C-SCAN Scheduling, Eschenbach Scheme and Rotational Optimization


C-SCAN Scheduling

Disk arm moves unidirectionally across the disk surface toward the inner track. When there are no more requests for service ahead of the arm, it jumps back to service the request nearest the outer track and proceeds inward again.

  • It eliminates the discrimination of earlier strategies against the innermost and outermost cylinders.
  • In this method, the disk arm moves from the outer cylinder to the inner cylinder, servicing requests on a shortest-seek basis.
  • When the arm has completed its inward sweep, it jumps to the request nearest the outermost cylinder and then resumes its inward sweep processing  requests.
  • The request arrived during current sweep are serviced on the next sweep.
  • Thus C-SCAN completely eliminates the discrimination against requests for the innermost or outermost cylinders. It has high throughput, less mean response time, very less variance in response times.

Eschenbach Scheme

Disk arm movement is circular as in C-SCAN, but with several important exceptions. Every cylinder is serviced for exactly one full track of information whether or not there is a request for that cylinder. Requests are reordered for service within a cylinder to take advantage of rotational position, but if two requests overlap sector positions within a cylinder, only one is serviced for the current sweep of the disk arm.

  • This strategy is designed for handling heavy loads. This is the only scheme which not only attempts to optimize seek time but also rotational delay.

Rotational Optimization

Under heavy load conditions, it becomes necessary to refer a particular cylinder several times, during these conditions it is necessary to consider the rotational optimization as well as seek optimization.

  • Shortest Latency Time Fist strategy is designed for rotational optimization.
  • When the disk arm arrives at a particular cylinder, there may be many requests pending on the various tracks of that cylinder.
  • The SLTF strategy examines all these requests and services the one with the shortest rotational delay first.
  • It is referred to as sector queueing since requests are queued by sector position around the disk and the nearest sectors are serviced first.

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