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Web server cluster has been widely used to increase the performance of Web server. Moreover, as the Web has become an important business service delivery infrastructure, the need for supporting quality of service (QoS) in Web service has become more strident recently. In this paper, we propose a solution for supporting QoS in Web server cluster called QWSC, and implement a prototype of QWSC. Our objective is to provide better services to high priority request classes without over-sacrificing low priority request classes. To achieve this objective, we equip Web server cluster with admission control, service differentiation, performance isolation, and content adaptation mechanisms. QWSC periodically repartitions servers so as to adapt to varying request resource demands. Realistic experiments show that QWSC satisfies the service level agreement targets for high priority request classes. In addition, the percentage of dropped requests for QWSC is very low, which mainly benefits from the content adaptation mechanism.