Hi Hsing-Foo,
Have you set the property below in your `properties-local.xml`? If so, when saving, say, from Form Builder, it should either save in your MariaDB if everything else is setup properly, or you should get an error (generic in the UI, more detailed in the `orbeon.log`).
<property as="xs:string" name="oxf.fr.persistence.provider.*.*.*" value="mysql"/>
If with this property in place data is still saved in eXist, you might add it to the wrong place. Check your `properties-local.xml`, maybe add a syntax error there and reload Orbeon Forms to check you get an error (if not, you're not modifying the right file in the right location).
I hope this helps, and you'll let us know if you're getting any error you need help with.
-Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet