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 (ie a dedicated host). Your virtual machine initially will have a single IP address allocated to it, but can host multiple web sites using name based hosting. If you need to host SSL sites, or if there are other technical reasons that make this necessary, we can supply additional IPs for you to bring up, in accordance with RIPE’s regualtions. The machine will have a Secure Shell Daemon (sshd) running so you can gain access to it through any Unix ssh or sftp client, or tools such as PuTTY and filezilla under Windows. These access methods (ssh and sftp) will enable you to configure the virtual machine’s software, install any additional software you need and transfer your data to it.

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 console shell?

The console 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 console 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.