In discussion with Lai and Jian, it makes sense that "lfs mkdir -i N" be "sticky" on a particular MDT, in the same way that it was before 2.15 introduced MDT space balance, since that is what users expect (the few that actually use "lfs mkdir -i" in the first place) and is the "more obvious" behavior. If a user is running "lfs mkdir -i N" then they want the directory to be on a specific MDT, and that matches the existing behavior better. The majority of users that don't know/use "lfs mkdir" will probably want (and get) the MDTs to be balanced automatically.

That means by default that the 2.15+ "lfs mkdir -i N" should also include a default directory layout for the same MDT. It would be possible to override this with "--max-inherit=0|-X 0".