Optimizing server placement for QoS requirements in hierarchical grid environments

This paper focuses on two problems related to QoS-aware 1/0 server placement in hierarchical Grid environments. Given a hierarchical network with requests from clients, the network latencies of links, constraints on servers' capabilities and the service quality requirement, the solution to the minimum server placement problem attempts to place the minimum number of servers that meet both the constrains on servers' capabilities and the service quality requirement. As our model considers both the different capabilities of servers and the network latencies, it is more general than similar works in the literatures. Instead of using a heuristic approach, we propose an optimal algorithm based on dynamic programming to solve the problem. We also consider the optimal service quality problem, which tries to place a given number of servers appropriately so that the maximum expected response time is minimized. We prove that an optimal server placement can be achieved by combining the dynamic programming algorithm with a binary search on the service quality requirement. The simulation results clearly show the improvement in the number of servers and the maximum expected response time.