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In environments where all guests belong to the same administrative domain, weak isolation may be acceptable
because the administrator can adjust the allocation of resources to alleviate any problems. This does not work
in environments such as multihosting environments where guests are owned and operated by different entities
with no specific incentive to collaborate fairly.
Implementations of operating system level virtualization include Virtuozzo, Linux VServers, OpenVZ, Solaris
Containers, FreeBSD jails, and HP UX 11i Secure Resource Partitions.
Other Types of Virtualization
Two remaining types of virtualization bear mentioning, although unlike the first four we discussed, they are not
capable of executing a full operating system. The first is library virtualization, which emulates operating systems
or subsystems via a special software library. An example of this type of virtualization is the Wine library
available on Linux systems. Wine provides a subset of the Win32 API as a library to allow Windows desktop
applications to be executed in a Linux environment.
The final type of virtualization discussed in this chapter is application virtualization (managed runtime).
Application virtualization is the approach of running applications inside a virtual execution environment. This is
different from running a normal application on the hardware. The virtual execution environment provides a
standard API for cross-platform execution and manages the application's consumption of local resources. It may
also supply resources such as the threading model, environment variables, user interface libraries, and objects
that aid in application programming. The most prevalent example of such a virtual execution environment is the
Sun Java Virtual Machine. It is important to keep in mind that this technique does not virtualize the full-blown
set of hardware necessary for a full operating system.
Overview of Virtualization Types
Table 1.1 contains a summary of the virtualization techniques discussed in this section.
Table 1.1. Virtualization Techniques at a Glance
Type Description Advantages Disadvantages
Emulation
The
hypervisor
presents a
complete
virtual
machine (of a
foreign
computing
architecture
to the host)
enabling
foreign
applications
to run in the
emulated
environment.
Simulates hardware that is not physically available.
Low performance
and low density.
Full
The
hypervisor
provides a
complete
virtual
machine (of
the same
Flexibility—run different versions of different operating systems
from multiple vendors.
Guest OS does
not know that it
is being
virtualized. Can
incur a sizable
performance hit
on commodity