lsyncd does the brunt of the work. it uses inotify, which is a facility of the linux kernel, to tell which files should be copied. but i think there is a problem.

inotify is not guaranteed to see every file system access. in fact, it often misses them when the file system is busy because there is an upper bound upon the number of inotify events that can be queued, set in /proc/sys/fs/inotify/max_queued_events.

i.e., you could get data loss using this mechanism alone. you need to also run rsync every so often, i think.

the lsyncd guys are aware of this: http://code.google.com/p/lsyncd/

(maybe this is how dropbox works under linux too, if so it presumably has the same problem..)