# create an 8G swap file
dd if=/dev/zero of=/swap bs=1G count=8
# set permissions
chmod 0600 /swap
# look for an unused md device (ie, not listed)
mdconfig -lv 
cp /etc/fstab /etc/fstab.save
# edit /etc/fstab and add the following line, using the correct md##
joe /etc/fstab
md42	none	swap	sw,file=/swap	0	0
# save your file
# turn on swap
swapon -aq
# look at swap information
swapinfo -k