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Sheetanshu
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Data Transfer Rate

ISM Book Chapter -2 Page -32

"The Data Transfer rate drops while accessing data from zones close to the center of the platter. " WHY ?

Are we asuming that Head always resides on the landing zone .before fetching the data ?.

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4 Replies

Re: Data Transfer Rate

Hello Sheetanshu,

No, the heads are not residing on the landing zone, the heads will be flying at the 'glide height' when the drive is spinning.

Because of zone bit recording, the outer tracks contain more sectors (and therefore more data) than the inner tracks. Since the angular velocity (effectively the RPM) is a constant for all tracks, then the raw data transfer rate of the outer tracks is greater than that of the inner tracks (in one revolution you read more data from an outer track than you read from an inner track, but the time taken for that one revolution is the same for both the outer track and the inner track).

Very often, if a drive is formatted at a lower capacity than it's real capability (and therefore to be sold at a lower price), then it is likely that the outer tracks are the ones that are formatted.

I hope that helps and makes sense.

Best regards, Richard.

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Hen2
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Re: Data Transfer Rate

"Very often, if a drive is formatted at a lower capacity than it's real capability (and therefore to be sold at a lower price), then it is likely that the outer tracks are the ones that are formatted."

Interesting point, Richard. Thanks.

Lets compare Seagate disks drives Cheetah 15K.6 450GB 15K RPM and Barracuda ES.2 1TB 7,2K RPM. Both of them have the same number of platters and r/w heads (4/8) and the same areal density (165 Gbits/inch2). But these disks have different capacity (450GB/1TB) and avg. seek latency (r-w 3,4-3,9ms / 8,5-9,5ms). Does this mean they use platters of different diameters (smaller platters are less inertial, that’s good for high rotation speed), or Cheetah has only outer tracks formatted?

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Re: Data Transfer Rate

Hi,

I cannot comment on these two particular drives without doing more research.

Having said that, drives spinning at 15k RPM often have smaller platters (and therefore a lower storage capacity), than drives spinning at a lower RPM rate. My understanding is that as the RPM increases, and the raidus of the platter increases, then the physical stresses on the platter become more difficult to manage (there are also issues around flight mechanics, fluid mechanics, drag, and power consumption).

So, I would expect that the 15k RPM platter has a smaller radius than the 7200 RPM platter (as I said, I have not researched this any further).

The Barracuda disk itself comes in four different formats (1TB, 750GB, 500GB, 250GB). I would expect that this is where my formatting point is more relevant.

Anybody on the forum with a more  direct experience of these particular drives..... ?

Best regards, Richard.

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Hen2
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Re: Data Transfer Rate

This drives was given just as an example of two similar products sold at the same time by one vendor.

I am rather inclined to agree with you. It seems the main difference is diameter of platters. This is the main reason why FC drives have less avg. seek latency in comparison with SATA disks.

In return smaller platter allows using higher rotational speed. That led to avg. rotational latency decrease.

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