Image is everything

Anyone who has experimented with the Linux operating system will be familiar with ISO image files, but even if you haven’t ventured down this route, it’s highly likely that you’ve heard the term.

But what are these files and what purpose do they serve? A good way to think of an ISO file ­ a shorthand term derived from the ISO 9660 standard that defines the file structure of CD-Rom discs ­ is as a disc image, because it contains the entire contents of a CD or DVD within a single file.

It’s effectively a snapshot of the disc contents, including the directory structure and other information, transposed into a single file. In much the same way that a compressed file such as a ZIP can contain hundreds or even thousands of files, an ISO is simply a container for many files and directories. Importantly, though, it can be used to recreate an an exact replica of the original disc it represents.

For many PCW readers this may seem old hat, but it’s easy to forget that you’ve got to start somewhere, hence we’ve started this occasional series with a ‘back to basics’ theme for those readers who are just getting to grips with the more advanced side of personal computing. In this month’s feature we’re going to look at the various uses for ISO files ­ and other similar formats ­ and the tools needed to work with them.

Introducing ISOs
The ISO format is most commonly used as a means of software distribution via the internet. It has historically been most commonly associated with Linux distributions, which can be packaged into a single big file for downloading.

Rather than making the individual files on a CD or DVD available for download, or supplying software on a physical disc, it is generally cheaper, easier and more reliable for companies to use this method to transfer large volumes of data.

The benefits of this method over transporting discs are obvious ­ not only are there no postage costs involved, but there is no risk of a disc being lost in transit. It is also quicker to download one big file than lots of individual files, and you can reduce the risk of files going missing or getting corrupted in the download process.

Finally, ISOs make it possible to download an entire directory structure of unlimited complexity, something that’s not possible when downloading individual files.

Why use ISO files?
Once an ISO file has been downloaded there are two basic uses for it. The first option is to use the ISO to burn a new CD or DVD in order to access the files it contains; the resulting disc can then be used in precisely the same way as any other CD or DVD. The alternative is to use image-mounting software to convert the image file into a virtual disc that can then be loaded into a virtual optical drive and used just as if it was a real disc.

The ISO format is also useful for the purposes of backing up data. With most modern PCs fitted with recordable CD or DVD drives, many people use these to create backups of important data. Having one backup copy of files is all well and good but more than one is advisable. Rather than wasting space by storing multiple copies of the same CDs and DVDs, discs can quickly and easily be converted into ISO images, which can then be stored on a hard disk in a safe place. This means that even if your original disc gets damaged or lost, you should be able to recreate it (as long as the disc isn’t copy-protected).

Of course, storing ISOs on a hard disk isn’t a fail-safe backup method ­ it’s an additional measure, not a substitute. A better use for ISO files might be to store backup copies of all your commercial software CDs and DVDs on the hard disk: this has the added benefit of giving access to these titles without having to reach for the original CD or DVD. We’ll come to this soon.