Virtualization Systems

The virtualization of computer systems is to operate on the same machine (physical) multiple operating systems simultaneously. Specifically, an operating system, so-called host is installed on the machine "physical" and emulates one or more machines "virtual" (with processor, memory, hard disk, network card, BIOS, etc. ...). Each virtual machine can host an operating system called "virtualized" (or "guest"). The host system provides the partitioning between virtualized systems and the sharing of physical resources: it is a "supervisor".

Note: the host and guest operating systems can all be different: there may be a supervisor Linux, a Windows 2003 server guest system and another system called Mac OS X.

Virtualized systems are manipulated at will by the supervisor: start, stop, freeze, saving context ... In addition, as the hard drive of a virtualized system is usually emulated by a file, it is easy to duplicate a virtualized system or do migrate from one host to another. Virtualization makes it possible to make optimal use of resources of a machine can add virtual machines if the physical machine is underutilized (saving money by pooling of resources) or delete if it is saturated (management of scalability). Finally, virtualization allows a distribution of services on multiple virtual machines and thus better security: if a system is compromised, the others can continue to function normally.

The virtualization software are numerous (QEMU, Bochs, VMWare, ...). Most of them emulate the complete Virtual Machines: the guest systems are unaware of work on virtual hardware. By cons, the emulation virtual machine induces an overload that strike the performance of virtualized systems.

Virtualization System
Solutions
Overview
Microsoft Virtual PC
Microsoft 		The worldwide leader in software acquired Connectix in February 2003, which allows him to propose this solution. Connectix was first made known by Virtual PC for Mac and Windows, allowing solutions to run multiple operating systems (OS / 2, Linux, Solaris, Netware, Windows) and applications on a single workstation .
VMware Workstation
VMware 		This version of workstation software. It allows the creation of one or more virtual machines within a single operating system (usually Windows or Linux). These can be connected to the LAN with a different IP address, while being on the same physical machine (actually existing machine). It is possible to run multiple virtual machines simultaneously, the limit being the performance of the host computer.
QEMU
Fabrice Bellard 		QEMU can run one or more operating systems (and applications) in isolation on a single physical machine. QEMU runs on x86, x86-64, PPC, Sparc and ARM. QEMU runs under the Linux operating system, FreeBSD, OpenBSD, Mac OS X, Unix and Windows. QEMU is a free virtualization tool reliability and performance. The guest OS and share the resources of the physical machine.