MariaDB is a fork of MySQL that started in 2009. It has replaced MySQL as the default database for Debian with the release of Debian 9 Stretch on June 17th, 2017.
MariaDB closely follows a free an open source development model. It is developed in the open on GitHub by an independent foundation. MySQL is developed by a private company known for its troubled relationship with the open source community. Its code is developed privately and published only with each release.
Many huge open source projects switched to MariaDB years ago. Have a look at https://github.com/MariaDB/server to review its growing adoption and rising popularity. Red Hat made MariaDB its default engine in 2015 with the release of RHEL 7.
As MariaDB 10.0 and 10.1 are fully compatible and available on all our targeted operating systems, we suggest to concentrate our testing efforts on MariaDB and phase out MySQL server.
Having only one database backend to support and test will allow us to concentrate developer time on resolving issues and expanding functionalities.
If some of our dear users cannot switch to MariaDB they should come forward now and contribute or fund testing and debugging for their required database backend. MySQL seems to work well with the next release 0.11 on Ubuntu 16.04 but we noticed problems with 17.04. MariaDB has been successfully tested for all supported OS. Switching from MySQL to MariaDB on Ubuntu also went fine, it was not necessary to dump the database (but you must perform backups before any risky operations and changing the database is obviously one of them).
Unless the community comes forward with the required resources to maintain both RDBMS engines, the switch to MariaDB only support will be effective with the next release, version 0.11.