Virtual Machines -- What Are They?
If you're an active computer user, by now you may have heard the
term Virtual Machine, or VM.
Chances are you already have some idea of what a Virtual Machine is. But
in case you don't, this introduction is a layman's view of what a Virtual
Machine is, and what it can be used for.
Basically, a Virtual Machine is an elaborate program that provides a virtual
environment for code running within it. By virtual environment, I mean that an
expected architecture is apparent to operating systems running in the Virtual
Machine, even though the various aspects of a real environment, such as
interrupts and hardware interfaces, are created by software in the VM, and are
not real or direct to the actual architecture.
What Virtual Machines try to do is let an operating system run inside a
synthesized environment and apparently see all the aspects of an actual
hardware architecture that it seeks, yet all of this is only simulated by the
The Virtual Machine allows controlled access by the enclosed operating
system (called the guest) to the actual hardware and other resources in
the host system. By controlled access I mean that the host is protected
from anything the guest may want to do. The guest, for example, is confined to
a provided virtual disk system within the machine, and cannot access the disk
space controlled by the host except perhaps by a networking method.
VMs give the multi-boot concept the boot
It's a relatively new concept in that computers of only a few years ago
didn't have enough resources to allow such a thing, except for some
specific and small cases. The old solution, if one wanted to run more
than one operating system on a computer, was to have lots of disk space,
and run a multi-boot setup.
That worked well enough, the disk being the only significant additional
resource needed. To get from one operating system to another, one would
have to exit the current operating system and boot to another. Not
exactly convenient, but doable and reliable.
The limited Virtual Machine efforts I spoke of early were things like
running MSDOS within Windows, or MSDOS (via dosemu) in an xterm
running in Linux, or even running a Windows program using wine on a
Linux operating system.
Getting dos to run within Windows and Linux was relatively easy, and
light on resources. But going further, like running Linux within Windows
or vice versa, had to wait until significant resources other than ample
disk space was available.
So how much resource do you need for a VM?
Obviously, in order to have one or more guest operating systems
installed on your host, you'll need enough disk space for each system.
Modern operating systems easily chew up a few gigabytes in a basic install,
and even more if a lot of applications for the guest are used.
The good news is that a couple of hundred gigabytes is becoming a
pretty standard amount of disk space on even a modest modern system. That's
easily enough room to house a few guest systems, even being generous with
the amount of disk given them.
But disk space alone isn't enough. A computer running guest operating
systems within a host has to have enough memory to keep them both functioning
simultaneously. Trying this with much less than a gigabyte of memory isn't
such a great idea. But with a gigabyte or more memory, you can generally
run a guest comfortably in a host computer. I commonly run Windows 2000
in my Debian Linux host with a gigabyte of memory.
But even memory isn't totally enough. Since a Virtual Machine is having
to synthesize much of its guest environment, there is inevitable slowdown
experienced by the guest OS. Again, modern computers are up to the task,
and some VMs run much faster than others. I am able to run Windows 2000
comfortably on my 1 gigahertz computer in some VMs, but very sluggishly in
Here's what I'm running, and I can successfully run Windows 2000 and
Windows XP within my Debian system:
1 gigahertz AMD processor|
1 gigabyte of memory
80 gigabytes of disk
What more could I use?
If I want to add more than the couple of VM installs I normally use, I'll
need more disk.
If I should want to run Vista, I'd need more memory.
A faster processor would always be nice, but I'm in the ballpark for
So what do you do with a Virtual Machine?
Today's Virtual Machines can offer many benefits to users, depending
upon their needs:
Allow a user access to favorite programs on another operating system.|
Allow a user to investigate other operating systems without harming his
or her host installation.
Provide a very handy environment for a software developer who does
development on multiple operating systems.
So how does it work?
I certainly haven't worked with all the different Virtual Machines
out there. But I have worked with three of the most common ones: Vmware
server, Virtualbox, and qemu.
These there machines aren't identical in all respects, but they do share
similar guest OS installation techniques.
Once a Virtual Machine of your choice is installed on your computer,
installing a guest OS is much like installing on a native machine
Each of the VMs I tested started by creating a VM file that was to become
the root disk drive for the guest OS. Some of the machines can allow this file
to grow as needed, others are fixed (though one can create and mount another VM
file as another disk).
Each of the VMs I tested have a way of indicating what the
boot device is, usually a cdrom during the guest install.
Once an install cdrom for the guest system is booted, installation
proceeds much as with a native machine install.
On the next boot, vmware and virtualbox assume the boot device is the
install disk, but qemu has to be told not to re-boot from the cdrom.
Once re-booted to a newly installed Windows guest, both
vmware and virtualbox have some proprietary augmentations that can be
installed in the guest to improve performance and add features.
All of the systems I've tested provide easy to configure network connections
to a guest OS. All of them also provide support for USB devices, and serial and
parallel port connectivity. So as you can see, you should be able to install
an OS as a guest have nearly all of the features working that the guest could
provide in a native environment.
The only feature I'm aware of that's missing on all of them is 3D
graphic support. So don't plan to install a guest to be used primarily