how do you calculate access time on a magnetic disk? explain with the help of a suitable example
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I didn't divide the average latency by 2. I divided the time it takes for one rotation of the platter by 2 ==> that's how you compute the average latency.
Remember, the disk is rotating. To access data there are fundamentally two things that have to happen [there are actually others, but they take so little time it's not worth considering]: (a) you have to move the heads to the right cylinder ("seek"), and then you have to wait until the platter rotates so the sector you want is under the heads so it can be read/written.
The seek time obviously varies depending on where the cylinder you want is relative to where the head is right now; but the published seek times are the average, so that's what you generally use. For more detailed calculations, you can check the manufacturer's site and get data for track-track seek; head settling time; max stroke time (time to move from the innermost cylinder to the outermost cylinder), etc. ==> but for your purposes just using the average seek time is fine.
The "latency" refers to how long you're waiting for the platter to rotate to the sector you want. On average, this will be 1/2 of the time it takes to rotate. It could be as little as zero ... i.e. the sector you want might just happen to be where it's at when the seek is completed, or it could be a full rotation ... if you "just missed" the sector when the seek finished. But statistically, it will be 1/2 of the rotation time.
Remember, the disk is rotating. To access data there are fundamentally two things that have to happen [there are actually others, but they take so little time it's not worth considering]: (a) you have to move the heads to the right cylinder ("seek"), and then you have to wait until the platter rotates so the sector you want is under the heads so it can be read/written.
The seek time obviously varies depending on where the cylinder you want is relative to where the head is right now; but the published seek times are the average, so that's what you generally use. For more detailed calculations, you can check the manufacturer's site and get data for track-track seek; head settling time; max stroke time (time to move from the innermost cylinder to the outermost cylinder), etc. ==> but for your purposes just using the average seek time is fine.
The "latency" refers to how long you're waiting for the platter to rotate to the sector you want. On average, this will be 1/2 of the time it takes to rotate. It could be as little as zero ... i.e. the sector you want might just happen to be where it's at when the seek is completed, or it could be a full rotation ... if you "just missed" the sector when the seek finished. But statistically, it will be 1/2 of the rotation time.
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