March 21, 2010 | Linux/Unix

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As I mentioned in a article on Ubuntu Boot CD images, there’s a new boot CD image every day because the new version is being tested out. When the new version is finally released, work starts on version next, and daily builds start to come out again.

The problem is downloading a new 700MB file daily seems wasteful, and that’s 21GB of download per month, which your ISP may or may not like.

The solution is zsync, a binary file sychronization program.

Introduction to zsync

zsync uses the same algorithm as another file-syncing program, rsync. But zsync is better than rsync for file distribution where one file is going to multiple clients outside of a single organization. Also:

  • zsync doesn’t require a program to run on the server. It runs on the client.
  • zsync handles compressed files well.

You might wonder how zsync could possibly work with binary files, since a single small change can render the whole file totally different, and thus not practicably syncable. The answer is the zsync has special handling for gzipped files (files compressed with the gzip program).

Installing zsync

First, if you haven’t already, add the universe repository to your list of software repositories. You can do this in System > Administration > Software Sources.

Then install zysync either by clicking here from Firefox or entering the following from the terminal:

sudo apt-get install zysnc

Using zsync

It only makes sense to use zysnc if you already have a full ISO image file that you want to update. So, first download an Ubuntu ISO image file.

Then, when there’s an update you want to get, go to the directory where the full image file is in a terminal. Then type zsync followed by the HTTP address of where the .zsync file is. (The .zsync file helps zsynch to correctly identify and apply updates.)

For the most often used Ubuntu CD image (the 32-bit desktop image), that would be:


You can find other .zsync files in the same place where the CD images are at Ubuntu downloads.

Note: At the end of this procedure, the updated file will have the same name as the old file. The old file is kept and saved with a “.zs-old” filename suffix.

zsync at work

zysnc first downloads the .zsync file.

Then it starts downloading the actual image file:

downloading from
###################- 96.3% 43.8 kBps

The image file is saved with a .part suffix during the download. If the download is interrupted, just run zsync again. It’ll pick up where it left off:

Read lucid-desktop-i386.iso.part. Target 92.4% complete.
downloading from
##################-- 94.8% 43.8 kBps         A

Once it’s done, it verifies the file:

verifying download...checksum matches OK
used 663175168 local, fetched 55365870

Saving the .zsync file

The .zsync file is about 1.4MB. You can avoid downloading it again by using the -k option followed by the .zsync file.

If the local file has a different than that referenced in the .zsync file, specify the local filename with the -i option.

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