Update a distro's iso to new one with zsync

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.
to install it on ubuntu
$ sudo apt-get install zsync
Suppose u have the alpha version of karmic koala and kept it as karmic.iso in your home folder. For updating this image to the final  release I typed the following command.
$ zsync -i karmic.iso http://releases.ubuntu.com/karmic/ubuntu-9.10-desktop-i386.iso.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:

• Client-side rsync — zsync uses the rsync algorithm, but runs it on the client side, thus avoiding the high server load associated with rsync.
• Rsync over HTTP — zsync provides transfers that are nearly as efficient as rsync -z or cvsup, without the need to run a special server application. All that is needed is an HTTP/1.1-compliant web server. So it works through firewalls and on shared hosting accounts, and gives less security worries.
• Handling for compressed files — rsync is ineffective on compressed files, unless they are compressed with a patched version of gzip. zsync has special handling for gzipped files, which enables update transfers of files which are distributed in compressed form.