Overview

zsync is a file transfer program. It allows you to download a file from a remote server, where you have a copy of an older version of the file on your computer already. zsync downloads only the new parts of the file. It uses the same algorithm as rsync. However, where rsync is designed for synchronising data from one computer to another within an organisation, zsync is designed for file distribution, with one file on a server to be distributed to thousands of downloaders. zsync requires no special server software — just a web server to host the files — and imposes no extra load on the server, making it ideal for large scale file distribution.

zsync is open source, distributed under version 2 of the Artistic License. Feedback, bugs reports and patches are welcome.

Advantages of zsync

zsync fills a gap in the technology available for large-scale file distribution. Three key points explain why zsync provides a genuinely new technique for file distribution:

The special handling of gzip files is, as far as I know, entirely new and unique to zsync.

I have put up technical papers. There is a current paper, which reflects zsync's latest development, and which I update with new figures as I run new tests and comparisons; there are also older snapshots of the technical paper, included for reference and citation.

Practical Engineering

zsync is only useful if people offer zsync downloads. I have made a deliberate effort to make zsync simple and reliable in use, and easy to deploy.

zsync can only be used to download files for which a .zsync file has been generated by someone who already has a copy of the file. So it requires that the download distributor, or third party, generates a .zsync file for the download.

If you are a server operator looking to use zsync for file distribution, see the server page, which gives some guidance on how to create and offer .zsync files.

Current Status

zsync was originally released in 2004, and has seen few changes in the last 15 years. While it never reached a v1.0 release, the program is stable and has been used by many projects for file distribution.

I am interested in hearing feedback about zsync. Bug reports are welcome. At this time I am not planning to make any large feature additions to zsync in its current form. Note in particular that: