# dd if=/dev/zero of=/usr4/swapfile bs=1024k count=1024
# echo "/dev/vn0c /usr4/swapfile swap" >> /etc/vntab:
# vnconfig -ae
# swapinfo

   1.      Be certain that your kernel configuration includes the memory disk driver (md(4)). It is default in GENERIC kernel.
	  device   md   # Memory "disks"

   2.      Create a swapfile (/usr/swap0):
	  # dd if=/dev/zero of=/usr/swap0 bs=1024k count=64

   3.      Set proper permissions on (/usr/swap0):
	  # chmod 0600 /usr/swap0

   4.      Enable the swap file in /etc/rc.conf:
	  swapfile="/usr/swap0"   # Set to name of swapfile if aux swapfile desired.

   5.      Reboot the machine or to enable the swap file immediately, type:
	  # mdconfig -a -t vnode -f /usr/swap0 -u 0 && swapon /dev/md0