Question

Suppose that a 10 MB file is stored on a disk on track number 50 in consecutive sectors. The disk arm is currently situated over track number 100. Assume that moving the arm from one cylinder to the next takes about 1 ms and it takes about 5 ms for the sector where the beginning of the file is stored to rotate under the head. Also assume that reading occurs at a rate of 100 MB/s.
How long will it take to retrieve the file from the disk?

Answer 1

The cylinder is a set of tracks one from each platter or surface. So, the number of cylinders is equal to the number of tracks. We are given that the time to move from one cylinder to another takes 1ms, this implies that the time to move from one track to another is also 1ms. The disk arm is currently situated over track number 100 but the data we need to retrieve is at track number 50.

The time taken to move from track number 100 to 50: 50ms.

A track is divided into sectors where data is stored.

The initial position of read/write head is not given so Let's assume that initially, the sector where the beginning of the file is stored is not under the read/write head. Time taken by sector where the beginning of the file is stored to rotate under the head: 5ms

Time to read the file: 10MB/(100MB/s) = 0.1s = 100ms

so, the time required to retrieve the file from the disk = (time taken to move from track number 100 to 50) + (time taken to move the sector where the beginning of the data is stored under the head) + (time taken to read the data)

= 50ms + 5ms + 100ms

= 155ms