JIGDO - create SME Server ISOs
jigdo
Introduction
jigdo is a method of downloading CD and DVD images over the Internet (and other large files) and is used by Debian and Ubuntu when new releases are created. jigdo files contain a list of the files that need to be included in the new CD or DVD ISO image and you can get the files from local sources and over the Internet. This means when you want to create ISO images for updated versions of Debian, you only need to download the changed files. This saves both bandwidth and and time.
jigdo's approach is unique due to the way it identifies the small pieces that the large file consists of: For instance, if the file is a CD image, the individual files on the CD are contained somewhere within the image file. jigdo-file is capable of finding these files, so if copies of the individual files on the CD are stored separately on the server, jigdo-lite can be made to download these files one by one, and assemble the CD image on the fly on the user's computer. This scheme has several advantages:
- The large image does not need to be stored on the server, instead only the many small files contained in the image. This works with CD images, DVD images (both ISO9660 and UDF format), uncompressed zip files, tar archives... jigdo-file is format independent.
- In spite of the above, jigdo creates a bit-exact copy of the original image on the user's machine. (To achieve this, the directory data, boot block etc. of the image is stored in a special .template file which is distributed alongside the .jigdo file.)
- There is full control over where jigdo-lite will download the individual parts. It is possible to define mirrors so users can choose their nearest mirror.
- jigdo relies on standard HTTP/FTP, no support for special protocols like rsync, and no installation of special software is required on the mirrors.
- jigdo-lite supports resuming aborted downloads, or continuing the download with another mirror if the current one proves slow.
- It is possible to "upgrade" a CD image: If a new version of an image is released, jigdo-lite can download only the data that has changed, the remaining data is read from the outdated version.
- It is possible to release modified versions: If someone distributes a CD image and you have made a few small modifications, e.g. added some files, then you only need to upload the data for your modifications to your webspace. All the data from the original CD image is still fetched from the original site.
If you have read this far, you may be confused now, asking yourself what jigdo does and doesn't do. :-) It seems that the jigdo concept is a bit difficult to grasp at first. If you're still interested, have a look at the respective section of the HOWTO and at the Examples section of the jigdo-file manual.
