If you read technical magazines regularly, you’re bound to have seen at least one article about virtualization in the past six months. Virtual data centers, virtual disaster recovery facilities, virtual servers, virtual switches and virtual networks seem to have become almost omnipresent.
Even with the abundance of coverage that virtualization receives in the press it is often still difficult to learn the basics of its various purposes and benefits. What is virtualization? How can virtualization really help in business or academic environments? What are the benefits of going this route? These are all questions that need to be answered before you begin digging deeper into specific products or technologies.
The purpose of this series of articles is to approach virtualization via its most basic of components, namely virtual machine systems. Ultimately, we’ll use this introduction as a stepping-stone to work our way to some of the more complicated aspects of virtualization.
Definition and History
Virtualization – sometimes referred to as virtual machine systems technology – makes use of a software layer to enable multiple, diverse, and independent operating systems to run simultaneously on a single set of hardware.
IBM first developed virtual machines in the 60s. At the time, their main goal was to correct some of the limitations of the company’s OS 360 multi-programming operating system. IBM’s virtual machines were basically fully protected, isolated copies of the underlying physical machine’s hardware. A software component ran directly on the “real” hardware. This software component could then be used to create multiple virtual machines, each of which could run its own operating system.
Popular during the 60s and 70s, virtual machines practically disappeared during the 80s and 90s. It was not until the end of the 90s that they truly came back on the scene, not only in the traditional area of servers, but also in many other areas of the computing world.
The Structure of Virtual Machine Systems
Current virtual machine systems are essentially built on the same theoretical grounds as their IBM ancestors. A thin layer of software – the virtual machine monitor (VMM) – is interposed between two of the layers of a computer (Figure 1) to create a virtual machine environment.
The VMM creates a layer of abstraction between the physical machine’s hardware and the virtual machine’s operating system. The VMM then manages the resources of the underlying physical machines (referred to as the host machine) in such a way that the user can create several virtual, “guest” machines on top of the physical host machine. The VMM also virtualizes the physical hardware of the host machine and presents to each virtual guest machine a hardware interface that is compatible with the operating system the user chose to install on it.
Each of the guest machines is composed of a combination of the host machine’s hardware and the VMM. The layer of abstraction created by the VMM gives each guest machine the illusion of being a complete physical machine, and fools each guest operating system into believing that it is running on the normal hardware environment it is used to.
How Virtual Machine Systems are built
There are currently two main approaches to the building of virtual machine systems. In the first approach, the VMM sits between the hardware of the real machine and the guest systems (Figure 2). This approach was used in the 60s by the original IBM virtual machine systems, and is also used nowadays by modern implementations like VMWare’s ESX Server.
In the second approach, the VMM is installed as a normal process between the underlying real operating system, called the host system, and the virtual machines created by the users (Figure 3). This approach is currently used by some of the most popular virtualization software, like VMWare’s Workstation and GSX Server, and Microsoft’s Virtual PC 2004 and Virtual Server 2005.
Common characteristics of Virtual Machines Sytems
Regardless of the approach used to build them, virtual machine systems share a certain number of necessary characteristics: faithful reproduction of the guest operating system’s normal environment, adequate performance, isolation between the guest machines (and between each guest and the host), centralized control of the host’s resources, and encapsulation of the virtual machines.
The main goal of virtualization is to enable applications and guest operating systems to run on hardware, or host operating systems with which they would normally not be compatible. To attain this goal, the VMM must first reproduce the system it is emulating as faithfully as possible.
Eventually, the VMM must be able to map into software parts of the original hardware architecture that no longer exist. If the virtual machines are used to test prototype and beta software, the VMM might even have to map into software parts of system architecture that do not exist yet.
The VMM must also be able to provide the guest operating systems and applications with an environment that is essentially identical to the original machine so that any program running on a guest machine will have the same behavior and the same effects as the same program running in its original environment.
The VMM represents an additional layer of software between the hardware, or the host operating system and the guest operating systems and applications. This additional layer is likely to add overhead to the system, and affect the performance of the software running on the guest machines. To be useful, however, the virtual machine system must exhibit a performance level comparable to that of the original real machine.
If the VMM really reproduces faithfully the real system it is emulating, and if the environment provided by the VMM is essentially identical with the original machine, the definitions of the two interfaces, real and virtual, should match, and the performance of the virtual machine should hardly suffer from the virtualization.
The first modern virtual machines systems implemented on common Intel-like computers, used to suffer performance losses sometimes as high as 50%. Nowadays, however, there is hardly any difference between the performances of real and virtual machines. At the end of last year, I personally tested the performance of virtual machines installed on a VMWare GSX server, and found it absolutely comparable to the performance of a “real” physical machine. In many cases, the virtual machines actually performed better than the physical machine.
Virtual machine systems must allow applications hosted in the different virtual machines to run concurrently without interfering with each other. To achieve this goal, the VMM must be able to completely isolate the virtual machines from each other and from the real machine.
This isolation must be twofold. On one hand, the applications and data of each machine, virtual or real, must be out of the reach of all the other machines. On the other hand, the VMM must be able to ensure that the use of host system resources by one virtual machine does not have a negative impact on the performance of other virtual machines. This means that the VMM must constantly have complete control over the resources, such as memory, peripherals, I/O, and even, eventually, processor time, used by the virtual machines. It must be in charge of allocating resources, and it must be able to dynamically allocate and remove them as needed.
Finally, virtual machine systems must encapsulate all the software of each virtual machine. This encapsulation enhances the isolation of the virtual machines from the host machine. It also allows users to easily migrate virtual machines from one hardware platform to another, different one. This allows users to “save” the state of a virtual machine at a certain moment in time, change the configuration of the machine, for example install new applications or security patches, test them, then return the virtual machine to its original state of it.
This finalizes this first article on virtualization. I know it is rather theoretical and dry beginning, but I believe in having at least a general idea of how things work before using them 🙂
In the next article, we’ll start getting our hands dirty, and installing virtualization software – that’s where the real fun begins!