磁道复制——多媒体数据读取的磁盘寻道优化策略

Track Replica—The Strategy for Disk Seeking Optimization in Retrieving Multimedia Data

由于寻道是磁盘IO操作中的主要浪费之一,采用磁道复制可以减少寻道.针对单盘与盘阵情况,磁道复制可以有两种不同的方法:nway和ndway,并经推导可得出二者平均寻道距离的解析表达式.进行的模拟实验不仅证明了解析表达式的准确性,而且表明在相同的空间开销下,磁道复制策略比传统的以空间换取时间的策略如nwaystriping和Dwaymirror有更少的寻道距离.

Because of mechanical operations such as head seeking and rotating, disk device based storage is a main bottleneck of a multimedia server. People can make full use of disk rotation time with track-aligned request service. So disk seeking is the main waste in disk I/O. Since multimedia data is almost read-only, the density of magnetic media is increased at speed of 60% per year. In this paper a new strategy to tradeoff between space and disk seeking is provided： cylinder replication is provided. The main work of the paper is： first, present the placement of n-way cylinder replica in single disk and n-d-way cylinder replica in disk array; then the expressions of mathematic expectation of mean seeking distance of the two cylinder replica placement strategies are derived; finally, lots of simulations are carried out. The simulation results not only show that the expressions are correct, but also tell us that cylinder replica strategy works more better than the traditional tradeoffs such as n-way striping and D-way disk mirror.