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Information storage and management (ISM) Impact
Given the following exercise in the course:
Exercise:
Consider an application that requires 1TB of storage
capacity and performs 4900 IOPS
–
Application I/O size is 4KB
As it is business critical application, response time must be within
acceptable range
Specification of available disk drive:
Drive capacity = 73 GB
15000 RPM
5 ms average seek time
40 MB/sec transfer rate
Calculate the number of disks required?
and its solution:
Calculate time required to perform one I/O
Seek time + (rotational delay)/speed in RPM + (block size/transfer rate)
Therefore, 5 ms + 0.5 /15000 + 4K/40MB = 7.1 msec
Calculate max. number of IOPS a disk can perform
1 / 7.1 ms = 140 IOPS
For acceptable response time disk controller utilization must be less
than 70%
Therefore, 140 X 0.7 = 98 IOPS
To meet application
Performance requirement we need 4900/98 i.e. 50 disk
Capacity requirement we need 1TB/ 73 GB i.e. 14 disk
Disk required = max (capacity, performance)
Can someone please explain how
5 ms + 0.5 /15000 + 4K/40MB = 7.1 ms
and how the 0.5 in the equation above was derived?
Anonymous
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May 20th, 2011 19:00
When I was preparing for the exam last year, I was confused like h3ll just like you.
I tried to figure out everything but failed consistently. .
Finally, I found an explanation online which made a lot of sense. Turned out that the explanation was not helpful at all missing many important factors. Forget what you are reading there... here is what you need to understand
The formula for 1 I/O is:
Avg. Seek Time + Avg. disk Latency(or rotational delay) + (BlockSize/Transfer Rate)
The issue here is how to find the Avg. disk Latency(or rotational delay)
Understanding this will help immensly:
Avg. Rotational Delay is 1/2 of the time it takes for ONE rotation.
We know that the RPM (Rotation Per minute) is 15000
so:
15000 Rotation in 60 seconds
=> 1 Rotation = 60/15000 seconds or (60/15000) x 1000 ms = 4 ms
so Avg. Rotational Delay is 1/2 of 4ms = 1/2 x 4 = 2 ms
Hope that helped
Anonymous
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May 23rd, 2011 12:00
Thanks AQ. I am very good at math and this problem threw me and got me mad and frustrated that I could not "reverse engineer" the numbers no matter how hard I tried. So, am I to assume that ALL of the problems that have to do with calculations are also going to be wrong?
Anonymous
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May 24th, 2011 20:00
GT:
I can't comment on the other problems involving calculation. I struggled with this one in particular hence remembered it so clearly...
Good luck with your first proven certification. I am sure you will do good.