• In my previous post titled “Disk Scheduling”, I tried to explain about the fundamentals of disk scheduling.
  • In this and upcoming posts, we will look some of the widely used disk scheduling algorithms.
  • In this post, I will start with the most simple and commonly used disk scheduling algorithm called First Come First Serve (FCFS).

  First Come First Serve (FCFS)

  • This is the simplest form of scheduling.
  • It is similar to the FIFO CPU scheduling algorithm.
  • According to FCFS algorithm, all requests will be serviced in the same order in which they arrive.
  • In other words, request will be serviced in the sequential manner.
  • It does not provide the fastest access and it is also not optimal.
  • It has higher seek time compare to other algorithms.
  • For example, A disk queue with request of input, output of two blocks on cylinders are as 98, 183, 37, 122, 14, 124, 65, 67.
  • If disk head is initially at cylinder 53 than it will move from 53 to 98 that is the end of the request of the block and to 183, 37 and so on.

FCFS Disk Scheduling

  • We can see total head movements and cylinders to be traveled from the request servicing queue.
  • If the request for cylinders is 37 and 14 to be serviced together before or after the request at 122 and 124, the total head movement could be decreased and performance could be improved.