Bukkit

The old backup plugin I used, Minebackup, had a system in which it only backed up a world if it is "dirty", aka changed since the last backup. This really helps to save processing and disk usage. It would be incredibly helpful if you could figure out how to implement such a feature.

This seems a good idea, I will try to implement this for all files (not just worlds) and maybe selectable individually (have certain files always backed up and other only when they're "dirty").

This ticket will be updated at the same time as I work on this feature.

PS: You said "This really helps to save processing and disk usage." I do agree that it saves disk usage, obviously but the processing wouldn't be correct: for each selected file, I would have to check for a previous backup containing a copy of this exact file, comparing their MD5 (or raw content but that would probably be more resource intensive), etc.
In the end, I don't think it would save RAM (it wouldn't use significantly more either though), but definitely disk space and usage.