2. Why jigdo?

2.1. How Does One Get A Debian ISO Image Set?

If you want your own set of Debian CDs there are many ways of getting them. One way is to buy them from vendors who sell Debian CDs. This has some merit since some of the vendors donate money back to the Debian project. Your donations help make sure that Debian is around for a long time.

Another way of getting a set of Debian CDs is to burn your own set. This first entails obtaining an ISO image and then burning that ISO image to a blank CD. Before jigdo, there were two ways of creating Debian CDs:

1. Downloading the entire ISO

2. Using the pseudo-image kit (PIK)

This document is about the new and better way of obtaining Debian ISO images, using a tool called jigdo. In fact, the PIK is now deprecated. The canonical method of getting Debian ISO images is with jigdo.

2.2. Why Not Download The Whole ISO Image?

There are mirrors which offer http and ftp downloads of Debian ISOs. The problem is that there are very few mirror sites, and their bandwidth can't support everyone who wants Debian ISOs. For example, fsn.hu has reportedly saturated the connection of its provider. The outgoing traffic reaches a few terabytes per month!

In addition, Debian testing and unstable get updated often. Your ISOs may become outdated the same day you download them unless you find some sneaky way of updating them like mounting the ISO on a loopback device and using rsync (which is what the PIK does). So if you want up-to-date ISO images, you must download a new set of ISO images every day. Clearly, this is not the way you want to obtain Debian ISOs!

Even if you want to download the stable ISO images, they still get updated every few months. Downloading the ISO images will give you up-to-date images for a few months, but every time a new revision of Debian stable is released, you'll need to go through the painful process of downloading the entire ISO set from scratch. This is not a good use of your time and the mirror's resources.

2.3. Why Not Use The Pseudo Image Kit (PIK)?

The PIK addresses most of the problems of downloading entire ISO images. The downloads are fast, and the PIK uses rsync to update only those portions of an ISO image that need to be updated, so it's an efficient way of keeping your ISO set up-to-date. However, there are some hefty problems with the PIK:

• It's difficult to use and not very user friendly.

• You can't use the PIK to download testing and unstable ISO sets.

• The PIK relies on rsync which is CPU-intensive for the server. If too many people use PIK with the same server, it would go up in smoke. Even if the PIK is made more friendly for the user, it's unacceptably unfriendly for the mirrors.

• The PIK uses rsync, which is blocked by many of the stricter firewalls. So even if you wanted to use that nice fast corporate network at work, you might run into problems using the PIK.

• Each image needs to be stored on the server. That was OK in the good old Potato days, when the 28 CD images "only" took 17 GB. Starting with Woody, the 96 CDs need 57 GB or so. Now imagine that we also want to offer DVDs and this figure doubles.

2.4. What Is Jigdo?

Jigdo (which stands for "Jigsaw Download") was written by Richard Atterer and is released under the GNU GPL. It's a tool that allows efficient downloading and updating of an ISO image. Any ISO image. Jigdo is not Debian specific, however Debian has chosen it to be the prefered method of downloading ISO images.

The jigdo tool comes with two utilities:

• jigdo-file is used by the person offering the ISO image. It enables anyone to download that image by creating a .jigdo and .template file for the image.

• jigdo-lite is used by people who want to download the ISO image. It downloads the image using the image's .jigdo and .template files which were created by jigdo-file. If your main concern is simply downloading Debian ISO images, you'll just be using jigdo-lite.

A common misconception is that jigdo creates ISO images; it doesn't. Jigdo-file simply allows people to download an ISO image by creating a .jigdo and .template file. The people who want to download the ISO image will get these two files and use jigdo-lite to download the image. The ISO image needs to be made in advance, before jigdo-file is used, and that's usually done with a utility like mkisofs or debian-cd.

Jigdo addresses all the problems with the other two methods of obtaining Debian ISO images:

• It's much faster than downloading the entire ISO image.

• Unlike downloading the entire ISO image, it can take an outdated CD (or a loop mounted outdated ISO image), download only the files that have changed since the CD (or ISO image) was created and create a new updated ISO. Very similar to how you use cvs to update source code.

• jigdo-lite is much easier to use than the PIK.

• jigdo-lite uses wget which, by default, uses http to transfer files. The PIK uses rsync. While rsync may be blocked by some firewalls, the only firewalls that block http are the ones from which you shouldn't be using jigdo to begin with. You'll almost never run into firewall problems with jigdo-lite.

• jigdo-lite is much more user friendly than the PIK.

• jigdo-lite is much more server friendly than the PIK.

Clearly, jigdo is the best method of obtaining Debian ISO images.