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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."
However, the term is an old one: It has been widely used since the 1960s or
earlier, and has been applied to many different aspects and scopes of computing
from entire computer systems 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:
Platform virtualization involves the simulation of computer environments.
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"
which may have little to do with the physical implementation.
An also useful concept is that which opposes virtualization to transparency:
something is virtual when it is visible, perceivable, but does not physically
exist in the form perceived (e.g. virtual storage); and is transparent when it
exists in the real world, but is not visible in use (e.g. location
transparency).
A Design Pattern
Most forms of virtualization involve a computing design pattern 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.
Platform virtualization
The original sense of the term virtualization, dating from the 1960s, is in the
creation of a virtual machine using a combination of hardware and software. For
convenience, we will call this platform virtualization. The term virtual machine
apparently dates from the IBM M44/44X experimental paging 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 given physical machine. For the "guest" system to function, the
simulation must be robust enough to support all the guest system's external
interfaces, which (depending on the type of virtualization) may include hardware
drivers.
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.)
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, 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. 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 Parallels
Workstation, Parallels Desktop for Mac, VirtualBox, Virtual Iron, Virtual PC,
VMware Workstation, VMware Server (formerly GSX Server),VMware ESX Server, QEMU,
Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro, and z/VM.
Hardware enabled virtualization
the virtual machine has its own hardware and allows a guest OS to be run in
isolation. In many instances the virtual machine runs an operating system
different than that of the host computer. Typically, the number of virtual
machines installed on a primary OS is limited only by the host computers
hardware and memory resources. In 2005 and 2006, Intel and AMD provided
additional hardware to support virtualization. Examples include VMware Fusion,
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 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 (which was the origin of the term hypervisor). Examples include Win4Lin
9x, Sun's Logical Domains, z/VM and TRANGO (embedded hypervisor for ARM, MIPS
and PowerPC architectures).
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; 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 Xenocode Virtual Appliance Studio, 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.
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:
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.
Multiprocessor and multi-core computer systems often present what appears as a
single, fast processor.
Computer clusters, grid computing, and virtual servers use the above techniques
to combine multiple discrete computers into larger metacomputers.
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 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.
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.
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
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