I need to sync two huge file systems. Both sides run linux with full root access.

My preferred solution: I can read the list of changed files and directories and sync only the changed files.

Here are some solutions and why they don't fit:

• rsync: Needs to check recursively all files. There are some million files and only little changes. The check takes too long.
• unison: the same: needs to check all files.
• inotify: I need a handler for every directory and there too many. Inotify was not build for "watch all files" scenarios.
• DRDB: Both sides should run independent.