Hardware Reference
In-Depth Information
Traditionally, hardware architectures have been designed with the expectation
that they will run one operating system at a time. The proliferation of shared com-
puting resources, such as cloud computing servers, benefit from having the ability
to run multiple operating systems at the same time. For example, Internet hosting
services typically provide a complete system to paying clients, upon which can be
built web services. It would be prohibitively expensive to install a new computer in
the server room each time a new customer enrolls. Instead, hosting services typi-
cally use
virtualization
to support the execution of multiple complete systems, in-
cluding the operating system, on one server. Only when the existing servers be-
come too overloaded does the hosting service have to install a new physical server
in the server pool.
While software-only approaches to virtualization do exist, they typically slow
down the virtual system, and they require specific operating system modifications
or utilize complex code analyzers to rewrite programs on the fly. These drawbacks
have led architects to enhance the OSM level of the architecture to support efficient
virtualization directly in hardware.
Hardware virtualization
, as illustrated in Fig. 6-19, is a combination of hard-
ware and software support that enables the simultaneous execution of multiple op-
erating systems on a single physical computer. To the user, each
virtual machine
running on the host computer appears to be a complete standalone computing sys-
tem. The
hypervisor
is a software component, much like an operating system ker-
nel, that creates and manages instances of virtual machines. The hardware provides
the software-visible events that are necessary for the hypervisor to implement shar-
ing policies for the CPU, storage, and I/O devices.
App
X
App
Y
App
Y
App
Z
App
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App
V
App
V
App
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OS A
H/W VM
OS A
OS B
H/W VM
OS C
H/W VM
H/W VM
Hypervisor Software
Host H/W Architecture and Peripherals
Figure 6-19.
Hardware virtualization allows multiple operating systems to run
simultaneously on the same host hardware. The hypervisor implements sharing of
host memory and I/O devices.
The existence of multiple virtual machines on one host computer, each possib-
ly running a different operating system, provides many benefits. In server systems,
