Disk Performance Parameters

When the disk is in use, a drive motor spins it at high speed. Most drives rotate 60 to 150 times per second. To read or write, the head must be positioned at the desired track and the beginning of the desired sector on the track. The time it takes to position the head at the desired track is called “Seek Time“.

Once the track is selected the disk controller waits until the desired sector reaches the read/write head. The time it takes to reach the desired sector is called “Latency time” or “Rotational Delay“. The sum of seek time and rotational delay is the access time. When the desired sector reached the read/write head, then the real data transfer starts.

Seek Time:

The time required to reach the desired track by read/write head is the ‘seek time‘. The seek time consists of two key components. The internal startup time and the time is taken to traverse the cylinders. The linear formula for seeking time is:

T_s=m*n+s
where
T_s=Estimated seek time
n=Number of tracks traversed
m=Constant that depends on the disk drive
s=startup time

Rotational Delay: The time required to reach the desired sector by the read/write head is called Rotational Delay.

Transfer time: The transfer time depends on the rotation speed of the disk. The formula for the transfer time is:

T=B/R*N
T=Transfer rate
B=Number of bytes to be transferred
Number of bytes on track
R=Rotation speed in revolutions per second
Thus the average access time can be expressed as:

T_a=T_s+1/2R+B/RN
where T_s is the average seek time.