Information Technology Reference
In-Depth Information
16.1 Introduction
Grids [1] take advantage of widely available network resources and provide
remote information services. They allow the network resources to be
shared by users and institutions, and can provide information services
remotely upon request. Current grid systems, however, provide limited
separation of the service and the computing platform. They allow users to
access shared network resources, but do not provide an infrastructure so
that the users can customize their own private computing environments
or enforce personalized policies. Recent studies show that system-level
virtualization can help to solve these issues by emulating the resources
and presenting a different view. In system-level virtualization the emu-
lated resources are at the system level. A system-level virtual environ-
ment has many advantages. It allows users to work in a virtualized but
familiar environment, provides an extra layer of hardware isolation and
security to defend computing environments against possible attacks from
malicious applications, and protects the privacy of virtual environments.
Virtualization can be provided at different levels. It could be at the service
level, such as grid service, at process/language level, such as Java virtual
machine, or at the system level. We focus on system-level virtualization.
Unless pointing out explicitly, virtualization means system-level virtual-
ization in this chapter. The following section explains the term “system”
or “system-level” virtualization.
16.1.1 Understanding System Virtualization
In a single statement, virtual machine technology allows the sharing of a
physical hardware between various virtual machines, each running the
same or different type/l avors of operating systems. The software that pro-
vides the features of virtualization is called a hypervisor or virtual machine
monitor (VMM). The VMM can run directly on hardware or in conjugation
with the operating system installed on that hardware. Usually virtual
machines can emulate different Instruction Set Architectures (ISA) at the
software level from what the original hardware gives them capability to
host multiple operating systems on the same hardware. There are many
virtualization techniques, but full virtualization and paravirtualization are
the two most widely used ones. In a fully virtualized system the guest sys-
tem (virtual machine) can have and run an unmodii ed operating system
on it, whereas in paravirtualization technique the guest operating system is
modii ed to efi ciently work with and integrate into the VMM software.
VMware provides virtualization solutions [2] generally based on the full
virtualization technique and Xen [3] uses paravirtualization techniques.
If we gaze into history virtual machine (VM) technology was i rst devel-
oped in the 1960s by IBM to support virtualization for multiple users on
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