Add swap space for Linux

Reference document:

• https://askubuntu.com/a/1075516
• How To Add Swap Space on Ubuntu 18.04

If you use a server with smaller memory, you can add a swap space for it.

1. execute sudo swapon --show, if there already exist swap file, you need delete it as following:

   1 2	$ sudo swapoff $swapfile $ sudo rm $swapfile

2. sudo dd if=/dev/zero of=$swapfile bs=1M count=8192
3. sudo chmod 600 $swapfile
4. sudo mkswap $swapfile
5. sudo swapon $swapfile

Now you can execute sudo swapon --show or free -m to ensure it.