An apparatus in one embodiment comprises a multi-tier storage system having at least a front-end storage tier and a back-end storage tier. The storage system comprises a software-defined storage controller configured to implement the front-end storage tier as a unified memory cluster accessible to respective processors of a plurality of compute nodes. The software-defined storage controller is implemented using at least one processing device comprising a processor coupled to a memory. The unified memory cluster in some embodiments comprises portions of respective memories of at least a subset of the compute nodes. The portions of respective memories of at least a subset of the compute nodes may comprise, for example, respective flash memories of those compute nodes, or respective dynamic random access memories of those compute nodes. Other illustrative embodiments include systems, methods and processor-readable storage media.