The scalability of spatial reuse based serial storage interfaces

Abstract

Due to the growing popularity of emerging applications such as digital libraries, Video-On Demand, distance learning, and Internet World-Wide Web, multimedia servers with a large capacity and high performance storage subsystem are in high demand. Serial storage interfaces are emerging technologies designed to improve the performance of such storage subsystems. They provide high bandwidth, fault tolerance, fair bandwidth sharing and long distance connection capability. All of these issues are critical in designing a scalable and high performance storage subsystem. Some of the serial storage interfaces provide the spatial reuse feature which allows multiple concurrent transmissions. That is, multiple hosts can access disks concurrently with full link bandwidth if their access paths are disjoint. Spatial reuse provides a way to build a storage subsystem whose aggregate bandwidth may be scaled up with the number of hosts. However, it is not clear how much the performance of a storage subsystem could be improved by the spatial reuse with different configurations and traffic scenarios. Both limitation and capability of this scalability need to be investigated. To understand their fundamental performance characteristics, we derive an analytic model for the serial storage interfaces with the spatial reuse feature. Based on this model, we investigate the maximum aggregate throughput from different system configurations and load distributions. We show how the number of disks needed to saturate a loop varies with different number of hosts and different load scenarios. We also show how the load balancing by uniformly distributing the load to all the disks on a loop may incur high overhead. This is because the accesses to far away disks need to go through many links and consume the bandwidth of each link it goes through. The results show the achievable throughput may be reduced by more than half in some cases.