Cannot start MariaDB on Plesk server: the directory /var/lib/mysql is not empty, so initialization cannot be done

Symptoms

• MariaDB fails to start:

  # systemctl status mariadb

  <..>
  Database MariaDB is not initialized, but the directory /var/lib/mysql is not empty, so initialization cannot be done.

• /var/lib/mysql does not have mysql directory in it:

  # ls -l /var/lib/mysql | grep mysql
  # (empty output)

Cause

MariaDB set up is corrupted or incomplete.

Resolution

1. Connect to the server via SSH

2. Stop MariaDB:

   # systemctl stop mariadb

3. Move all content out of /var/lib/mysql directory:

   # mkdir /var/lib/mysql_bk && mv /var/lib/mysql/* /var/lib/mysql_bk

4. Start and then stop MariaDB:

   # systemctl start mariadb
   # systemctl stop mariadb

5. Copy contents from backed-up directory to /var/lib/mysql:

   # cp -a /var/lib/mysql_bk/* /var/lib/mysql

6. Open the my.cnf file with a text editor:

   1. on CentOS/RHEL-based distributions

      # vi /etc/my.cnf

   2. on Debian/Ubuntu-based distributions

      # vi /etc/mysql/my.cnf

7. Add the skip-grant-tables directive under the [mysqld] section:

   [mysqld]
   skip-grant-tables
   bind-address = ::

8. Restart MariaDB and restore the mysql database from daily dump:

   # zcat /var/lib/psa/dumps/mysql.daily.dump.0.gz | sed -n '/-- Current Database: mysql/,/-- Current Database:*/p' | plesk db

9. Restore the admin user by following the instructions from the article "Unable to access Plesk UI or its database on Linux: Access denied for user 'admin'@'localhost' (using password: YES)";
10. Remove the skip-grant-tables directive under the [mysqld] section of the my.cnf file.

11. Restart MariaDB:

    # systemctl restart mysql && systemctl restart mariadb