MySQL/MariaDB error on the server with Plesk: Could not increase number of max_open_files to more than 16384

Symptoms

• The following error can be seen when checking MySQL/MariaDB status:

  # systemctl status mariadb
  MySQL error: Could not increase number of max_open_files to more than 16384 (request: 32335).

• MySQL/MariaDB performance is slow.

Cause

Hard limit of the process exceeded the number set for open files limit (16384) in /etc/systemd/system/mysqld.service:

#Number of files limit
LimitNOFILE=16384

Resolution

1. Connect to the server via SSH.

2. Edit file /etc/systemd/system/mysqld.service.

   Note: Depending on the OS, this can be found at /lib/systemd/system/mariadb.service instead.

3. Change the line LimitNOFILE as follows:

   LimitNOFILE=32335

4. Reload daemons:

   # systemctl daemon-reload

5. Restart mariadb service:

   # systemctl restart mariadb