MDE has been successfully adopted in the production of software for several domains. As the models that need to be handled in MDE grow in scale, it becomes necessary to design scalable algorithms for model transformation (MT) as well as suitable frameworks for storing and retrieving models efficiently. One way to cope with scalability is to exploit the wide availability of distributed clusters in the Cloud for the parallel execution of MT. However, because of the dense interconnectivity of models and the complexity of transformation logic, the efficient use of these solutions in distributed model processing and persistence is not trivial. This paper exploits the high level of abstraction of an existing relational MT language, ATL, and the semantics of a distributed programming model, MapReduce, to build an ATL engine with implicitly distributed execution. The syntax of the language is not modified and no primitive for distribution is added. Efficient distribution of model elements is achieved thanks to a distributed persistence layer, specifically designed for relational MT. We demonstrate the effectiveness of our approach by making an implementation of our solution publicly available and using it to experimentally measure the speed-up of the transformation system while scaling to larger models and clusters.