Fedora has decided to replace MySQL with MariaDB as have other Linux distributions while Arch Linux is now following suit. Many Linux distribution vendors are not satisfied with how Oracle is maintaining the open-source, community version of MySQL. With that in mind, many are turning to the MariaDB fork of the community project.
MariaDB is mostly a drop-in replacement to MySQL with many of its developers before former MySQL developers from the Sun Microsystems days.
For more information on the Arch Linux upgrade path from MySQL to MariaDB, see this ArchLinux.org posting.