1. Rotational Latency:
Rotational Latency is the amount of time it takes for the desired sector of a to rotate under the read-write heads of the disk drive.
In other words, the disk is divided into many circular tracks, and these tracks are further divided into blocks knows as sectors. So Rotational Latency is the time required by the read/write head to rotate to the requested sector from the current position.
The average rotational latency for a disk is half the amount of time it takes for the disk to make one revolution.
Rotational Latency time depends on the Rotational speed of a disk, measured in revolutions per minute(RPM).
2. Transfer Time:
The time it takes to transmit or move data from one place to another. It is the time interval between the start of the transfer and the completion of the transfer.
It varies on the rotational speed of the disk, the faster a disk rotates the faster it reads the data.
It not only depends on the rotational Speed of a disk. But also depends track and sector density. More the density, lesser is the time. Disk transfer time also depends on the amount to data to be transferred.
Difference between Rotational Latency and Transfer Time in Disk Scheduling :
|Rotational Latency Time||Transfer Time|
|Rotational Latency is the amount of time it takes for the desired sector of a to rotate under the read-write heads of the disk drive.||The time it takes to transmit or move data from one place to another. It is the time interval between the start of the transfer and the completion of the transfer.|
|Rotational Latency = (Angle between current position and the required sector) / (Rotational frequency).|| T = b/rN
T is Transfer time, b is number of bytes to be transfered, N is number of bytes on a track and r is rotational speed in Rps
| Maximum latency = 60/rpm
Average latency = 0.5*Latency Time
|It takes about 1 sec to transfer 30-60MB of data|
|It can be reduced if subsequent request belongs to adjacent sector.||It depends on the respective machine.|
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