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I have this situation where I need to calculate some things. I have following things with me,

Disk characteristics: number of cylinders; number of heads; # sectors/track; 10 ms average seek time; 3250 KBs transfer speed; 7200 RPM rotational speed

File characteristics: # bytes/file; record size 1244 bytes

I need to calculate the following things, number of records in file; number of bytes in one cylinder; Capacity of disk in bytes; Time to read one record randomly

I just need the formulae, like how to. Thanks for help!

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I just need the formulae, like how to. This is what professors and class notes are for. – jcolebrand♦ Jan 18 at 2:34
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Hi user1890338, as you can see from the question closure, as it stands this is off-topic here. It's not far off-topic but it is that side of the line (incidentally it is probably too broad too: we prefer one question at a time). The community is the best judge of these things, but I don't want you to be discouraged from asking future on-topic questions here: it seems likely that you will have them. I'd also like to draw your attention to the community response to the one answer you have received—it's normally our aim to be polite and respectful. – Jack Douglas♦ Jan 18 at 8:41

closed as off topic by ypercube, Mark Storey-Smith, Phil, jcolebrand♦ Jan 18 at 2:34

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1 Answer

up vote -4 down vote

We'd like to help, but unfortunately, there are so many variables here that it's a not really a question that is answerable in a meaningful way. Here's some questions you could ask back to the teacher.

What database platform? How much header information per page? per row? What's the database page size? Is the data in the database page compressed? If so what kind/level of compression is being used? How much space per file is taken up by system objects?

Also please tell your teacher that things like this will serve you 0 use in the real world. Bytes per page, per file sure. Number of pages (and records) per file sure. Figuring out the disk size based on the number of cylinders, nope totally useless. The computer will do that for you.

The time to read one record at random? That'll totally depend on where the head is at the time, where the record is at the time, if the record is in the buffer, if the record is in the disk cache, what else the disk is doing at the time (odds are it isn't handling just your request).

Sorry this answer isn't of much more help than giving you some smart ass responses for your teacher. But in reality you don't have enough information unless I'm going to start making a whole lot of assumptions.

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Why the downvotes? The information given was valid. If you're going to downvote it's customary to at least reply so we can learn from your reasoning. – Ali Razeghi Jan 18 at 1:27
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@AliRazeghi "Crap question" is not appropriate. "What database platform" is irrelevant. "crap like this will serve you 0 in the real world" is not appropriate and many would argue nonsense (this is a student with a study assignment). "The time to read one record at random" rant is wrong (RPM/latency/seek are used to calculate IOPS). A poorly researched and phrased question this may be but it did not warrant this response. – Mark Storey-Smith Jan 18 at 1:34
But Mark wouldn't you agree that 'the time to read one record at random' does actually depend on the needle position, outstanding requests, disk/app features such as read ahead as Denny stated? Needle position is one of the most important factors in latency on magnetic disks. At the end of the sentence Mr. Denny did state that there's a whole log of assumptions they'll have to make. I do agree the answer could have been given nicer though :) – Ali Razeghi Jan 18 at 1:41
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@AliRazeghi The question looks to be a simplistic study question, not a quantum mechanical study of which particular atom the read head is hovering over at a certain point in time. Can't you see that? Also, your first comment on this answer is inappropriate and you should consider deleting it. – Phil Jan 18 at 1:44
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mrdenny, my take for what it is worth: this answer is mostly useful but could easily be read as an aggressive response and especially for a first-time question that outweighs the good points. -1 but I'd reverse that if you make a minor edit to change the tone to be "firm but welcoming". – Jack Douglas♦ Jan 18 at 8:55
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