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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 504
SEC. 7.1
amount of critical state information about every process is kept in operating system
tables, including information relating to open files, alarms, signal handlers, and
more. When migrating a virtual machine, all that have to be moved are the memory
and disk images, since all the operating system tables move, too.
Another use for virtual machines is to run legacy applications on operating sys-
tems (or operating system versions) no longer supported or which do not work on
current hardware. These can run at the same time and on the same hardware as cur-
rent applications. In fact, the ability to run at the same time applications that use
different operating systems is a big argument in favor of virtual machines.
Yet another important use of virtual machines is for software development. A
programmer who wants to make sure his software works on Windows 7, Windows
8, several versions of Linux, FreeBSD, OpenBSD, NetBSD, and OS X, among
other systems no longer has to get a dozen computers and install different operat-
ing systems on all of them. Instead, he merely creates a dozen virtual machines on
a single computer and installs a different operating system on each one.
Of course,
he could have partitioned the hard disk and installed a different operating system in
each partition, but that approach is more difficult. First of all, standard PCs sup-
port only four primary disk partitions, no matter how big the disk is.
Second, al-
though a multiboot program could be installed in the boot block, it would be neces-
sary to reboot the computer to work on a new operating system.
With virtual ma-
chines, all of them can run at once, since they are really just glorified processes.
Perhaps the most important and buzzword-compliant use case for virtualization
nowadays is found in the
The key idea of a cloud is straightforward: out-
source your computation or storage needs to a well-managed data center run by a
company specializing in this and staffed by experts in the area. Because the data
center typically belongs to someone else, you will probably have to pay for the use
of the resources, but at least you will not have to worry about the physical ma-
chines, power, cooling, and maintenance. Because of the isolation offered by virtu-
alizaton, cloud-providers can allow multiple clients, even competitors, to share a
single physical machine. Each client gets a piece of the pie.
At the risk of stretch-
ing the cloud metaphor, we mention that early critics maintained that the pie was
only in the sky and that real organizations would not want to put their sensitive
data and computations on someone else’sresources. By now, however, virtualized
machines in the cloud are used by countless organization for countless applica-
tions, and while it may not be for all organizations and all data, there is no doubt
that cloud computing has been a success.
With all the hype surrounding virtualizaton in recent years, we sometimes for-
get that by Internet standards virtual machines are ancient. As early as the 1960s.
IBM experimented with not just one but two independently developed hypervisors:

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