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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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 526
SEC. 7.10
the software on multiple virtual machines all running on the same physical ma-
chine? Many software vendors are somewhat unsure of what to do here.
The problem is much worse in companies that have a license allowing them to
machines running the software at the same time, especially when virtual
machines come and go on demand.
In some cases, software vendors have put an explicit clause in the license for-
bidding the licensee from running the software on a virtual machine or on an unau-
thorized virtual machine.
For companies that run all their software exclusively on
virtual machines, this could be a real problem.
Whether any of these restrictions
will hold up in court and how users respond to them remains to be seen.
Virtualization technology played a crucial role in the dizzying rise of cloud
computing. There are many clouds. Some clouds are public and available to any-
one willing to pay for the use of resources, others are private to an organization.
Likewise, different clouds offer different things. Some give their users access to
physical hardware, but most virtualize their environments. Some offer the bare ma-
chines, virtual or not, and nothing more, but others offer software that is ready to
use and can be combined in interesting ways, or platforms that make it easy for
their users to develop new services. Cloud providers typically offer different cate-
gories of resources, such as ‘‘big machines’’ versus ‘‘little machines,’’ etc.
For all the talk about clouds, few people seem really sure about what they are
exactly. The National Institute of Standards and Technology, always a good source
to fall back on, lists five essential characteristics:
On-demand self-service.
Users should be able to provision re-
sources automatically, without requiring human interaction.
Broad network access.
All these resources should be available over
the network via standard mechanisms so that heterogeneous devices
can make use of them.
Resource pooling.
The computing resource owned by the provider
should be pooled to serve multiple users and with the ability to assign
and reassign resources dynamically. The users generally do not even
know the exact location of ‘‘their’’ resources or even which country
they are located in.
Rapid elasticity.
It should be possible to acquire and release re-
sources elastically, perhaps even automatically, to scale immediately
with the users’ demands.
Measured service.
The cloud provider meters the resources used in a
way that matches the type of service agreed upon.

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