
compatibility to the end user.
1
In fact, in an unusual move for the industry giant, Microsoft has gone so far as to virtually abandon its existing non-hypervisor product, Virtual Server.
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