If I make the source=>dest one-way, such as by truncating, I can store the real filename inside the file. That provides the other way.

However, next time it goes the first way, it may not be unique, if suffixes are added to disambiguate. It may have to check several candidates to see which matches. Perhaps that will be rare, especially if I set the limits to match the real destination filesystem (so it doesn't fold more than it has to).

Not as elegant as 1:1, but probably not over-burdened. Hmm, but it would mean it always has to open the file to check the real name, not just compare dates to see if the backup needs freshening.

—John