A system comprises a controller and a buffer accessible to the controller. The controller is configured to prefetch data from a storage medium in advance of such prefetch data being requested by a host device, some of such prefetch data being retrieved from the storage medium and stored in the buffer ready for access by the host device (“ready data”) and a remainder of such prefetch data in process of being retrieved from the storage medium but not yet stored in the buffer (“not ready data”). The controller alters a depth of the prefetch data based on a ratio of the ready data to a combined total of the ready data and not ready data.