labels.
Please test the slice/label features:
cd /dev; sh MAKEDEV svn0
cd /var/tmp; dd if=/dev/zero of=vnfile bs=8192 count=1024
vnconfig -c /dev/rvn0 /var/tmp/vnfile
fdisk /dev/rvn0 # invent a geometry, create one BSD partition
disklabel -r -w vn0 floppy # a convenient (bogus) label
disklabel -e vn0 # edit label to match device
newfs /dev/rvn0a
mount /dev/vn0a /mnt
...
The steps after the fdisk could name the device vn0s1 instead of vn0.
20aaa0b802