UnionFS is a filesystem driver for Linux that allows you to make changes to a certain read-only file system, while it is actually stores these changed files on another partion (for example an USB memory stick). Is this useful at all? Yes, it is.
“UnionFS”:http://www.fsl.cs.sunysb.edu/project-unionfs.html is interesting to the live-CD Linux distributions in this world; the Linux distributions that you don’t have to install, but just boot directly from the CD. The main advantage of such a Linux distribution is that you can’t fall asleep during installation with your head on your keyboard, while in the partition manager accidentally “typing in” exactly the keystrokes that remove all your Windows partitions. These Live CD distributions, as will you understand, must be developed by non-GNU-religious people, as those people would do everything to “accidentally” remove your Windows partitions. Don’t believe me? See how many keys you have to press in your Linux distribution’s installer to wipe out your harddisk and convert it to GNU-purism.
But I’m running off track. One problem with Live CD distributions, beside being quite slow, is that they don’t allow you to install or upgrade software. All the software and nearly all other files are read from the CD, which is read-only. That’s where UnionFS comes in. If you plug-in your USB memory stick and do some magic (which probably consists of first saying the magic words, “I shan’t run evil non-GNU software”, followed by typing in some UNIX commands), the file system of your USB memory stick and the CD will magically unify. You can now change things in the file system, these changes are stored on the USB memory stick. This means that you can just go and apt-get install wine away. Only if you do it very silently, of course, _or Richard S. will know what you’re typing, he reads keys you know._
The first Live CD distribution to use UnionFS is Knoppix, which “will introduce this feature in its Knoppix 3.8 release”:http://www.oreillynet.com/pub/wlg/6654.