Want to protect your cyber security and still get fast solutions? Ask a secure question today.Go Premium

x
?
Solved

How to Calculate access time of Disk.....

Posted on 2013-06-08
9
Medium Priority
?
17,838 Views
Last Modified: 2013-07-15
Seek Time on a hard disk is 60 ms.It rotates at the rate of 12000 rpm.Each tract has 300 sectors.Calculate the access time of disk?
0
Comment
Question by:sarang8180
  • 4
  • 2
  • 2
  • +1
9 Comments
 
LVL 28

Expert Comment

by:MacroShadow
ID: 39232534
average seek time + average rotational delay + transfer time + controller overhead + queuing delay

See: http://www.csc.villanova.edu/~japaridz/8400/sld012.htm
0
 

Author Comment

by:sarang8180
ID: 39232542
Calculation is: Access time=Seek time+latency time

    =60 ms+(300/2)/12000*100

how 100 they have put here?Each time i have to divide 2 from number of sectors ie. 300
0
 
LVL 93

Expert Comment

by:nobus
ID: 39232589
where does that calculation come from??
0
What is SQL Server and how does it work?

The purpose of this paper is to provide you background on SQL Server. It’s your self-study guide for learning fundamentals. It includes both the history of SQL and its technical basics. Concepts and definitions will form the solid foundation of your future DBA expertise.

 

Author Comment

by:sarang8180
ID: 39232597
from my book..Another example..

Seek time of dsk is 20ms.It rotates at the rate of 6000 r.pm.Each track on this disk has 200 sectors. Calculate access time of disk.

Solution: (200/2 /6000*200)*60

     now please tell how above calculation formed?....
0
 

Author Comment

by:sarang8180
ID: 39232599
book name is dotcombooks4u.com
1
 
LVL 70

Expert Comment

by:Gary Case
ID: 39232818
The statement that "... Calculation is: Access time=Seek time+latency time " is correct, but the formula does not match that.

Average latency time is simply 1/2 the time it takes to rotate the platter ... i.e. on average you will get to the data quicker sometimes, a bit longer others, but will average 1/2 of the rotation.

The number of sectors/cylinder is irrelevant (and is in fact DIFFERENT on different sections of the disk).

So ... for the first example in your question ...

Average seek time = 60 ms

Disk rotates at 12,000 rpm, so that's 12000/60 = 200 rotations/second.   So it takes 1/200 th of second to rotate once, which is 5ms.     1/2 of that is 2.5ms, so the average latency is 2.5ms.

So the average access time is 60 + 2.5 = 62.5ms


In your 2nd example, the seek time is 20ms;  the average latency for a 6000rpm disk will be 5ms (since it takes 10ms to rotate once);  so the average access = 20ms + 5ms = 25ms
1
 

Author Comment

by:sarang8180
ID: 39232980
why have you divided avg. latency by 2?
0
 
LVL 70

Accepted Solution

by:
Gary Case earned 1500 total points
ID: 39233355
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.
1

Featured Post

Concerto Cloud for Software Providers & ISVs

Can Concerto Cloud Services help you focus on evolving your application offerings, while delivering the best cloud experience to your customers? From DevOps to revenue models and customer support, the answer is yes!

Learn how Concerto can help you.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Many businesses neglect disaster recovery and treat it as an after-thought. I can tell you first hand that data will be lost, hard drives die, servers will be hacked, and careless (or malicious) employees can ruin your data.
Compliance and data security require steps be taken to prevent unauthorized users from copying data.  Here's one method to prevent data theft via USB drives (and writable optical media).
This video Micro Tutorial explains how to clone a hard drive using a commercial software product for Windows systems called Casper from Future Systems Solutions (FSS). Cloning makes an exact, complete copy of one hard disk drive (HDD) onto another d…
This tutorial will walk an individual through the process of installing the necessary services and then configuring a Windows Server 2012 system as an iSCSI target. To install the necessary roles, go to Server Manager, and select Add Roles and Featu…
Suggested Courses
Course of the Month15 days, 21 hours left to enroll

580 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question