librsync is a library for calculating and applying network deltas, with an interface designed to ease integration into diverse network applications. librsync encapsulates the core algorithms of the rsync protocol, which help with efficient calculation of the differences between two files. The rsync algorithm is different from most differencing algorithms because it does not require the presence of the two files to calculate the delta. Instead, it requires a set of checksums of each block of one file, which together form a signature for that file. Blocks at any in the other file which have the same checksum are likely to be identical, and whatever remains is the difference.
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Convenient and transparent local/remote incremental mirror/backup
rdiff-backup is a script, written in Python, that backs up one directory to another and is intended to be run periodically (nightly from cron for instance). The target directory ends up a copy of the source directory, but extra reverse diffs are stored in the target directory, so you can still recover files lost some time ago. The idea is to combine the best features of a mirror and an incremental backup. rdiff-backup can also operate in a bandwidth efficient manner over a pipe, like rsync. Thus you can use rdiff-backup and ssh to securely back a hard drive up to a remote location, and only the differences from the previous backup will be transmitted.