創建 Swap 交換空間 (補完) 2019-06-12
備註：DO 的教學文章會卡在 `# swapon /swapfile` 這行指令
錯誤訊息是： swapon: swapfile: swapon failed: Invalid argument
參考文章：
https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-centos-7
https://superuser.com/questions/539287/swapon-failed-invalid-argument-on-a-linux-system-with-btrfs-filesystem
http://carlislebear.blogspot.com/2015/02/arch-swap-tips.html