Introduction
The my.cnf open_files_limit setting may not be taken as a variable and needs to be set via systemd on CentOS 7.
Procedure
Create the file
/etc/systemd/system/mariadb.service.d/limits.conf
and paste the following into that file
[Service]
LimitNOFILE=40000
Then run the following to load that change and restart MairaDB.
systemctl daemon-reload
/scripts/restartsrv_mysql