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Affinity-Based Routing in Zoned Mirrored Disks
1 Computer Science Department, New Jersey Institute of Technology (NJIT), Newark, NJ 07102, USA, 2 Hitachi Global Storage Technologies, San Jose, CA, USA
Magnetic disk capacity is increasing rapidly, while disk positioning time is decreasing very slowly. Disks are a system bottleneck for applications generating accesses to random disk blocks, which do not benefit from caching and prefetching. Disk mirroring is a solution to this problem, since it provides twice the access bandwidth of a single disk in processing read requests and in addition tolerates single disk failures. Performance can be further improved by affinity-based routing (ABR), i.e. sending a request to a disk which will yield a reduced positioning time. We analytically evaluate a specific instance of ABR in zoned disks, which improves performance (reduces the seek time) by routing read requests intended for outer disk cylinders to one disk and those for inner disk cylinders to another. We evaluate several criteria for specifying the pivot point, which delineates inner and outer disk cylinders. This is accomplished by analyzing the performance of a single zoned disk with FCFS scheduling. Analytic and numerical results show that balancing disk utilizations achieves the same performance as methods that improve the response time. A transposed disk organization, which stores data in reversed directions on two disks, is also proposed and shown to achieve the best performance for fully loaded disks. We also discuss an adaptive approach to implement ABR, so that the pivot point is determined by the router based on the history of routed requests. The simple criterion of balancing disk loads can then be adopted easily in this context.
Received 6 August 2004. revised 7 January 2005.