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zchunk is a compressed file format that splits the file into independent chunks. This allows you to only download changed chunks when downloading a new version of the file, and also makes zchunk files efficient over rsync.

zchunk files are protected with strong checksums to verify that the file you downloaded is, in fact, the file you wanted.

As of zchunk-1.0, the ABI and API have been marked stable, and the only changes allowed are backwards-compatible additions


To build and install zchunk, first install meson and run

meson build
cd build
ninja test
sudo ninja install

If you're building on an operating system where some libraries are stored in /usr/local, you'll need to replace meson build above as follows:

CFLAGS=-I/usr/local/include CXXFLAGS=-I/usr/local/include LDFLAGS=-L/usr/local/lib meson build

Using the utilities

To decompress a zchunk file, simply run:

unzck <filename>

To compress a new zchunk file, run:

zck <filename>

To download a zchunk file, run:

zckdl -s <source> <url of target>

To read a zchunk header, run:

zck_read_header <file>

Zchunk dictionaries

By default, each chunk in a zchunk file is compressed independently. However, if you're creating a zchunk file that has any repetitive data, you may be able to reduce the overall file size by using a zstd dictionary. The dictionary takes up extra space at the beginning of the zchunk file, but is used as an identical initial dictionary for compressing each chunk, which can give a significant overall savings.

It is important that all further revisions of the zchunk file use the same dictionary. If the dictionary changes, none of the chunks will match from the old file, and the full new file will be downloaded.

Zchunk can use any zstd dictionary, but also includes a utility to generate the ideal zstd dictionary for a zchunk file.

To create an ideal dictionary for a zchunk file, run:

zck_gen_zdict <file.zck>

The dictionary will be saved as <file.zdict>.

You will then need to recompress the file with the dictionary:

zck -D <uncompressed file>

Note that zck_gen_zdict does require that the zstd binary be installed on your system.



A file format designed for highly efficient deltas while maintaining good compression




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