The /etc/my.cnf.d/ and /etc/mysql/conf.d/ directories are special locations for the MariaDB configuration files. They are found on the MariaDB releases for Linux such as Debian, Ubuntu, Fedora, Red Hat, and CentOS.

We will only have one or the other of them, never both, and regardless of which one we have, their function is the same. The basic idea behind these directories is to allow the package manager (APT or YUM) to be able to install packages for MariaDB, which include additions to MariaDB's configuration without needing to edit or change the main my.cnf configuration file. It's easy to imagine the harm that would be caused if we installed a new plugin package and it overwrote a carefully crafted and tuned configuration file. With these special directories, the package manager can simply add a file to the appropriate directory and be done.

When the MariaDB server and the clients and utilities included with MariaDB start up, they first read the main my.cnf...