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Steve McIntyre - Debian

Jigdo - Spreading the load of CD and DVD downloads

Jigsaw Download (jigdo) is a tool designed to ease the distribution of very large files over the internet, for example CD or DVD images. Its aim is to make downloading the images as easy for users as a click on a direct download link in a browser, while avoiding all the problems that server administrators have with hosting such large files.

Jigdo was originally developed by Richard Atterer in 2002. Since then, it has been used by Debian as the primary method of distribution for ISO images. It generally works well, and is an effective way to allow people to download these ISO images quickly and efficiently.

I started working on improving the performance of jigdo creation in 2004. The process of turning an ISO image into jigdo files was incredibly slow, causing painful release delays. I fixed this by working jigdo support into mkisofs directly, leading to a twenty-fold performance improvement.

The jigdo format itself is reasonable, but the implementation of the jigdo programs in C++ was difficult to understand and extend. I wrote a parallel implementation in C, and started adding more features.

I plan to talk about:

• the internals of jigdo - how it works
• where the big performance problem came from, and how I fixed it (JTE)
• other enhancements along the way, and a re-implementation of the tools (jigit)
• first attempt at tackling the download problem (iso-image.pl)
• further work: better compression; jigdoofus - create images on the fly!