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 via the 

  systemctl status mariadb

   command:

  [Warning] Could not increase number of max_open_files to more than 16384 (request: 32335).

• MySQL/MariaDB performance is very slow.

Cause

Hard limit of the process exceeded the number set for open files limit (16384) in database server configuration files:

#Number of files limit
LimitNOFILE=16384

Resolution

1. Connect to the server via SSH

2. Find in which exact file the limit is set by executing the following commands:
   

   # egrep -r LimitNOFILE /etc/systemd/system/m*

   # egrep -r LimitNOFILE /lib/systemd/system/m*

   Note: You are looking for a line that does not have a # in its beginning. That line contains the configuration that is active.

3. Once you find in which file the line 

   LimitNOFILE

    is active, edit that file with your favorite command-line text editor and adjust the value:

   LimitNOFILE=32335

4. Save the changes and close the file

5. Reload all daemons on the server:

   # systemctl daemon-reload

6. Restart the  mariadb  or  mysql  service:

   # systemctl restart mariadb