Data storage is an integral part of IT infrastructures, where Quality of Service (QoS) differentiation amongst customers and their applications is essential for many. Achieving this objective in a production environment is nontrivial, because these environments are complex and dynamic. Numerous practical and engineering constraints render the task even more challenging. This paper presents SLED-2, a QoS differentiation solution that meets these challenges in offering effective protection to the performance of important workloads at the expense of less important workloads when needed. SLED-2 uses a customized feedback heuristic that rate-limits selected I/O streams. This approach is unique in that it accounts for a number of important practical considerations, including fine-grained controls, errors in storage systems models, and inexpensive and safe QoS management. SLED-2 has been implemented for the IBM DS8000 series storage servers and shown to be highly effective in a set of hostile and practical scenarios using test facilities for IBM storage products. © 2010 IEEE.