On Sat, 2014-08-09 at 13:24 -0400, Theodore Ts'o wrote:

The last time I asked why in the world anyone would want to use this
feature, the only use case that I heard was people who were using
containers, and where the all of the project id's were inside a
chroot.  Hence, any questions I asked about what happens when a file
gets moved out from the hierarchy were hand-waved away, since inside a
chroot, it could never happen.

Actually, I don't believe that's entirely accurate.  The performance
problem with shared filesystem roots for containers has meant OpenVZ has
been using a block root for a while.  However, we still support the old
shared filesystem root, but for quota's within the chroot, we use a subtree
quota system (not a project quota) for which Dmitry Monakhov
posted the patches several times a couple of years ago.

James