VIRTUAL MACHINES

A Virtual Machine(VM) is an extension to an older idea known as software emulation. Through emulation, a computer could emulate another type of computer. More specifically, the emulator would translate the instructions of some piece of incompatible software into instructions native to the computer. This would allow a user to run programs compiled for another computer with the right emulator.

The VM is a related idea to the emulator. The VM, as the name implies, creates an illusionary computer in your physical computer. The physical computer is set up to run a specific operating system and specific software. However, through emulation, the VM then can provide the user with a different operating system running different software.

One form of VM that you might be familiar with is the Java Virtual Machine (JVM),which is built into web browsers. Through the JVM, most web browsers can execute Java Applets. The JVM takes each Java Applet instruction, stored in an intermediate form called byte code, decodes the instruction into the machine language of the host computer, and executes it. Thus, the JVM is an interpreter rather than a compiler. The JVM became so successful that other forms of interpreters are now commonly available in a variety of software so that you can run, for instance, Java or Ruby code. Today, just about all web browsers contain a JVM.

Today’s VMs are a combination of :

• software :  is a program that can perform emulation
• data       :  consist of the operating system, applications software, and data files that the user uses in the virtual environment.

With VM software, you install an operating system. This creates a new VM. You run your VM software and boot to a specific VM from within. This gives you access to a non-native operating system, and any software you wish to install inside of it. Interacting with the VM is like interacting with a computer running that particular operating system. In this way, a Windows-based machine could run the Mac OS X or a Macintosh could run Windows 7.

Commonly, VMs are set up to run some version of Linux. Therefore, as a Linux user, you can access both your physical machine’s operating system (e.g., Windows) and also Linux without having to reboot the computer.

The cost of a VM is as follows:

• The VM software itself—although some are free, VM software is typically commercially marketed and can be expensive.
• The operating system(s)—if you want to place Windows 7 in a VM, you will have to purchase a Windows 7 installation CD to have a license to use it. Fortunately, mostly  versions of Linux are free and easily installed in a VM.
• The load on the computer—a VM requires a great deal of  computational and memory overhead, however modern multicore processors are more than capable of handling the load.
• The size of the VM on hard disk—the image of the VM must be stored on hard disk and the size of the VM will be similar in size to that of the real operating system, so for instance, 8 GBytes is reasonable for a Linux image and 30 GBytes for a Windows 7 image.

 You could create a Linux VM, a Windows VM, even a mainframe’s operating system, all accessible from your computer. Your computer could literally be several or dozens of different computers.

Each VM could have its own operating system, its own software, and its own file system space. Or, you could run several VMs where each VM is the same operating system and the same software, but each VM has different data files giving you a means of experimentation.

VM software is now available to run on Windows computers, Mac OS X, and Linux/Unix computers. VM software titles include vSphere Client and Server, VMware Workstation, VMware Player, Virtual Box, CoLinux, Windows Virtual PC, Parallels Desktop, VM from IBM, Virtual Iron, QEMU, and Xen. The latter two titles are open source and many of these titles have free versions available.

With VM software readily available, we can expand on its capabilities by implementing virtualization.
With virtualization, an organization hosts a number of VMs through one or more VM servers. The servers operate in a client–server networking model where a client runs a VM program on the user machine and requests access to one of the stored VMs.
The VM servers typically store the VMs on a storage area network (SAN). The collection of VM servers, the SAN, and the data that make up individual VMs can be called a VM Farm.
Now, accessibility to your VM is one of starting your VM client software and logging into the VM server. You select your VM from a list of choices, log into it, and the VM server then runs (emulates) the VM for you. Your client operates as an input/output device while the processing and storage take place on the VM server. As you make modifications to your VM, they are saved in the SAN.

There are numerous advantages to virtualization.

1. First is accessibility. You are able to access your VM from any computer that has Internet access and runs the proper VM client software. 
2. Second is cost savings. If your company is small and you cannot afford all of the hardware needed for your employees, you can lease or purchase time and space in a VM farm where the VM servers and SAN become the hardware that you use.
3. You can also rely on the company hosting your VMs to handle security and backups alleviating some of the IT needs from your own organization. 

Today, more and more organizations are taking advantage of virtualization to improve their efficiency and lower costs.

Reasons to Use Virtualization

Resources

Linux with Operating System Concepts by Richard Fox
isbn:9781482235906, goodreads:20792170