Finding the size of directory with multiply hardlinked files

[Dean S. Messing]

I have a sequence of directories that have many multiply
(cross)hardlinked files, and I'd like to see what their
"true" sizes are.  These dirs are backup directories
created with something like:

rsync -a --delete --link-dest=/mnt/bkup/backup_A/ \
      /dir_to_back_up /mnt/bkup/backup_B/

By "true" sizes I mean this:

Say backup_A was created first.  If I do `du_true
backup_A' I shd. see its size.  Now I delete some files
in "/dir_to_back_up" and create some other new files.
Now I create "backup_B" with the above rsync command.  I'd
like for my mythical `du_true' to compute the
incremental change in size from backup_A to backup_B.

This seems like a non-trivial problem but maybe I'm just
missing something simple.

Dean