Virtualization

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See also: Virtual machine, Hypervisor, and Comparison of virtual machines
This article is about a real world technology involving computer systems. For a fictional use of this term, see Scanners (Code Lyoko).

In computing, virtualization is a broad term that refers to the abstraction of computer resources. One useful definition is "a technique for hiding the physical characteristics of computing resources from the way in which other systems, applications, or end users interact with those resources. This includes making a single physical resource (such as a server, an operating system, an application, or storage device) appear to function as multiple logical resources; or it can include making multiple physical resources (such as storage devices or servers) appear as a single logical resource."[1] It can also include making one physical resource to appear, with somewhat different characteristics, as one logical resource.

The term is an old one: it has been widely used since the 1960s, and has been applied to many different aspects and scopes of computing—from entire networks to individual capabilities or components. The common theme of all virtualization technologies is the hiding of technical detail, through encapsulation. Virtualization creates an external interface that hides an underlying implementation, e.g. by multiplexing access, by combining resources at different physical locations, or by simplifying a control system. Recent development of new virtualization platforms and technologies has refocused attention on this mature concept.

Like such terms as abstraction and object orientation, virtualization is used in many different contexts. This article reviews common uses of the term, divided into two main parts:[citation needed]

  • Platform virtualization involves the simulation of whole computers.
  • Resource virtualization involves the simulation of combined, fragmented, or simplified resources.

Of course, virtualization is also an important concept in non-computer contexts. Many control systems implement a virtualized interface to a complex device; thus a modern car's gas pedal does much more than just increase the flow of fuel to the engine; and a fly-by-wire system presents a simplified "virtual airplane"[citation needed] which may have little to do with the physical implementation.

From a business structure perspective, Virtualization is the process of converting a traditional, brick & mortar business to a Virtual Business.[citation needed]

Another useful concept, opposing virtualization, is transparency: a virtual artifact is visible, perceivable, but does not physically exist; a transparent one exists physically, but is invisible in use.

Contents

[edit] A design pattern

Most forms of virtualization involve a computing design pattern[citation needed] that relate a consumer and provider. A consumer and provider interact using some interface. Virtualization places an intermediary between consumer and provider that acts on both sides of the interface, providing the interface for the actual consumer and consuming the interface of the actual provider. Usually this is done either to allow a multiplicity of consumers to interact with one provider, or one consumer to interact with a multiplicity of providers, or both, with only the intermediary being aware of multiplicities.

Ideally, consumers and providers that work together directly, in an unvirtualized environment, should work without modification in a virtualized context. An example is virtual address spaces, in which an intermediary (the virtual memory manager or address space manager) is inserted between a real address space and its consumer. The intermediary provides a multiplicity of virtual address spaces to support multiple consumers; the consumers and the provider (the real memory) generally remain unaware of these multiplicities.

[edit] Platform virtualization

The original sense of the term virtualization, dating from the 1960s, is in the creation of a virtual machine[citation needed] using a combination of hardware and software. For convenience, we will call this platform virtualization. The term virtual machine apparently dates from the experimental IBM M44/44X system. The creation and management of virtual machines has also been referred to as creating pseudo machines, in the early CP-40 days, and server virtualization more recently. The terms virtualization and virtual machine have both also acquired additional meanings through the years.

Platform virtualization is performed on a given hardware platform by host software (a control program), which creates a simulated computer environment, a virtual machine, for its guest software. The guest software, which is often itself a complete operating system, runs just as if it were installed on a stand-alone hardware platform. Typically, many such virtual machines are simulated on a single physical machine, their number limited only by the host’s hardware resources. Typically there is no requirement for a guest OS to be the same as the host one. The guest system often requires access to specific peripheral devices to function, so the simulation must support the guest's interfaces to those devices. Trivial examples of such devices are hard disk drive or network interface card.

There are several approaches to platform virtualization, listed below based on how complete a hardware simulation is implemented. (The following terms are not universally-recognized as such, but the underlying concepts are all found in the literature.[citation needed])

Emulation or simulation
the virtual machine simulates[clarify] 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, PowerPC 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. This approach was pioneered in 1966 with IBM CP-40 and CP-67, predecessors of VM family. Examples outside mainframe field include Parallels Workstation, Parallels Desktop for Mac, VirtualBox, Virtual Iron, Virtual PC, VMware Workstation, VMware Server (formerly GSX Server), QEMU, Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro.
Hardware enabled virtualization
the virtual machine has its own hardware and allows a guest OS to be run in isolation.[citation needed] In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Examples include VMware Workstation, VMware Fusion, Microsoft Virtual PC, Parallels Desktop for Mac, and Parallels Workstation.
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 systems like 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[clarify] the "guest" OS. This system call to the hypervisor is called a "hypercall" in TRANGO and Xen; it is implemented via a DIAG ("diagnose") hardware instruction in IBM's CMS under VM[clarify] (which was the origin of the term hypervisor). Examples include VMware ESX Server, IBM's LPARs,[citation needed] Win4Lin 9x, Sun's Logical Domains, z/VM,[citation needed] and TRANGO.
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 (for Windows or Linux), OpenVZ, Solaris Containers, and FreeBSD Jails.
Application Virtualization
running a desktop or server application locally, using local resources, within an appropriate virtual machine;[citation needed] 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.)

As of 2006, recent tools and technologies for virtualization are providing new alternatives for application virtualization and application streaming.

[edit] Resource virtualization

The basic concept of platform virtualization, described above, was later extended to the virtualization of specific system resources, such as storage volumes, name spaces, and network resources.

  • Resource aggregation, spanning, or concatenation combines individual components into larger resources or resource pools. For example:
    • Virtual memory, usually providing a different virtual address space for each application, which allows uniform, contiguous addressing of physically separate and non-contiguous memory and disk areas
    • RAID and volume managers combine many disks into one large logical disk.
    • Storage Virtualization refers to the process of completely abstracting logical storage from physical storage, and is commonly used in SANs. The physical storage resources are aggregated into storage pools, from which the logical storage is created. Multiple independent storage devices, which may be scattered over a network, appear to the user as a single, location-independent, monolithic storage device, which can be managed centrally.
    • Channel bonding and network equipment use multiple links combined to work as though they offered a single, higher-bandwidth link.
    • Virtual Private Network (VPN), Network Address Translation (NAT), and similar networking technologies create a virtualized network namespace within or across network subnets.
  • Partitioning is the splitting of a single resource (usually large), such as disk space or network bandwidth, into a number of smaller, more easily utilized resources of the same type. This is sometimes also called "zoning," especially in storage networks.
  • Encapsulation[citation needed] is the hiding of resource complexity by the creation of a simplified interface. For example, CPUs often incorporate cache memory or pipelines to improve performance, but these elements are not reflected in their virtualized external interface. Similar virtualized interfaces hiding complex implementations are found in disk drives, modems, routers, and many other "smart" devices.

[edit] Virtualization examples

The following examples illustrate applications of virtualization.

Server consolidation 
Virtual machines are used to consolidate many physical servers into fewer servers, which in turn host virtual machines. Each physical server is reflected as a virtual machine "guest" residing on a virtual machine host system. This is also known as Physical-to-Virtual or 'P2V' transformation.
Disaster recovery 
Virtual machines can be used as "hot standby" environments for physical production servers. This changes the classical "backup-and-restore" philosophy, by providing backup images that can "boot" into live virtual machines, capable of taking over workload for a production server experiencing an outage.
Testing and training 
Hardware virtualization can give root access to a virtual machine. This can be very useful such as in kernel development and operating system courses.[2]
Portable applications 
Certain application configuration mechanisms such as the registry on the Microsoft Windows platform lead to well-known issues involving the creation of portable applications. For example, many applications cannot be run from a removable drive without installing them on the system's main disk drive. This is a particular issue with USB drives. Virtualization can be used to encapsulate the application with a redirection layer that stores temporary files, Windows Registry entries, and other state information in the application's installation directory – and not within the system's permanent file system. See portable applications for further details. It is unclear whether such implementations are currently available.
Portable workspaces 
Recent technologies have used virtualization to create portable workspaces on devices like iPods and USB memory sticks. These products include:
  • Application Level – Thinstal – which is a driver-less solution for running "Thinstalled" applications directly from removable storage without system changes or needing Admin rights
  • OS-level – MojoPac, Ceedo, Aargo and U3 – which allows end users to install some applications onto a storage device for use on another PC.
  • Machine-level – moka5 and LivePC – which delivers an operating system with a full software suite, including isolation and security protections.
Hardware virtualization technologies
  • Intel Vanderpool x86 virtualization
  • AMD Pacifica x86 virtualization
  • Sun UltraSPARC T1 hypervisor
  • IBM Advanced POWER virtualization

[edit] References

  1. ^ Mann, Andi, Virtualization 101, Enterprise Management Associates (EMA), <http://www.emausa.com/ema_lead.php?ls=virtwpws0806&bs=virtwp0806>. Retrieved on 2007-10-29
  2. ^ Examining VMware Dr. Dobb’s Journal August 2000 By Jason Nieh and Ozgur Can Leonard

For historical sources, including many seminal computer science papers dealing with the topic, see CP/CMS references.

[edit] See also

[edit] External links

Skeptics about recent virtualization trends

de:Virtualisierung (Informatik) es:Virtualización fr:Virtualisation (informatique) id:Virtualisasi it:Virtualizzazione ko:가상화 ja:仮想化 nl:Virtualisatie pl:Wirtualizacja ru:Виртуализация sk:Virtualizácia fi:Virtualisointi zh:虛擬化

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