It is relatively common to use a "hard link tree" to create a second backup of a folder that is effectively just a copy of the files that have changed since the original backup.
rsync has a command line option
--link-dest to achieve this.
My question is whether there is an easy way to see the extra disk space used by such a "hard link tree"?
Because the hard links are the same file, just performing
du on the new tree will show the total size of all the files, including the ones that are hard links, and thus share the disk space of the original files.