MySQL crashes periodically on Plesk: Out of memory

Symptoms

• Plesk and websites are down from time to time:

  503 Service Unavailable

  ---

  Server Error
  500
  Zend_Db_Adapter_Exception
  SQLSTATE[HY000] [2002] Connection refused

  ---

  ERROR: PleskDBException: Unable to connect to database: mysql_connect(): Connection refused /var/lib/mysql/mysql.sock (Error code: 2002). Please check that database server is started and accessible. (Abstract.php:69)

Cause

Insufficient amount of free RAM on the server.

• In the log file /var/log/mariadb/mariadb.log or /var/log/mysql/mysql.log, the following error messages are found:

  InnoDB: Fatal error: cannot allocate memory for the buffer pool
  [ERROR] Plugin 'InnoDB' init function returned error.
  [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
  [ERROR] mysqld: Out of memory (Needed 128917504 bytes)
  [ERROR] mysqld: Out of memory (Needed 96681984 bytes)

• The following error messages can be found in the log file /var/log/messages:

  kernel: Free swap = 0kB kernel: Total swap = 2097148kB ...
  kernel: Out of memory: Kill process 21770 (mysqld) score 17 or sacrifice child
  kernel: Killed process 21770 (mysqld) total-vm:2698392kB, anon-rss:24616kB, file-rss:0kB, shmem-rss:0kB

Resolution

Contact the server administrator or provider to increase the amount of the server's RAM and add swap if it is not enabled.