How about a background cron of diff -br copyX copyY , once per week, for each X and Y .. if they are hot/cold-accessible

Although, in my case, the original is evolving, and renaming a folder and few files makes that diff go awry.. needing manual intervention. Or maybe i need a content-based-naming - $ ln -f x123 /all/sha256-of-x123 then compare those /all