First create a 2 GB file
dd if=/dev/zero of=/swapfile bs=1M count=4096
For 8 GB, use count=8192. You can also use fallocate command instead of dd.
fallocate -l 8G /swapfile
Make it swap
mkswap /swapfile chmod 0600 /swapfile swapon /swapfile
Add it to /etc/fstab
echo "/swapfile swap swap defaults 0 0" >> /etc/fstab