Unable to login to Plesk after MySQL upgrade: unknown variable 'innodb_additional_mem_pool_size='

Symptoms

• The error message may be found at MySQL log file /var/log/mysqld.log or /var/log/mariadb/mariadb.log:
  

  CONFIG_TEXT: [ERROR] unknown variable 'innodb_additional_mem_pool_size='

• Unable to login to Plesk after MySQL upgrade to version 5.7.4+ or Mariadb 10.2:
  

  PLESK_INFO: ERROR: Zend_Db_Adapter_Exception: SQLSTATE[HY000] [2002] No such file or directory

• Unable to start MySQL service:
  

  # service mysqld start
  Starting mysqld: [FAILED]
  # service mariadb status
  ERROR! MariaDB is not running, but lock file (/var/lock/subsys/mysql) exists

Cause

After MySQL upgrade, the variable innodb_additional_mem_pool_size became obsolete.

Resolution

Note: if you don't have root access to Plesk via SSH, contact your hosting provider regarding the issue

1. Connect to the server using SSH.
2. Open /etc/my.cnf with any text editor and comment the innodb_additional_mem_pool_size variable using #:
   

   # grep innodb_additional_mem_pool_size /etc/my.cnf
   #innodb_additional_mem_pool_size=500k
   # grep innodb_additional_mem_pool_size /etc/my.cnf.d/server.cnf
   #innodb_additional_mem_pool_size=100M

3. Start MySQL or Mariadb service:
   

   # service mysqld start
   # systemctl restart mariadb.service