Video DataCentre Economics : Virtualization
There are several approaches to platform virtualization, listed below based on how complete a hardware simulation is implemented.
Emulation or simulation
the virtual machine simulates the complete hardware, allowing an unmodified "guest" OS for a completely different CPU to be run. This approach has long been used to enable the creation of software for new processors before they were physically available. Examples include Bochs, PearPC, PPC version of Virtual PC, QEMU without acceleration, and the Hercules emulator. Emulation is implemented using a variety of techniques, from state machines to the use of dynamic recompilation on a full virtualization platform.
Native virtualization and full virtualization
the virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same CPU) to be run in isolation. Typically, many instances can be run at once. This approach was pioneered in 1966 with CP-40 and CP[-67]/CMS, predecessors of IBM's VM family. Examples include Virtual Iron, VMware Workstation, VMware Server (formerly GSX Server), Parallels Desktop, Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro, and z/VM.
Partial virtualization (and including "address space virtualization")
the virtual machine simulates multiple instances of much (but not all) of an underlying hardware environment, particularly address spaces. Such an environment supports resource sharing and process isolation, but does not allow separate "guest" operating system instances. Although not generally viewed as a virtual machine category per se, this was an important approach historically, and was used in such systems as CTSS, the experimental IBM M44/44X, and arguably such systems as OS/VS1, OS/VS2, and MVS. (Many more recent systems, such as Microsoft Windows and Linux, as well as the remaining categories below, also use this basic approach.)
Paravirtualization
the virtual machine does not necessarily simulate hardware, but instead (or in addition) offers a special API that can only be used by modifying the "guest" OS. This system call to the hypervisor is called a "hypercall" in Xen, Parallels Workstation and Enomalism; it is implemented via a DIAG ("diagnose") hardware instruction in IBM's CMS under VM (which was the origin of the term hypervisor). Examples include VMware ESX Server, Win4Lin 9x, and z/VM.
Operating system-level virtualization
virtualizing a physical server at the operating system level, enabling multiple isolated and secure virtualized servers to run on a single physical server. The "guest" OS environments share the same OS as the host system – i.e. the same OS kernel is used to implement the "guest" environments. Applications running in a given "guest" environment view it as a stand-alone system. Examples are Linux-VServer, Virtuozzo, OpenVZ, Solaris Containers, and FreeBSD Jails.
Application Virtualization
running a desktop or server application locally, using local resources, within an appropriate virtual machine; this is in contrast with running the application as conventional local software, i.e. software that has been 'installed' on the system. (Compare this approach with Software installation and Terminal Services.) Such a virtualized application runs in a small virtual environment containing the components needed to execute – such as registry entries, files, environment variables, user interface elements, and global objects. This virtual environment acts as a layer between the application and the operating system, and eliminates application conflicts and application-OS conflicts. Examples include the Sun Java Virtual Machine, Softricity, Thinstall, Altiris, and Trigence. (This approach to virtualization is clearly different from the preceding ones; only an arbitrary line separates it from such virtual machine environments as Smalltalk, FORTH, Tcl, P-code, or any interpreted language.)
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