Differential backup

Thinking about allowing a backup to tell which files have changed in the
database since last backup. This would allow an external utility to copy
away only changed files.

Now there's a few ways of doing this and many will say this is already
possible using file access times.

An explicit mechanism where Postgres could authoritatively say which
files have changed would make many feel safer, especially when other
databases also do this.

We keep track of which files require fsync(), so we could also keep
track of changed files using that same information.

Is this route worthwhile? Or in some way unacceptable?

-- Simon Riggs           www.2ndQuadrant.com