Fix MySQL issue "Fatal error: cannot allocate memory for the buffer pool" on Digital Ocean server

Add swap

// Check swap
$ sudo swapon -s

  Filename				Type		Size	Used	Priority --> empty

// Create and enable swap file
$ sudo dd if=/dev/zero of=/swapfile bs=1024 count=512k
$ sudo mkswap /swapfile
  Setting up swapspace version 1, size = 262140 KiB
  no label, UUID=103c4545-5fc5-47f3-a8b3-dfbdb64fd7eb
  
// Activate swap
$ sudo swapon /swapfile

// View swap summary
$ swapon -s
  Filename				Type		Size	Used	Priority
  /swapfile                               file		262140	0	-1
  
$ echo 0 | sudo tee /proc/sys/vm/swappiness  

// Set swap file permission
$ sudo chown root:root /swapfile 
$ sudo chmod 0600 /swapfile