[[http://librsync.sourceforge.net/|Librsync]] is a library that implements a remote-delta algorithm, which efficiently creates diffs of files separated by a network connection.  This allows for greatly reduced network IO to send an updated version of a file that is mostly the same as the previous version.  

===== Case Studies =====
  * FolderShare used librsync 0.9.7 to transfer files up to 2GB.  The code ran on Linux, PC, and OSX without any problems.  

===== Gotchas =====
  * The larger the file, the larger the amount of memory necessary.  It may require a significant amount of memory for gigabyte+ files.  This can be mitigated by adjusting the window size.
  * Be sure to test your product with files greater than 2 and 4 GB.  


===== Alternatives =====
  * If you have a copy of the content that exists on the remote side [[http://xdelta.org/|xdelta]] can be a more efficient (and less arcane) choice.
  * Another remote syncing algorithm, RDC, is described in [[http://research.microsoft.com/~gurevich/Opera/183.pdf|this Microsoft Research paper]].

===== Other Resources ==== 
  * [[http://rsync.samba.org/tech_report/|Description of the rsync algorithm]]