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 file and then any files that they find under the /etc/my.cnf.d/ or /etc/mysql/conf.d/ directories that have the extension .cnf because of a line at the end of the default configuration files. For example, MariaDB includes a plugin called feedback whose sole purpose is to send back anonymous statistical information to the MariaDB developers. They use this information to help guide future development efforts. It is disabled by default but can easily be enabled by adding feedback=on to a [mysqld] group of the MariaDB configuration file (we'll talk about configuration groups in the following section). We could add the required lines to our main my.cnf file or, better yet, we can create a file called feedback.cnf (MariaDB doesn't care what the actual filename is, apart from the .cnf extension) with the following content:

[mysqld]
feedback=on

All we have to do is put our feedback.cnf file in the /etc/my.cnf.d/ or /etc/mysql/conf.d/ directory and when we start or restart the server, the feedback.cnf file will be read and the plugin will be turned on. Doing this for a single plugin on a solitary MariaDB server may seem like too much work, but suppose we have 100 servers, and if we further assume that since the servers are doing different things, each of them has a slightly different my.cnf configuration file. Without using our small feedback.cnf file to turn on the feedback plugin on all of them, we would have to connect to each server in turn and manually add feedback=on to the [mysqld] group of the file. This would get tiresome and there is also a chance that we might make a mistake with one or several of the files that we edit, even if we try to automate the editing in some way. Copying a single file to each server that only does one thing (turning on the feedback plugin in our example) is much faster and much safer. And, if we have an automated deployment system in place, copying the file to every server can be almost instant.