Hard drives - capacity and speed

This is a discussion on Hard drives - capacity and speed within the Tech Board forums, part of the Community Boards category; I'm just wondering about something. Let's say there are two identical hard drives with only the capacity being different. One ...

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    Math wizard
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    Hard drives - capacity and speed

    I'm just wondering about something. Let's say there are two identical hard drives with only the capacity being different. One hard drive is 120 GB and the other is 500 GB. Would the hard drive of 500 GB capacity read data faster than the 120 GB hard drive since the data would otherwise be packed closer together? If so, how much faster would it be? Double? Quadruple?
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    Registered User whiteflags's Avatar
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    > Let's say there are two identical hard drives with only the capacity being different.
    Hard drives with different capacities are never identical. I also think that most disk heads are going to be pretty boring, and work on to the same degree.

    It's not really the capacity that matters as much. Most HDD platters are like 3.5" across, and better reading technologies or materials give you that extra few gigabytes. It also depends on the file system to a degree - because if I were using FAT 32 right now, I would only be able to use like half of my disk.

    There's also the caveat that you can't really improve one part of the machine cycle and get a huge performance boost. The disk might be able to load a lot of stuff into RAM for processing, but the processor would have to be pretty fast at calculating, too.

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    Math wizard
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    Well, assume you've got a top-of-the-line $5000 computer with the greatest and fastest of hardware. I forgot to mention that.
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    If you look at the technologies used to make the drive, it may make more sense as to what the difference could be. Speed is affected by several factors, here are a few of the general ones.

    Access time: Of the two drives you propose in hypothesis, it could be that the manufacturer rates these two the same - many 120G and 500G drives are rated around 9ms to about 8.5ms. This rating is not well standardized, either, and can be misleading, but it generally describes how quickly the read/write head can find a track. The farther away the target data is, the longer it takes for the head to get there.

    Latency: This is related to the spin rate of the drive. It measures the time it CAN take for the data you're interested in to swing around and travel under the head after it's positioned. The faster the spin, the shorter that delay. So, since you didn't mention drive speed, you'd have to compare that and thus the latency.

    These two factors combine (wait for the position of the head, the wait for the data to pass under the head) to roughly measure a largely undiscussed delay, for which I can't really say there is a formal name. Latency plus access delay equates to how quickly the drive can find the starting point of a piece of data on the drive. Depending on what is happening in the application and OS, this delay can be paramount.


    Rotation speed: Tied to latency, this is simply the RPM of the drive. Obviously the faster the drive spins, the faster data is traveling under the head, but there's more to it.


    Density: The larger capacity drives GENERALLY use higher density platters. There are 'jump' points related to each manufacturer's history. In the old days (say, late 80s) drives were very tall and had many platters. These days, drives are thin, and may have only 2 or 3 platters of much higher density recording. For WD, for example, the 120 and 160 G models have the same density, but the 250G shares the same density (I think) with the 320G models, but the 500G drives are even denser.

    Density and rotational speed combine to provide the sustained read speed of data. For any given speed, the higher the density, the more data that is passing under the head at any given moment. Said another way, for each rotation of the platter, a larger volume of data flies by.

    So, the higher the density, the faster the drive's data rate for any comparative rotational speed (same RPM).

    Caveat's apply to the point where you really have to compare the graph of performance on the drive (like HDTACH's output) to compare.

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    Registered User Frobozz's Avatar
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    Quote Originally Posted by JVene
    may have only 2 or 3 platters of much higher density recording
    Or as few as one in the case of this Seagate 160GB drive on NewEgg.

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    Malum in se abachler's Avatar
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    Ultimately it depend on what the performance bottleneck of your system is. For web servers, its almost always I/O, so they generally spring for the high end drives. RAID can help in this department, since the right RAID level can dramatically increase your read speeds (while poop-canning your write speeds). So it depends on what you are using yoru computer for. Personally, I use mine mostly for app development and games. Id rather spend $600 on a G-Force 8800 than $300 on a new harddrive. Id definately see the improvement from the G-Force, but the faster harddrive would only be noticeable when the app was loading.

    Most commercial HDD use 3 platters, of which only 5 sides are used. Low end drives usually have one head per side. Ive taken apart high end drives that had as many as 5 heads per side.
    Last edited by abachler; 06-01-2007 at 09:58 AM.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

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