The key point missing from the other current answers is that the
minfree space is not solely reserved for root. While it is true that the root user is exempt from
minfree restrictions, this space is not there just for this reason.
The space reserved for
minfree is used by the filesystem code to keep performance good - in particular, it keeps fragmentation on the filesystem down. The value of
minfree also affects whether the filesystem tries to be fast or space-efficient by default; if minfree is below 8%, and you haven't altered the optimisation method using the
-o flag to
tunefs(8), you'll see filesystem performance drop.
This comes up from time to time on the FreeBSD lists; there is a reasonable summary (and thread) from May 2003. There is a last word not attached to that page, too:
"The steady state of disks is full"
-- Marshall Kirk McKusick