Plesk Debian 8 General error: 23 Out of resources when opening

On a Debian 8 server with Plesk control panel, on the Plesk login page, I got the following error message

500 Plesk\Exception\Database
DB query failed: SQLSTATE[HY000]: General error: 23 Out of resources when opening file '/tmp/#sql_353d_0.MAI' (Errcode: 24 "Too many open files"), query was: DESCRIBE `sessions`

This error is due to MySQL open_files_limit value is set low. To verify, run the command

SHOW VARIABLES LIKE 'open%';

You need to increase the limit.

On Debian 8, edit the file

vi /etc/mysql/my.cnf

Find

[mysqld]

Add below

open_files_limit = 102400

Restart MySQL

systemctl restart mysql

If you have other version of Operating System, see MariaDB Change Open Files Limit.

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