Maybe can save someone a little hassle .... you can tell xfsdump to not back up a directory tree or individual files by using <chattr -R +d (directory name)> or <chattr +d (filename)>.
Yay, saves space, avoids backing up junk you don't want to back up. Also nice if you are setting up a root disk but don't want to copy over all the user files.
The only thing they don't bother to mention is that unless you specifically tell xfsdump to pay attention to that attribute (with an -e switch), it will ignore the flags you just went to all the trouble to set and copy everything anyway.
There went 45 minutes of my life. Hope it saves someone else some hassle. And oh yeah, if you notice it's copying things you don't want, don't hit ctrl-C to stop. Then it thinks you are giving it a rest and will come back later. If you try to do another dump of the same structure it will insist that the dump was just interrrupted and you want to start all over on a dump that wasn't what you wanted in the first place. Catch-22.
Otherwise, this is a fine piece of high-tech engineering.