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N-Step-SCAN disk scheduling

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  • Last Updated : 10 May, 2020

Prerequisite – Disk Scheduling Algorithms

The input output requests that is coming for the disk is scheduled by operating system and that scheduling of disk is known as disk scheduling. Disk scheduling is important since multiple requests comes from processes for disk but only one disk assigned to process at a time. Seek time is one of the crucial parameter in operating system. Requests are linked in queues henceforth seek time gets increased due to which system becomes slow. Algorithm which is used for disk scheduling is known as Disk Scheduling Algorithm whose purpose is to reduce total seek time.

N-Step-SCAN Disk Scheduling :
N-Step-SCAN also called as N-Step-Look which is actually a Disk Scheduling Algorithm. It helps in determining motion of Disk’s arm and also helps in servicing read and write requests. It divides the request queue into sub queues of length N. By doing this it ensures that the service guarantee objective is achieved.

After this subsequent request are done they can not be allocated into N size sub queues since they are full by elevator algorithm. Therefore starvation is completely eliminated and the service within N requests is guaranteed.

Algorithm for N-Step-SCAN Disk Scheduling :

  1. A buffer is created for N requests.
  2. All the requests that are kept in this buffer is serviced in any specific wipe.
  3. During this time all the new incoming requests can not be added to this buffer, this new requests will be kept in a separate buffer.
  4. Now here comes the role of I/O (Input Output) scheduler because when these top N requests are serviced, I/O (Input Output) scheduler chooses next N requests and this process goes on and on.

By doing this N-Step-SCAN allows better throughput and its devoid of thrust.

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