Create empty file

Traditional:

dd if=/dev/zero of=/mnt/swap16G bs=1G count=16

Fast:

fallocate -l 16G /mnt/swap16G

Make Swapfile and enable

mkswap /mnt/swap16G
chmod 600 /mnt/swap16G
swapon /mnt/swap16G

Enable on boot

Add to the /etc/fstab if you want to keep the settings

/mnt/swap16G   none    swap    sw    0   0

Note

If using things like AWS instance-store-volume adding the file to the /etc/fstab is not appropriate. You could create the swapfile on the fly or you could create one swapfile, compress it, and decompress on each boot. In Ubuntu and other probably others using the /etc/rc.local file can be a decent choice (there are others).

gunzip -c /swap16G.gz > /mnt/swap16G
chmod 600 /mnt/swap16G
swapon /mnt/swap16G