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1. Introduction Virtualization is a proven software technology that is rapidly transforming the IT landscape and fundamentally changing the way that people compute. Today’s powerful x86 computer hardware was designed to run a single operating system and a single application. This leaves most machines vastly underutilized. Virtualization lets you run multiple virtual machines on a single physical machine, sharing the resources of that single computer across multiple environments. Different virtual machines can run different operating systems and multiple applications on the same physical computer. Virtualization is a framework or methodology of dividing the resources of a computer into multiple execution environments, by applying one or more concepts or technologies such as hardware and software partitioning, time-sharing, partial or complete machine simulation, emulation, quality of service, and many others. Virtualization is technology for supporting execution of computer program code, from applications to entire operating systems, in a software-controlled environment. Such a Virtual Machine (VM) environment abstracts available system resources (memory, storage, CPU core(s), I/O, etc.) and presents them in a regular fashion, such that “guest” software cannot distinguish VM-based execution from running on bare physical hardware.

Fig (1): Virtual Machine

Virtualization commonly refers to native virtualization, where the VM platform and the guest software target the same microprocessor instruction set and comparable system architectures. Virtualization can also involve execution of guest software cross-compiled for a

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different instruction set or CPU architecture; such emulation or simulation environments help developers bring up new processors and cross-debug embedded hardware. A virtual machine provides a software environment that allows software to run on bare hardware. This environment is created by a virtual-machine monitor, also known as a hypervisor. A virtual machine is an efficient, isolated duplicate of the real machine. The hypervisor presents an interface that looks like hardware to the “guest” operating system. It allows multiple operating system instances to run concurrently on a single computer; it is a means of separating hardware from a single operating system. it can control the guests’ use of CPU, memory, and storage, even allowing a guest OS to migrate from one machine to another. It is also a method of partitioning one physical server computer into multiple “virtual” servers, giving each the appearance and capabilities of running on its own dedicated machine. Each virtual server functions as a full-fledged server and can be independently rebooted.

How Does Virtualization Work? Virtualization platform transform or “virtualize” the hardware resources of an x86based computer—including the CPU, RAM, hard disk and network controller—to create a fully functional virtual machine that can run its own operating system and applications just like a “real” computer. Each virtual machine contains a complete system, eliminating potential conflicts. Virtualization works by inserting a thin layer of software directly on the computer hardware or on a host operating system. This contains a virtual machine monitor or “hypervisor” that allocates hardware resources dynamically and transparently. Multiple operating systems run concurrently on a single physical computer and share hardware resources with each other. By encapsulating an entire machine, including CPU, memory, operating system, and network devices, a virtual machine is completely compatible with all standard x86 operating systems, applications, and device drivers. You can safely run several operating systems and applications at the same time on a single computer, with each having access to the resources it needs when it needs them.

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Virtual Machine

Fig(2): VMware Virtual Machine

A virtual machine is a tightly isolated software container that can run its own operating systems and applications as if it were a physical computer. A virtual machine behaves exactly like a physical computer and contains it own virtual (ie, software-based) CPU, RAM hard disk and network interface card (NIC).

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An operating system can’t tell the difference between a virtual machine and a physical machine, nor can applications or other computers on a network. Even the virtual machine thinks it is a “real” computer. Nevertheless, a virtual machine is composed entirely of software and contains no hardware components whatsoever. As a result, virtual machines offer a number of distinct advantages over physical hardware. Virtual Machines Benefits Virtual machines possess four key characteristics that benefit the user: •

Compatibility: Virtual machines are compatible with all standard x86 computers

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Isolation: Virtual machines are isolated from each other as if physically separated

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Encapsulation: Virtual machines encapsulate a complete computing environment

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Hardware independence: Virtual machines run independently of underlying hardware

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Virtual Infrastructure A virtual infrastructure lets you share your physical resources of multiple machines across your entire infrastructure. A virtual machine lets you share the resources of a single physical computer across multiple virtual machines for maximum efficiency. Resources are shared across multiple virtual machines and applications. This resource optimization drives greater flexibility in the organization and results in lower capital and operational costs.

Fig(3): Virtual Infrastructure

A virtual infrastructure consists of the following components: •

Bare-metal hypervisors to enable full virtualization of each x86 computer.

•

Virtual infrastructure services such as resource management and consolidated backup to optimize available resources among virtual machines

Virtual Infrastructure Benefits Delivering built–in availability, security and scalability to applications. It supports a wide range of operating system and application environments, as well as networking and storage infrastructure.

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2. History of Virtualization Virtualization is a proven concept that was first developed in the 1960s by IBM as a way to logically partition large, mainframe hardware into separate virtual machines. These partitions allowed mainframes to "multitask"; run multiple applications and processes at the same time. Virtualization was effectively abandoned during the 1980s and 1990s when client-server applications and inexpensive x86 servers and desktops established the model of distributed computing. The growth in x86 server and desktop deployments has introduced new IT infrastructure and operational challenges. These challenges include: •

Low Infrastructure Utilization - Typical x86 server deployments achieve an average utilization of only 10% to 15% of total capacity. Organizations typically run one application per server to avoid the risk of vulnerabilities in one application affecting the availability of another application on the same server.

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Increasing Physical Infrastructure Costs - The operational costs to support growing physical infrastructure have steadily increased. Most computing infrastructure must remain operational at all times, resulting in power consumption, cooling and facilities costs that do not vary with utilization levels.

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Increasing IT Management Costs - As computing environments become more complex, the level of specialized education and experience required for infrastructure management personnel and the associated costs of such personnel have increased. Organizations spend disproportionate time and resources on manual tasks associated with server maintenance, and thus require more personnel to complete these tasks.

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Insufficient Failover and Disaster Protection - Organizations are increasingly affected by the downtime of critical server applications and inaccessibility of critical end user desktops. The threat of security attacks, natural disasters, health pandemics and terrorism has elevated the importance of business continuity planning for both desktops and servers.

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High Maintenance end-user desktops - Managing and securing enterprise desktops present numerous challenges. Controlling a distributed desktop environment and enforcing management, access and security policies without impairing users' ability to work effectively is complex and expensive.

Present Day Today, computers based on x86 architecture are faced with the same problems of rigidity and underutilization that mainframes faced in the 1960s. Today's powerful x86 computer hardware was originally designed to run only a single operating system and a single application, but virtualization breaks that bond, making it possible to run multiple operating systems and multiple applications on the same computer at the same time, increasing the utilization and flexibility of hardware.

Why Virtualization: A List of Reasons Following are some reasons for and benefits of virtualization: 

Virtual machines can be used to consolidate the workloads of several under-utilized servers to fewer machines, perhaps a single machine (server consolidation). Related benefits are savings on hardware, environmental costs, management, and administration of the server infrastructure.

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The need to run legacy applications is served well by virtual machines. A legacy application might simply not run on newer hardware and/or operating systems. Even if it does, if may under-utilize the server,

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Virtual machines can be used to provide secure, isolated sandboxes for running untrusted applications. You could even create such an execution environment dynamically - on the fly - as you download something from the Internet and run it.

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Virtual machines can be used to create operating systems, or execution environments with resource limits, and given the right schedulers, resource guarantees.

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Virtual machines can provide the illusion of hardware, or hardware configuration that you do not have (such as SCSI devices, multiple processors,) Virtualization can also be used to simulate networks of independent computers.

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Virtual machines can be used to run multiple operating systems simultaneously: different versions, or even entirely different systems, which can be on hot standby. Some such systems may be hard or impossible to run on newer real hardware.

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Virtual machines allow for powerful debugging and performance monitoring.

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Virtual machines can isolate what they run, so they provide fault and error containment. You can inject faults proactively into software to study its subsequent behavior.

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Virtual machines are great tools for research and academic experiments. Since they provide isolation, they are safer to work with. They encapsulate the entire state of a running system: you can save the state, examine it, modify it, reload it, and so on. The state also provides an abstraction of the workload being run.

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Virtualization can enable existing operating systems to run on shared memory multiprocessors.

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Driving out the cost of IT infrastructure through more efficient use of available resources

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Simplifying the infrastructure.

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Increasing system availability.

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Delivering consistently good performance.

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Centralizing systems, data, and infrastructure

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3.Virtual machine & Hypervisor VIRTUAL MACHINE Virtual machine (VM) is a software implementation of a machine (computer) that executes programs like a real machine.

Fig(4): Connectix Virtual PC version 3 in Mac OS 9, running a Windows 95

A virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Virtual machines are separated into 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). 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. System virtual machines

System virtual machines (sometimes called hardware virtual machines) allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is

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called a virtual machine monitor or hypervisor. A hypervisor can run on bare hardware (Type 1 or native VM) or on top of an operating system (Type 2 or hosted VM). The main advantages of system VMs are: •

multiple OS environments can co-exist on the same computer, in strong isolation from each other

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the virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine

The guest OS’s do not have to be all the same, making it possible to run different OS’s on the same computer (e.g., Microsoft Windows and Linux, or older versions of an OS in order to support software that has not yet been ported to the latest version). Process virtual machines

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 — that of a high-level programming language (compared to the low-level ISA abstraction of the system VM). 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 (JVM). And .NET Framework, which runs on a VM called the Common Language Runtime.

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Techniques Emulation of the underlying raw hardware (native execution)

Fig (5): VMware Workstation running Ubuntu, on Windows Vista

This approach is described as full virtualization of the hardware, and can be implemented using a Type 1 or Type 2 hypervisor. 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. 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. 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: •

The specification of the Java virtual machine.

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The Common Language Infrastructure virtual machine at the heart of the Microsoft .NET initiative.

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•

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.

Hypervisor A hypervisor, also called virtual machine monitor (VMM), is a computer hardware platform virtualization software that allows multiple operating systems to run on a host computer concurrently.

Classifications Hypervisors are classified in two types: •

Type 1 (or native, bare-metal) hypervisors are software systems that run directly on the host's hardware as a hardware control and guest operating system monitor. A guest operating system thus runs on another level above the hypervisor.

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Type 2 (or hosted) hypervisors are software applications running within a conventional operating system environment. Considering the hypervisor layer being a distinct software layer, guest operating systems thus run at the third level above the hardware.

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4. Popek and Goldberg virtualization requirements The Popek and Goldberg virtualization requirements are a set of sufficient conditions for computer architecture to efficiently support system virtualization. They were introduced by Gerald J. Popek and Robert P. Goldberg in their 1974 article "Formal Requirements for Virtualizable Third Generation Architectures". Even though the requirements are derived under simplifying assumptions, they still represent a convenient way of determining whether computer architecture supports efficient virtualization and provide guidelines for the design of virtualized computer architectures. There are three properties of interest when analyzing the environment created by a VMM: Equivalence: A program running under the VMM should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly. Resource control: The VMM must be in complete control of the virtualized resources. Efficiency: A statistically dominant fraction of machine instructions must be executed without VMM intervention.

In Popek and Goldberg terminology, a VMM must present all three properties. VMM are typically assumed to satisfy the equivalence and resource control properties. So, in a sense, Popek and Goldberg's VMMs are today's efficient VMM. The main result of Popek and Goldberg's analysis can then be expressed as follows. Theorem 1. For any conventional third-generation computer, a VMM may be constructed if the set of sensitive instructions for that computer is a subset of the set of privileged instructions. Theorem 2. A conventional third-generation computer is recursively Virtualizable if 1. it is Virtualizable and 2. a VMM without any timing dependencies can be constructed for it.

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5. Classification of Virtualization Here we discuss about different types of virtualization •

Platform virtualization, which separates an operating system from the underlying platform resources

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Full virtualization

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Hardware-assisted virtualization

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Partial virtualization

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Para virtualization

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Operating system-level virtualization

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Hosted environment

Resource virtualization, the virtualization of specific system resources, such as storage volumes, name spaces, and network resources o

Storage virtualization, the process of completely abstracting logical storage from physical storage

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RAID - redundant array of independent disks

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Disk partitioning

Network virtualization, creation of a virtualized network addressing space within or across network subnets

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Computer clusters and grid computing, the combination of multiple discrete computers into larger meta computers

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Application virtualization, the hosting of individual applications on alien hardware/software

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Portable application

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Cross-platform virtualization

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Emulation or simulation

Desktop virtualization, the remote manipulation of a computer desktop

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6. Platform virtualization Platform virtualization is a virtualization of computers or operating systems. It hides the physical characteristics of computing platform from the users, instead showing another abstract, emulated computing platform. •

Fig (6): VMware Workstation running Ubuntu, on Windows, an example of platform virtualization

Concept The creation and management of virtual machines has been called platform virtualization, or server virtualization. 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 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.

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There are several approaches to platform virtualization.

Full virtualization In full virtualization, the virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same instruction set) to be run in isolation. This approach was pioneered in 1966 with IBM CP-40 and CP-67, predecessors of VM family.

Hardware-assisted virtualization In hardware-assisted virtualization, the hardware provides architectural support that facilitates building a virtual machine monitor and allows guest OSes to be run in isolation. In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Examples include Linux KVM, VMware Workstation, VMware Fusion, Microsoft Virtual PC, Xen, Parallels Desktop for Mac,VirtualBox and Parallels Workstation. Hardware virtualization technologies include: •

AMD-V x86 virtualization (previously known as Pacifica)

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IBM Advanced POWER virtualization

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Intel VT x86 virtualization (previously known as Vanderpool)

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UltraSPARC T1 and UltraSPARC T2 processors from Sun Microsystems have the HyperPrivileged execution mode

Partial virtualization In partial virtualization (and also "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.

Para virtualization In 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;

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Operating system-level virtualization In operating system-level virtualization, a physical server is virtualized 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.

Hosted environment Applications that hosted by the third party servers and that can be called or can be used by a remote system’s environment.

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7. Resource virtualization The virtualization of specific system resources, such as storage volumes, name spaces, and network resources is the resource virtualization,

Storage Virtualization Storage virtualization is the pooling of multiple physical storage resources into what appears to be a single storage resource that is centrally managed. Storage virtualization automates tedious and extremely time-consuming storage administration tasks. This means the storage administrator can perform the tasks of backup, archiving, and recovery more easily and in less time, because the overall complexity of the storage infrastructure is disguised. Storage virtualization is commonly used in file systems, storage area networks (SANs), switches and virtual tape systems. Users can implement storage virtualization with software, hybrid hardware or software appliances. Virtualization hides the physical complexity of storage from storage administrators and applications, making it possible to manage all storage as a single resource. In addition to easing the storage management burden, this approach dramatically improves the efficiency and cuts overall costs. The Advantages of Storage Virtualization Storage virtualization provides many advantages. First, it enables the pooling of multiple physical resources into a smaller number of resources or even a single resource, which reduces complexity. Many environments have become complex, which increases the storage management gap. With regard to resources, pooling is an important way to achieve simplicity. A second advantage of using storage virtualization is that it automates many time-consuming tasks. In other words, policy-driven virtualization tools take people out of the loop of addressing each alert or interrupt in the storage business. A third advantage of storage virtualization is that it can be used to disguise the overall complexity of the infrastructure.

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Network virtualization Network virtualization is the process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization involves platform virtualization, often combined with resource virtualization. Network virtualization is categorized as either external, combining many networks, or parts of networks, into a virtual unit, or internal, providing network-like functionality to the software containers on a single system. Whether virtualization is internal or external depends on the implementation provided by vendors that support the technology.

Components of a virtual network Various equipment and software vendors offer network virtualization by combining any of the following: •

Network hardware, such as switches and network adapters, also known as network interface cards (NICs)

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Networks, such as virtual LANs (VLANs) and containers such as virtual machines and Solaris Containers

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Network storage devices

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Network media, such as Ethernet and Fibre Channel

External network virtualization External network virtualization, in which one or more local networks are combined or subdivided into virtual networks, with the goal of improving the efficiency of a large corporate network or data center. The key components of an external virtual network are the VLAN and the network switch. Using VLAN and switch technology, the system administrator can configure systems physically attached to the same local network into different virtual networks. Conversely, VLAN technology enables the system administrator to combine systems on separate local networks into a VLAN spanning the segments of a large corporate network.

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Internal network virtualization In internal network virtualization, a single system is configured with containers, such as the Xen domain, combined with hypervisor control programs or pseudo-interfaces such as the VNIC, to create a “network in a box.” This solution improves overall efficiency of a single system by isolating applications to separate containers and/or pseudo interfaces.

Combined internal and external network virtualization Some VMM offer both internal and external network virtualization. Basic approach is network in the box on a single system, using virtual machines that are managed by hypervisor software. Infrastructure software connects and combines networks in multiple boxes into an external virtualization scenario.

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8. Cluster and Grid computing Cluster computing

Fig (7): An example of a computer cluster

A computer cluster is a group of linked computers, working together closely so that in many respects they form a single computer. The components of a cluster are commonly, but not always, connected to each other through fast local area networks. Clusters are usually deployed to improve performance and/or availability over that provided by a single computer, while typically being much more cost-effective than single computers of comparable speed or availability.

Cluster categorizations High-availability (HA) clusters

High-availability clusters (also known as failover clusters) are implemented primarily for the purpose of improving the availability of services which the cluster provides. They operate by having redundant nodes, which are then used to provide service when system components fail.. Load-balancing clusters

Load-balancing clusters operate by distributing a workload evenly over multiple back end nodes. Typically the cluster will be configured with multiple redundant load-balancing front ends.

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Compute clusters

Clusters are used for primarily computational purposes, rather than handling IOoriented operations such as web service or databases. For instance, a cluster might support computational simulations of weather or vehicle crashes.

Grid computing Grids are usually compute clusters, but more focused on throughput like a computing utility rather than running fewer, tightly-coupled jobs. grids will incorporate heterogeneous collections of computers, possibly distributed geographically distributed nodes, sometimes administered by unrelated organizations. Grid computing is optimized for workloads which consist of many independent jobs or packets of work, which do not have to share data between the jobs during the computation process. Grids serve to manage the allocation of jobs to computers which will perform the work independently of the rest of the grid cluster. Resources such as storage may be shared by all the nodes, but intermediate results of one job do not affect other jobs in progress on other nodes of the grid.

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9. Application virtualization Application virtualization is a term that describes software technologies that improve portability, manageability and compatibility of applications by encapsulating them from the underlying operating system on which they are executed. A fully virtualized application is not installed in the traditional sense, although it is still executed as if it is. The application is fooled at runtime into believing that it is directly interfacing with the original operating system and all the resources managed by it, when in reality it is not. Application virtualization differs from operating system virtualization in that in the latter case, the whole operating system is virtualized rather than only specific applications.

Description Limited application virtualization is used in modern operating systems such as Microsoft Windows and Linux. For example, IniFileMappings were introduced with Windows NT to virtualize (into the Registry) the legacy INI files of applications originally written for Windows 3.1. Full application virtualization requires a virtualization layer. This layer must be installed on a machine to intercept all file and Registry operations of virtualized applications and transparently redirect these operations into a virtualised location. The application performing the file operations never knows that it's not accessing the physical resource it believes it is. In this way, applications with many dependent files and settings can be made portable by redirecting all their input/output to a single physical file, and traditionally incompatible applications can be executed side-by-side.

Benefits of application virtualization •

Allows applications to run in environments that do not suit the native application (e.g. Wine allows Microsoft Windows applications to run on Linux).

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Uses fewer resources than a separate virtual machine.

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Run incompatible applications side-by-side, at the same time and with minimal regression testing against one another.

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Implement the security principle of least privilege by removing the requirement for end-users to have Administrator privileges in order to run poorly written applications.

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Simplified operating system migrations.

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Accelerated application deployment, through on-demand application streaming.

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Improved security, by isolating applications from the operating system.

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Fast application provisioning to the desktop based upon user's roaming profile.

Disadvantages of application virtualization •

Applications have to be "packaged" or "sequenced" before they will run in a virtualized way.

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Minimal increased resource requirements (memory and disk storage).

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Not all software can be virtualized. Some examples include applications that require a device driver and 16-bit applications that need to run in shared memory space.

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Some types of software such as anti-virus packages are difficult to virtualize.

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Some compatibility issues between legacy applications and newer operating systems cannot be addressed by application virtualization (although they can still be run on an older operating system under a virtual machine).

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10. Cross-platform virtualization Cross-platform virtualization is a form of computer virtualization that allows software compiled for a specific CPU and operating system to run unmodified on computers with different CPUs and/or operating systems, through a combination of dynamic binary translation and operating system call mapping. Since the software runs on a virtualized equivalent of the original computer, it does not require recompilation or porting, thus saving time and development resources. However, the processing overhead of binary translation and call mapping imposes a performance penalty, when compared to natively-compiled software. For this reason, cross-platform virtualization may be used as a temporary solution until resources are available to port the software. By creating an abstraction layer capable of running software compiled for a different computer system, cross-platform virtualization characterizes the Popek and Goldberg virtualization requirements outlined. Cross-platform virtualization is distinct from emulation and binary translation - which involve the direct translation of one CPU instruction set to another - since the inclusion of operating system call mapping provides a more complete virtualized environment. Cross-platform virtualization is also complementary to server virtualization and desktop virtualization solutions, since these are typically constrained to a single CPU type, such as x86 or POWER.

Emulation Emulation or Emulator may refer to as imitation of behavior of a computer or other electronic system with the help of another type of computer/system. Console emulator, a program that allows a computer or modern console to emulate another video game console. Hardware emulation, the use of special purpose hardware to emulate the behavior of a yet-tobe-built system, with greater speed than pure software emulation

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Simulation Simulation is the imitation of some real thing, state of affairs, or process. The act of simulating something generally entails representing certain key characteristics or behaviors of a selected physical or abstract system. A computer simulation (or "sim") is an attempt to model a real-life or hypothetical situation on a computer so that it can be studied to see how the system works. By changing variables, predictions may be made about the behaviour of the system. Computer simulation has become a useful part of modeling many natural systems in physics, chemistry and biology, and human systems in economics as well as in engineering to gain insight into the operation of those systems. Simulation in Computer science

In Computer science, simulation has some specialized meanings: Alan Turing used the term "simulation" to refer to what happens when a universal machine executes a state transition table (in modern terminology, a computer runs a program) that describes the state transitions, inputs and outputs of a subject discrete-state machine. Less theoretically, an interesting application of computer simulation is to simulate computers using computers. In computer architecture, a type of simulator, typically called an emulator, is often used to execute a program that has to run on some inconvenient type of computer, or in a tightly controlled testing environment Since the operation of the computer is simulated, all of the information about the computer's operation is directly available to the programmer, and the speed and execution of the simulation can be varied at will.

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11.Desktop virtualization Desktop virtualization or virtual desktop infrastructure (VDI) is a server-centric computing model that borrows from the traditional thin-client model but is designed to give system administrators and end-users the ability to host and centrally manage desktop virtual machines in the data center while giving end users a full PC desktop experience.

Rationale Installing and maintaining separate PC workstations is complex, and traditionally users have almost unlimited ability to install or remove software. Desktop virtualization provides many of the advantages of a terminal server, but (if so desired and configured by system administrators) can provide users much more flexibility. Each, for instance might be allowed to install and configure their own applications. Users also gain the ability to access their server-based virtual desktop from other locations.

Advantages •

Instant provisioning of new desktops

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Near-zero downtime in the event of hardware failures

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Significant reduction in the cost of new application deployment

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Robust desktop image management capabilities

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Normal 2-3 year PC refresh cycle extended to 5–6 years or more

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Existing desktop-like performance including multiple monitors, bi-directional audio/video, streaming video, USB support etc.

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Ability to access the users' enterprise desktop environment from any PC, (including the employee's home PC)

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Desktop computing power on demand

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Multiple desktops on demand

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Self provisioning of desktops (controlled by policies)

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12.Virtualization Softwares

VMware Workstation

Fig (8): VMware Workstation 6.5 running Ubuntu

The Snapshot Manager in VMware Workstation 6

VMware Workstation is a virtual machine software suite for x86 and x86-64 computers from VMware, a division of EMC Corporation. This software suite allows users to set up multiple x86 and x86-64 virtual computers and to use one or more of these virtual machines simultaneously with the hosting operating system. Each virtual machine instance can execute its own guest operating system, such as Windows, Linux, BSD variants, or others. In simple terms, VMware Workstation allows one physical machine to run multiple operating systems simultaneously.

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Microsoft Virtual Server Microsoft Virtual Server is a virtualization solution that facilitates the creation of virtual machines on the Windows XP, Windows Vista and Windows Server 2003 operating systems. Originally developed by Connectix, it was acquired by Microsoft prior to release. Virtual PC is Microsoft's related desktop virtualization software package. Virtual machines are created and managed through an IIS web-based interface or through a Windows client application tool called VMRCplus. The current version is Microsoft Virtual Server 2005 R2 SP1. New features in R2 SP1 include Linux guest operating system support, Virtual Disk Precompactor, SMP (but not for the Guest OS), x86-64 (x64) Host OS support (but not Guest OS support), the ability to mount virtual hard drives on the host OS and additional operating systems including Windows Vista. It also provides a Volume Shadow Copy writer which enables live backups of the Guest OS on a Windows Server 2003 or Windows Server 2008 Host. A utility to mount VHD images is also included since SP1. Officially supported Linux guest operating systems include Red Hat Enterprise Linux versions 2.1-5.0, Red Hat Linux 9.0, SUSE Linux and SUSE Linux Enterprise Server versions 9 and 10.

Microsoft Virtual PC Microsoft Virtual PC is a virtualization suite for Microsoft Windows operating systems, and an emulation suite for Mac OS X on PowerPC-based systems. The software was originally written by Connectix, and was subsequently acquired by Microsoft. In July 2006 Microsoft released the Windows-hosted version as a free product. In August 2006 Microsoft announced the Macintosh-hosted version would not be ported to Intel-based Macintoshes, effectively discontinuing the product as PowerPC-based Macintoshes are no longer manufactured. Virtual PC virtualizes a standard PC and its associated hardware. Supported Windows operating systems can run inside Virtual PC. However, other operating systems like Linux may run, but are not officially supported (for example, Ubuntu, a popular Linux distribution,

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can get past the boot screen of the Live CD (and function fully) when using Safe Graphics Mode).

VirtualBox VirtualBox is an x86 virtualization software package, originally created by German software company innotek, now developed by Sun Microsystems as part of its Sun xVM virtualization platform. It is installed on an existing host operating system; within this application, additional operating systems, each known as a Guest OS, can be loaded and run, each with its own virtual environment. Supported host operating systems include Linux, Mac OS X, OS/2 Warp, Windows XP or Vista, and Solaris, while supported guest operating systems include FreeBSD, Linux, OpenBSD, OS/2 Warp, Windows and Solaris. According to a 2007 survey ,Virtual Box is the third most popular software package for running Windows programs on Linux desktops.

Xen Xen is a virtual machine monitor for IA-32, x86, x86-64, IA-64 and PowerPC 970 architectures. It allows several guest operating systems to be executed on the same computer hardware concurrently. Xen was initially created by the University of Cambridge Computer Laboratory and is now developed and maintained by the Xen community as free software, licensed under the GNU General Public License (GPL2). A Xen system is structured with the Xen hypervisor as the lowest and most privileged layer. Above this layer are one or more guest operating systems, which the hypervisor schedules across the physical CPUs. The first guest operating system, called in Xen terminology "domain 0" (dom0), is booted automatically when the hypervisor boots and given special management privileges and direct access to the physical hardware. The system administrator logs into dom0 in order to start any further guest operating systems, called "domain U" (domU) in Xen terminology.

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13.Conclusion Virtualization dramatically improves the efficiency and availability of resources and applications. Earlier Internal resources are underutilized under the old “one server, one application” model and users spend too much time managing servers rather innovating. By virtualization platform, users can respond faster and more efficiently than ever before. Users can save

50-70% on overall IT costs by consolidating their resource pools and delivering

highly available machines. Other major improvements by using virtualization are that they can: •

Reduce capital costs by requiring less hardware and lowering operational costs while increasing your server to admin ratio

•

Ensure enterprise applications perform with the highest availability and performance

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Build up business continuity through improved disaster recovery solutions and deliver high availability throughout the datacenter

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Improve desktop management with faster deployment of desktops and fewer support calls due to application conflicts.

Even after the implementations of distributed computing and other technologies, virtualization proved to be an effective in using the available resources of a system fully in an efficient way.

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14.References Websites: 1) http:// www. wikipedia.com 2) http:// www.vmware.com 3) http:// www.kernalthread.com 4) http:// www.virtualizationadmin.com 5) http:// www.virtualization.org 6) http:// www.microsft.com/virtualization.aspx

Books: 1) “Virtualization : From Beginners to Professionals”, Apress Publications

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