Live CDs Part I: Why Do We Care? [LWN.net]

May 31, 2006

This article was contributed by Michael J. Hammel

[Editor's note: this is the first in a four-part series; the next installment will appear in the next week or two.]

A live CD is a custom Linux environment that boots and runs entirely from a CD - no hard disk required. Live CDs are used for many purposes, including showcasing desktop distributions, providing useful tools for system recovery, and providing target-specific environments such as games, multimedia, GIS and security. Linux user groups often create demo live CDs for use at trade shows, install fests and other events to show that Linux isn't just a toy for hackers.

The usefulness of a live CD can be compared to the old DOS diskette used to run diagnostics on your PC. Since the floppy drive is a soon to be an extinct beast, technological evolution would have us using CDs for the same purpose. But a CD is to a floppy what a dump truck is to a spoon, and the extra space offers live CD creators nearly limitless options for customization.

There are many live CDs ISO images available for download for end users and developers. One list available from Frozentech.com lists 309 versions. The list shows versions available for varying categories, from desktop replacements to clustering environments and home entertainment. End users need only download an ISO image, burn it to a CD and boot the CD. You'll need to verify that your computer is configured to allow booting from a CD - check your BIOS configuration to be sure. Some live CD's also have minimum hardware requirements. Check the web site for that CD for details.

You might ask yourself why you're going to care about live CDs if you have a running desktop. First, live CDs are useful for specialized environments. A laptop configured for desktop use at home can boot an astronomy based live CD at night for field observations and then during the day at school use an educational live CD. No need to change the desktop configuration for three different environments.

Many live CDs also offer the option of saving user data to USB-attached drives, leaving the hard disk (if available) untouched. This makes a live CD perfect for setting up demonstrations for trade shows, customer contacts and conferences. If your group needs a demonstration of a particular application but you don't know who will give the demonstration or what hardware they will be using, all you need do is set up the live CD to handle the situation.

Second, a live CD can be used for system administration. If you've trashed your boot partition or accidentally overwritten important parts of the filesystem you can use a rescue CD to recover the partition or reinstall the OS without losing your user data. Live CDs can also be used, when appropriate, on public systems that don't offer the environment you need. A library kiosk or Internet cafe might offer you this option, for example.

Finally, live CDs are a good way to work with embedded systems. Embedded systems often have limited memory and little or no local storage. A live CD can be used to test the embedded system or manage it. Imagine a consumer media device that needs customer controlled upgrades. They can download a live CD to their computer, burn the CD and boot it to automatically run an upgrade even if the consumer device is not network connected.

More importantly, technologies used in live CDs often have important relationships with embedded systems. Compressed filesystems, read-only devices, and the use of ramdisks are all issues that are common between the two system types. Learning about live CDs can be a stepping stone into the interesting world of consumer devices.

Creating a live CD

Since a CD can hold around 700MB of data and a typical desktop installation can require more than 10-20GB, it won't be possible to duplicate your entire operating system (much less your personal data files) on a live CD. However, with compression and kernel tricks you can get very close to that.

Creating a personal live CD from your installed desktop is possible using the Linux live Scripts or similar tools. These tools make the assumption that the CD will be used on the same or very similar hardware that you're currently running on. For most desktop environments this is a safe assumption.

Another method is to build your own distribution from source and use it to create your live CD. The best place to learn how to do that is the LinuxFromScratch project. This project provides a recipe-driven process for creating your own Linux distribution from source code inside a directory on your current system. Recipes here include options for doing cross compiled builds of your distribution so that you can use your x86 desktop to build for a different architecture device, like a consumer media box.

Live CD Reviews

While it is possible to create your own live CD, it makes sense to first take a look at a few ready made versions to get an idea of what you can get now and what you might want in your own live CD. In the coming weeks I'll review a series of related live CDs from three different classes: desktop replacements, small footprint and special purpose live CDs. The goal of these reviews is not to compare one against another but to give you some idea of the variety of live CDs that area available so you can make an informed choice when you pick an existing version or take on the challenge of creating your own.

Most of the live CDs that will be reviewed are designed to allow end users to customize them with add-on packages, often packaged in project specific formats, such as compressed filesystem images, that you don't normally use with desktop distributions. I tested each of these on an EPIA M10000 board with 256MB of memory. This is an x86 compatible machine that requires the Via video drivers for both the kernel and X Window System - something that might be a little non-standard - just to see how each CD handles it. I'm also using the Linux Cool Keyboard which looks pretty much like a typical US QWERTY keyboard.

In the reviews I'll be looking for a number of things:

Cleanliness - How professional and uncluttered does it appear to the end user?
Originality - What makes this live CD unique?
On Target - How well does this CD stay true to the target audience?
Extensibility - How easy is it to add to this live CD?

Cleanliness is just a matter of taste. I prefer clean boots without much user interaction. Once I login I want to know where to go next to make the best use of the environment. For example, if this is a Games CD, where do I find the list of games and how do I start them? If this is a desktop CD, how clean is the desktop and how easy is it to find applications?

Originality is very important in these reviews. There are literally hundreds of live CD's available on the net. Each of these needs to have something that makes people want to use it. The live CD may be original because it has been targeted at a particular audience. Perhaps the CD boots quickly and offers an easy to use graphical interface that no one else offers. If they all look like a typical Red Hat or SuSE installation, there isn't much reason to choose one over another. Why is this so important? When you have a need for a CD, knowing there are 200 versions that boot to a typical desktop will let you know you can choose any one of them instead of making your own. But if only one CD boots on your TurboNator 3000 processor, maybe you will want to make your own.

Rating the CDs "On Target" value will be subjective - my interpretation of what category this CD belongs in (based in no small part on where FrozenTech.com lists the CD) and how well it stays true to that target. If a small footprint live CD takes up most of memory, that doesn't help with the small footprint problem I may be trying to solve.

Extensibility will be very important for developers and users who need to customize the CD. Most live CDs offer some way to extend the features on the CD. In some cases this will be done at runtime only with changes saved to hard disk or a USB connected storage device. In other cases, the ISO image can be extended with additional packages. The ease of adding new packages, either at runtime or in the ISO image, will determine the value of this rating.

The Chosen Few

If you want to get an early start, here is the list of live CDs I'll be looking at. Note that I've already downloaded these, before publication, so that they didn't have time to try and update just to make me happy.

Desktop Replacements
- KNOPPIX
- The GNOME live CD (with Ubuntu)
- Berry Linux
Small Footprint Systems
Specialized Systems

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