Advanced Database Management System - Tutorials and Notes: find the disk capacity, rotational latency, and number of cylinders

## Saturday, 10 February 2018

### Find the disk capacity, rotational latency, and number of cylinders, how to calculate the disk capacity, how to calculate the number of cylinders if number of tracks are known, how to compute the rotational latency

Question:
Consider a disk with a sector size of 512 bytes, 1000 tracks per surface, 25 sectors per track, 5 double-sided platters, and average seek time of 10 msec. Choose the correct answer for the following;
(i) capacity of tracks in bytes,
(ii) number of cylinders the disk has,
(iii) the average rotational latency if disk rotates at 5400 rpm.
(a) 12.5 K, 1000, 5.5 msec
(b) 25 K, 2000, 11 msec
(c) 12.5 K, 1000, 11 msec
(d) 25 K, 1000, 100 msec

(a) 12.5 K, 1000, 5.5 msec

Given,
Sector size (minimum storage unit of a hard drive) – 512 bytes
Number of double sided disk platters - 5
(One can read/write on both sides in a Double sided platters unlike DVD for example)
Number of tracks per each side of each disk platter – 1000
Number of sectors per track – 25
Average seek time – 10 msec
The time needed for moving head of a disk drive from one track to another is the seek time. The average of many seeks is average seek time.
(i) Capacity of track can be calculated as follows;
Capacity of track = bytes/track = bytes/sector * sectors/track
= 512 bytes/sector * 25 sectors/track = 12800 bytes
= 12800/1024 KB = 12.5 KB

(ii) The number of cylinders is the same as the number of tracks on each platter.
Number of cylinders = Number of tracks = 1000

(iii) Maximum rotational delay can be calculated as follows;
Maximum rotational delay = (1/rpm) * 60
= (1/5400) * 60 = 0.011 seconds or 11 msec
Average rotational latency is half of the rotation time, ie., 5.5 msec