A virtual machine is software that behaves like a complete computer, so one physical machine can run several operating systems at once. This lesson explains the concept, gives examples of the roles virtual machines play, and weighs up their benefits and limitations.
A virtual machine (VM) is an emulation of a computer system in software. The virtual machine software (a hypervisor) runs on the real hardware, usually on top of a host operating system, and presents emulated hardware to one or more guest operating systems, each of which behaves as if it had a whole computer to itself. Guest instructions are passed to, or translated for, the real hardware by the virtual machine software. VMs are used to run several operating systems on one computer, to run legacy software, to test software safely and to share servers between many customers. The main benefits are isolation (a crash or a virus in one guest does not affect the host or the others) and saving hardware; the main limitations are reduced performance and the need for a powerful host with plenty of memory.
Fig. 1: the virtual machine software sits between the guests and the host, giving each guest operating system its own emulated hardware
Section 01
The concept of a virtual machine.
A guest operating system cannot tell the difference between real hardware and the emulated hardware that the virtual machine software gives it. That is the whole idea.
Host: the real, physical computer and the operating system installed on it.
Guest: an operating system (with its applications) running inside a virtual machine.
Virtual machine software (hypervisor): creates and manages the virtual machines. It shares out the real processor time, memory and storage between the guests, and emulates devices such as disks and network cards for each one.
Translation: when a guest executes an instruction that needs the hardware, the virtual machine software intercepts it and carries it out on the real hardware (translating it if the guest was written for a different processor).
Process virtual machines: a different kind of VM runs a single program rather than a whole OS. The Java Virtual Machine executes Java bytecode, so the same compiled program runs on any computer that has a JVM.
Examiner trap
A virtual machine is not extra hardware and it is not the same as virtual memory. It is software that emulates a whole computer.
Section 02
The roles of virtual machines.
Questions often ask you to give examples of what virtual machines are used for. Each one below comes with the reason a VM helps.
Role
Why a virtual machine helps
Running more than one operating system on one computer
A user can run, for example, Linux as a guest on a Windows host, without a second computer or restarting.
Running legacy software
An old program that only runs on an out-of-date operating system can still be used in a guest running that OS.
Testing new software
Developers can test a program on many operating systems, or test an update, without risking the host; a snapshot lets the VM be reset after each test.
Studying malware safely
A suspected virus can be run in an isolated guest so it cannot damage the host or the network.
Server consolidation and cloud computing
One powerful physical server runs many virtual servers, each rented to a different customer or used for a different service.
Portable programs
A process virtual machine such as the JVM lets the same bytecode run on any platform.
Worked example Explaining a role in context
A company's payroll program only runs on an operating system that is no longer supported, and the old computer is failing. Explain how a virtual machine could help.
Virtual machine software is installed on a new computer with a modern host operating system.
A virtual machine is created and the old operating system is installed as a guest.
The payroll program is installed and run in the guest, which behaves like the old computer.
The company can keep using the program while the old hardware is retired.
Answer: Run the old OS as a guest on new hardware, and run the legacy program inside it
Stage 1 · Learn
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What is a virtual machine?
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In a virtual machine setup, what is the guest operating system?
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Why is a virtual machine a safe place to run a suspected virus?
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Which is a limitation of virtual machines?
Section 03
Benefits and limitations.
For a full answer, give a benefit or limitation and then explain it: what it means for the user or the organisation.
Benefits
Limitations
Isolation: a crash, a faulty program or a virus in one guest does not affect the host or the other guests.
Reduced performance: instructions pass through the virtual machine software, so the guest runs more slowly than it would on real hardware.
Less hardware: several systems run on one physical computer, saving money, space and energy.
A powerful host is needed: each guest needs its own share of memory, storage and processor time.
Several operating systems can be used on one computer at the same time.
Some hardware cannot be emulated well, so certain devices or software (for example some games or specialist drivers) may not work in a guest.
Easy backup and recovery: a whole VM is a file that can be copied, moved to another server or rolled back to a snapshot.
Complexity: virtual machine software must be set up and managed, and a failure of the physical host stops every guest on it.
Your turn
A software developer tests every new version of her app in a virtual machine. Give one benefit and one limitation of doing this.
Benefit: if the new version crashes or corrupts files, only the guest is affected, and it can be reset from a snapshot; she can also test on several different operating systems on one computer. Limitation: the app runs more slowly in the VM than on real hardware, so performance testing may not be accurate.
Examiner trap
"It is slower" earns little on its own. Say why: the virtual machine software has to intercept and translate the guest's instructions, which adds an overhead.
Stage 2 · Exam
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Which is an example of the role of a virtual machine?
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Why does a computer that hosts several virtual machines need a large amount of RAM?
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What does the Java Virtual Machine allow?
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A web hosting company runs 40 virtual servers on 4 physical servers. Which is a benefit for the company?
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Virtual machines, mastered.
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