In the era of big data and cloud, distributed key-value stores are increasingly used as building blocks of large-scale applications. Comparing to traditional relational databases, key-value stores are particularly compelling due to their low latency and excellent scalability. Many big companies, such as Facebook and Amazon, run multiple different applications and services on top of a single key-value store deployment to reduce the deployment and maintenance complexity as well as economic cost. However, every application has its performance requirement but most current key-value store systems are designed to serve every application request equally. This design works well when a single application accesses the key-value store, but it is not as good for the emerging concurrent multi-application scenario. In this paper, we present ZHT/Q, a flexible QoS (Quality of Service) fortified distributed key-value storage system for clouds and data centers. It improves the overall throughput by an order of magnitude and still satisfies different applications' latency requirements with QoS using dynamic and adaptive request batching mechanisms. The experiment results show that our new system delivers up to 28 times higher throughput than the base solution while more than 99% of requests' latency requirements are satisfied.