Cost models have been following the life cycle of databases. In the first generation, they have been used by query optimizers, where the cost-based optimization paradigm has been developed and supported by most of important optimizers. The spectacular development of complex decision queries amplifies the interest of the physical design phase (PhD), where cost models are used to select the relevant optimization techniques such as indexes, materialized views, etc. Most of these cost models are usually developed for one storage device (usually disk) with a well identified storage model and ignore the interaction between the different components of databases: interaction between optimization techniques, interaction between queries, interaction between devices, etc. In this paper, we propose a generic cost model for the physical design that can be instantiated for each need. We contribute an ontology describing storage devices. Furthermore, we provide an instantiation of our meta model for two interdependent problems: query scheduling and buffer management. The evaluation results show the applicability of our model as well as its effectiveness.