MySQL is the database of choice for most OpenStack components (Ceilometer is a notable exception). If you start with a small deployment, it will probably run like a charm. But as soon as the dataset grows, you will suddenly face several challenges. We will write a series of blog posts explaining the issues you may hit and how to overcome them.
Where is MySQL used in OpenStack?
Have a look at the logical diagram of OpenStack below (click the image for a larger view).