Presentation

binary-sync is a collection of tools to synchronize large binary files between distant systems.
There are two key ideas:

• synchronise: we are trying to minimize the amount of exchanged data between two systems to get the exact same file
• large file: We only have a local copy of the file on each system. As files are potentially large (or potentially block devices), a system cannot have two version of a it.

How it works

It works by exchanging messages between a master and a remote.
Supposing the master has the version B and the remote has the version A, you want the remote to get the version B.

1. The remote builds a checksum file and sends it to the master
2. The master builds its own checksum file
3. Comparing the two files, the master write needed blocks data in a data file
4. The remote can write blocks from the data file

Here are the convetion fot file extensions

• .sum will represent a checksum file
• .data will represent a data file

The format of each file is documented here.

Alternatives

Some tools already exists to manage diff and patch on binary files

• xdelta site
• bsdiff site

They work using the two versions of the file and finding differences to generate a patch. Once you have the patch file, anyone with the version A or B of the file can get the other one.

Pros

• Unlike these tools, with binary-sync you do not need the two versions of the file on a system to find the differences and synchronise it.
• The memory footprint is very low unlike traditionnal diff tools because there is no difference search algorithm.

Cons

• On the other side, there is no difference search algorithm and therefore you may request a block even if only one byte difference.
• Another limitation is that the file on master should not be updated between the checksum file generation and the data file creation to guarantee the data coherence.
• As none of the systems have the two versions of the file, they need to communicate at least twice to synchronise it, one for the remote to send its checksum file, and when the master sends the data file (computed from its own checksum file and the remote one).

Using

See the manual page for a complete description of the binaries.