Lsyncd watches a local directory trees event monitor interface 
(inotify).  It aggregates and combines events for a few seconds 
and then spawns one (or more) process(es) to synchronize the changes. 
By default this is rsync.  Lsyncd is thus a lightweight live mirror 
solution that is comparatively easy to install not requiring new 
filesystems or blockdevices and does not hamper local filesystem 
performance. 

This requires lua.