A hierarchical data-storage system has a volatile storage medium, a first non-volatile storage medium, and a controller including a ranking engine tracking data writes to each of the memory mediums. Each medium is associated with a pre-set capacity threshold, and the controller, upon the volatile medium reaching its pre-set threshold, identifies one or more blocks of data as least-frequently written to the volatile medium, copies the data in those blocks to the non-volatile medium, and marks those blocks as available for new data writes, and the controller, upon the non-volatile medium reaching its pre-set threshold, identifies one or more blocks of data as least-frequently written to the non-volatile medium, and marks those blocks as available for new data writes from the volatile medium.