Rsync

rsync

File:Newrsynclogo.jpg
Original author(s)	Andrew Tridgell, Paul Mackerras
Developer(s)	Wayne Davison
Stable release	2.6.9 / 6 November 2006
Repository	github.com/RsyncProject/rsync
Operating system	Cross-platform
Type	Data transfer/ differential backup
License	GNU General Public License
Website	http://rsync.samba.org/

In computing, rsync is a software application for Unix systems which synchronizes files and directories from one location to another while minimizing data transfer using delta encoding when appropriate. An important feature of rsync not found in most similar programs/protocols is that the mirroring takes place with only one transmission in each direction. rsync can copy or display directory contents and copy files, optionally using compression and recursion.

rsyncd, the rsync protocol daemon, uses the default TCP port of 873. rsync can also be used to synchronize local directories, or via a remote shell such as RSH or SSH. In the latter case, the rsync client executable must be installed on both the local and the remote host.

Released under the GNU General Public License, rsync is free software.

Algorithm

The rsync utility uses an algorithm (invented by Australian computer programmer Andrew Tridgell) for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a different version of the same structure.

The recipient splits its copy of the file into fixed-size non-overlapping chunks, of size $S$, and computes two checksums for each chunk: the MD4 hash, and a weaker 'rolling checksum'. It sends these checksums to the sender.

The sender computes the rolling checksum for every chunk of size $S$ in its own version of the file, even overlapping chunks. This can be calculated efficiently because of a special property of the rolling checksum: if the rolling checksum of bytes $n$ through $n+S-1$ is $R$, the rolling checksum of bytes $n+1$ through $n+S$ can be computed from $R$, byte $n$, and byte $n+S$ without having to examine the intervening bytes. Thus, if one had already calculated the rolling checksum of bytes 1–25, one could calculate the rolling checksum of bytes 2–26 solely from the previous checksum, and from bytes 1 and 26.

The rolling checksum used in rsync is based on Mark Adler's adler-32 checksum, which is used in zlib, and which itself is based on Fletcher's checksum.

The sender then compares its rolling checksums with the set sent by the recipient to determine if any matches exist. If they do, it verifies the match by computing the MD4 checksum for the matching block and by comparing it with the MD4 checksum sent by the recipient.

The sender then sends the recipient those parts of its file that didn't match any of the recipient's blocks, along with assembly instructions on how to merge these blocks into the recipient's version to create a file identical to the sender's copy.

If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files.

While the rsync algorithm forms the heart of the rsync application that essentially optimizes transfers between two computers over TCP/IP, the rsync application supports other key features that aid significantly in data transfers or backup. They include compression and decompression of data block by block using zlib at sending and receiving ends, respectively, and support for protocols such as ssh that enables encrypted transmission of compressed and efficient differential data using rsync algorithm. Instead of ssh, stunnel can also be used to create an encrypted tunnel to secure the data transmitted.

Uses

One of the earliest applications of rsync was to implement mirroring or backup for multiple Unix clients onto a central Unix server using rsync/ssh and standard Unix accounts. With a scheduling utility such as cron, one can even schedule automated encrypted rsync-based mirroring between multiple host computers and a central server.

Variations

A utility called rdiff uses the rsync algorithm to generate delta files with the difference from file A to file B (like the utility diff, but in a different delta format). The delta file can then be applied to file A, turning it into file B (similar to the patch utility).

Unlike diff, the process of creating a delta file has two steps: first a signature file is created from file A, and then this (relatively small) signature and file B is used to create the delta file. Also unlike diff, rdiff works well with binary files.

Using rdiff, a utility called rdiff-backup has been created, capable of maintaining a backup mirror of a file or directory over the network, on another server. rdiff-backup stores incremental rdiff deltas with the backup, with which it is possible to recreate any backup point.

duplicity is a variation on rdiff-backup that allows for backups without cooperation from the storage server, as with simple storage services like Amazon S3. It works by generating the hashes for each block in advance, encrypting them, and storing them on the server, then retrieving them when doing an incremental backup. The rest of the data is also stored encrypted for security purposes.

History

rsync was first announced on 19 June 1996. The original authors were Andrew Tridgell and Paul Mackerras.

See also

• CVSup
• List of delta encoding software
• Unison (file synchronizer)
• xdelta
• Jigdo
• Grsync

External links

• rsync homepage
• rsync algorithm
• HeatSoft Sync