Introduction to Virtual Machines
This is intended as an overview to server virtualisation and how virtual machines relate to the world around them.
What’s a virtual machine?
Your virtual machine is your own Linux server, and is intended to be indistinguishable from that provided by a more expensive co-location facility. Your virtual machine initially will have a single IP address allocated to it, but can host as many IPs as you need (see here for prices). The machine will have a Secure Shell Daemon (sshd) running so you can gain access to it through any Unix ssh client, or a tool such as PuTTY under Windows, and begin to install and configure the virtual machine’s software.
What’s a host machine?
We run all our customers’ virtual machines (which we call VMs or just ‘machines’) on top of a set of physical host machines (which we call ‘hosts’). Because virtual machines are simply programs themselves, they can be backed up and migrated to a secondary host machine. If a host machine fails for more than a few hours, we can bring up the affected customers’ virtual machines on a secondary host while we fix the primary host.
What’s the administration shell?
The administration shell is a program running on the host machine which allows you external control over your VM, and allows a level of control over your machine that co-location providers usually charge extra for. Through the administration shell, you can do the following (more details):
- halt or reboot your machine, even if the kernel has crashed;
- upgrade your Linux kernel to newer versions as they become available;
- set up the watchdog (see below) to recover your machine from crashes, or to alert you if software systems within the machine;
- access system or boot consoles.
Nearly all of the above would require that you have physical access to the machine, and are potentially “dangerous” operations to perform remotely; that is to say, if they went wrong, you would have to physically travel to the machine to rescue it, or pay someone to do the same.
Our administration shell allows you to experiment with different kernel versions, and try kernel programming on a well-connected host without needing to bother anyone if the kernel crashes.
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