Upcoming HPC clusters will feature hybrid memories and storage devices per compute node. In this work, we propose to use the MPI one-sided communication model and MPI windows as unique interface for programming memory and storage. We describe the design and implementation of MPI windows on storage, and present its benefits for out-of-core execution, parallel I/O and fault-tolerance. Using a modified STREAM micro-benchmark, we measure the sustained bandwidth of MPI windows on storage against MPI memory windows and observe that only a 10% performance penalty is incurred. When using parallel file systems such as Lustre, asymmetric performance is observed with a 10% performance penalty in reading operations and a 90% in writing operations. Nonetheless, experimental results of a Distributed Hash Table and the HACC I/O kernel mini-application show that the overall penalty of MPI windows on storage can be negligible in most cases on real-world applications.