With Group Sweeping
Scheduling (GSS ), requests are served in cycles, in roundrobin manner. To reduce disk arm movements, the set of n streams is divided into g
groups. Groups are served in fixed order. Individual streams within a group are served according to SCAN; therefore, it is not fixed at wich time or order individual streams within a group are served. In one cycle, a specific stream may be the first to be served; in another cycle, it may be the last in the same group.A smoothin buffer which is sized according to the cycle time and data rate of the stream assures continuity. If the SCAN scheduling strategy is applied to all streams of a cycle without any grouping, the playout of a stream cannot be started until the end of the cycle of its first retrieval ( where all requests are served once ) because the next service may be in the last slot of the following cycle. As the data must be buffered in GSS, the palyout can be started at the end of the group in which the first retrieval takes place. Whereas SCAN reuqires buffers for all streams, in GSS, the buffer can be reused for each group. Further optimizations of this scheme are proposed in. In this method, it is ensured that each stream is served once in each cycle. GSS is a trade-off between the optimization of buffer space and arm movements. To provide the requested guarantees for continuous media data, we propose here to introduce a “joint deadline” mechanism: we assign to each group of streams one deadline, the “joint deadline.” This deadline is specified as being the earliest one out of the deadlines of all streams in the respective group. Streams are grouped in such a way that all of them comprise similar deadlines.
