Plesk is not accessible due to stuck mysqld/mariadbd process: "Connection refused" or "No such file or directory"

Symptoms

• Plesk interface is not accessible with this message in a web browser:

  PLESK_INFO: Plesk\Exception\Database: DB query failed: SQLSTATE[HY000] [2002] Connection refused

  ---

  PLESK_INFO: DB query failed: SQLSTATE[HY000] [2002] No such file or directory

• MySQL/MariaDB service status reports this error:

  # service mysql status
  ...
  Active: deactivating (stop-sigterm) (Result: exit-code) since Thu 2024-06-20 06:01:50 EEST; 36min ago
  ...
  mariadbd[561691]: [ERROR] InnoDB: Unable to lock ./ibdata1 error: 11
  mariadbd[561691]: [Note] InnoDB: Check that you do not already have another mariadbd process using the same InnoDB data or log files.

  ---

  # systemctl status mariadb.service
  ● mariadb.service - MariaDB 10.3 database server
  Loaded: loaded (/usr/lib/systemd/system/mariadb.service; enabled; vendor preset: disabled)
  Active: failed (Result: exit-code)
  .....
  plesk.example.com systemd[1]: mariadb.service: Found left-over process 3037766 (mysqld) in control group while starting unit. Ignoring.
  plesk.example.com systemd[1]: This usually indicates unclean termination of a previous run, or service implementation deficiencies.
  plesk.example.com systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
  plesk.example.com systemd[1]: mariadb.service: Failed with result 'exit-code'.
  plesk.example.com systemd[1]: Failed to start MariaDB 10.3 database server.

Cause

Stuck mysqld process:

# ps auxfffwww | grep mysql
mysql 2222309 20.6 9.3 17574576 6182096 ? Sl Apr25 12999:45 /usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid

or mariadbd process:

# ps aux | grep mariadb
mysql 456786 86.6 3.9 9417400 2557260 ? Ssl Jun19 826:49 /usr/sbin/mariadbd

Resolution

1. Connect to the Plesk server via SSH.

2. Find the PID of the stuck MySQL/MariaDB service:

   # ps aux | grep -E 'mysql|mariadb' | grep -v 'grep'
   mysql 456786 86.6 3.9 9417400 2557260 ? Ssl Jun19 826:49 /usr/sbin/mariadbd

3. Using the process PID, kill the stuck process:

   # kill -9 456786

4. Start the MySQL/MariaDB service:

   # service mysql start

   OR

   # service mariadb start