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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 527
7.11.1 Clouds as a Service
In this section, we will look at clouds with a focus on virtualization and operat-
ing systems. Specifically, we consider clouds that offer direct access to a virtual
machine, which the user can use in any way he sees fit. Thus, the same cloud may
run different operating systems, possibly on the same hardware. In cloud terms,
this is known as
Infrastructure As A Service
), as opposed to
form As A Service
, which delivers an environment that includes things such as a
specific OS, database, Web server, and so on),
Software As A Service
which offers access to specific software, such as Microsoft Office 365, or Google
Apps), and many other types of as-a-service. One example of an IAAS cloud is
Amazon EC2, which happens to be based on the Xen hypervisor and counts multi-
ple hundreds of thousands of physical machines. Provided you have the cash, you
can have as much computing power as you need.
Clouds can transform the way companies do computing. Overall, consolidating
the computing resources in a small number of places (conveniently located near a
power source and cheap cooling) benefits from economy of scale.
your processing means that you need not worry so much about managing your IT
infrastructure, backups, maintenance, depreciation, scalability, reliability, perfor-
mance, and perhaps security. All of that is done in one place and, assuming the
cloud provider is competent, done well. You would think that IT managers are hap-
pier today than ten years ago. However, as these worries disappeared, new ones
emerged. Can you really trust your cloud provider to keep your sensitive data safe?
Will a competitor running on the same infrastructure be able to infer information
you wanted to keep private? What law(s) apply to your data (for instance, if the
cloud provider is from the United States, is your data subject to the PATRIOTAct,
even if your company is in Europe)? Once you store all your data in cloud X, will
you be able to get them out again, or will you be tied to that cloud and its provider
forever, something known as
vendor lock-in
7.11.2 Virtual Machine Migration
Virtualization technology not only allows IAAS clouds to run multiple dif-
ferent operating systems on the same hardware at the same time, it also permits
clever management. We have already discussed the ability to overcommit re-
sources, especially in combination with deduplication.
Now we will look at anoth-
er management issue: what if a machine needs servicing (or even replacement)
while it is running lots of important machines? Probably, clients will not be happy
if their systems go down because the cloud provider wants to replace a disk drive.
Hypervisors decouple the virtual machine from the physical hardware. In other
words, it does not really matter to the virtual machine if it runs on this machine or
that machine. Thus, the administrator could simply shut down all the virtual ma-
chines and restart them again on a shiny new machine. Doing so, however, results

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