A virtual machine was originally defined by Popek & Goldberg as an efficient, isolated duplicate of a real machine. Current use includes virtual machines which have no direct correspondence to any real hardware. Virtual machines are separated in two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine -- it cannot break out of its virtual world.
Example: A program written in Java would receive services from the Java Runtime Environment software by issuing commands from which the expected result is returned by the Java software. By providing these services to the program, the Java software is acting as a "virtual machine," taking the place of the operating system or hardware for which the program would ordinarily have had to have been specifically written.
Process virtual machine:
A process VM, sometimes called an application virtual machine, runs as a normal application inside an OS and supports a single process. It is created when that process is started and destroyed when it exits. Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.
A process VM provides a high-level abstraction (compared to the low-level ISA abstraction of the system VM) — that of a high-level programming language. Process VMs are implemented using an interpreter; performance comparable to compiled programming languages is achieved by the use of just-in-time compilation.
This type of VM has become popular with the Java programming language, which is implemented using the Java virtual machine. Another example is the .NET Framework, which runs on a VM called the Common Language Runtime.
A special case of process VMs are systems that abstract over the communication mechanisms of a (potentially heterogeneous) computer cluster. Such a VM does not consist of a single process, but one process per physical machine in the cluster. They are designed to ease the task of programming parallel applications by letting the programmer focus on algorithms rather than the communication mechanisms provided by the interconnect and the OS. They do not hide the fact that communication takes place, and as such do not attempt to present the cluster as a single parallel machine.
Unlike other process VMs, these systems do not provide a specific programming language, but are embedded in an existing language; typically such a system provides bindings for several languages (e.g. C and FORTRAN). Examples are PVM (Parallel Virtual Machine) and MPI (Message Passing Interface). They are not strictly virtual machines, as the applications running on top still have access to all OS services, and are therefore not confined to the system model provided by the "VM".
Emulation of the underlying raw hardware (native execution)This approach is described as full virtualization of the hardware, and can be implemented using a Type 1 or Type 2 hypervisor. (A Type 1 hypervisor runs directly on the hardware; a Type 2 hypervisor runs on another operating system, such as Linux). Each virtual machine can run any operating system supported by the underlying hardware. Users can thus run two or more different "guest" operating systems simultaneously, in separate "private" virtual computers.
The pioneer system using this concept was IBM's CP-40, the first (1967) version of IBM's CP/CMS (1967-1972) and the precursor to IBM's VM family (1972-present). With the VM architecture, most users run a relatively simple interactive computing single-user operating system, CMS, as a "guest" on top of the VM control program (VM-CP). This approach kept the CMS design simple, as if it were running alone; the control program quietly provides multitasking and resource management services "behind the scenes". In addition to CMS, VM users can run any of the other IBM operating systems, such as MVS or z/OS. z/VM is the current version of VM, and is used to support hundreds or thousands of virtual machines on a given mainframe. Some installations use Linux for zSeries to run Web servers, where Linux runs as the operating system within many virtual machines.
Full virtualization is particularly helpful in operating system development, when experimental new code can be run at the same time as older, more stable, versions, each in a separate virtual machine. The process can even be recursive: IBM debugged new versions of its virtual machine operating system, VM, in a virtual machine running under an older version of VM, and even used this technique to simulate new hardware.[2]
The standard x86 processor architecture as used in modern PCs does not actually meet the Popek and Goldberg virtualization requirements. Notably, there is no execution mode where all sensitive machine instructions always trap, which would allow per-instruction virtualization.
Despite these limitations, several software packages have managed to provide virtualization on the x86 architecture, even though dynamic recompilation of privileged code, as first implemented by VMware, incurs some performance overhead as compared to a VM running on a natively virtualizable architecture such as the IBM System/370 or Motorola MC68020. By now, several other software packages such as Virtual PC, VirtualBox, Parallels Workstation and Virtual Iron manage to implement virtualization on x86 hardware.
Intel and AMD have introduced features to their x86 processors to enable virtualization in hardware.
Emulation of a non-native system
Virtual machines can also perform the role of an emulator, allowing software applications and operating systems written for another computer processor architecture to be run.
Some virtual machines emulate hardware that only exists as a detailed specification. For
example:One of the first was the p-code machine specification, which allowed programmers to write Pascal programs that would run on any computer running virtual machine software that correctly implemented the specification.
The specification of the Java virtual machine.
The Common Language Infrastructure virtual machine at the heart of the Microsoft .NET initiative.
Open Firmware allows plug-in hardware to include boot-time diagnostics, configuration code, and device drivers that will run on any kind of CPU.
This technique allows diverse computers to run any software written to that specification; only the virtual machine software itself must be written separately for each type of computer on which it runs.
Operating system-level virtualization
VMware Workstation running Ubuntu, on Windows Vista
Operating System-level Virtualization is a server virtualization technology which virtualizes servers on an operating system (kernel) layer. It can be thought of as partitioning: a single physical server is sliced into multiple small partitions (otherwise called virtual environments (VE), virtual private servers (VPS), guests, zones, etc); each such partition looks and feels like a real server, from the point of view of its users. One example here is Solaris zones. You can have multiple guest OS running under the same OS (This is allowed on Solaris 10). But All guest OS have to use the same kernel level--you cannot run a different version; And also you cannot run a different OS than Solaris. Also AIX provides the same technique called Micro Partitioning
The operating system level architecture has low overhead that helps to maximize efficient use of server resources. The virtualization introduces only a negligible overhead and allows running hundreds of virtual private servers on a single physical server. In contrast, approaches such as virtualisation (like VMware) and paravirtualization (like Xen or UML) cannot achieve such level of density, due to overhead of running multiple kernels. From the other side, operating system-level virtualization does not allow running different operating systems (i.e. different kernels), although different libraries, distributions etc. are possible.
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To bo continued..
--Ronnei
