I discoverd an issue with mount_nullfs binary. When phisical drive eg. /dev/da0p2 is mounted to / and i mount_nullfs eg. /home/user /mnt (or any other place), there is a problem with updating space in df -h. What i know, when i do dd if=/dev/zero of=bigfile bs=1024 count=5120000 in /mnt and do df -h i get properly announced space (-500mb of free space), but when i deleted bigfile in /mnt that 500MB wont back to Avail neither on nullfs mount or in phisical drive mount. What helps is unmounting nullfs and then space info get updated on phisical drive mount.
I did a tests on 2 diferent machines, both with latest stable FreeBSD. If needed i can provide additional info.
StepByStep to reproduce this bug
mount_nullfs /home/<user>/ /mnt/
dd if=/dev/zero of=bigfile bs=1024 count=1024000
compare if 500mb of space get back to Avail