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Old 16th June 2011
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rocket357 rocket357 is offline
Real Name: Jonathon
Wannabe OpenBSD porter
 
Join Date: Jun 2010
Location: 127.0.0.1
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Sharris, forget the noise and politics. Get a hard drive. Run zcav on it. Plot it with gnuplot.

Then tell me, where are the lowest logical addresses? They're on the fastest part of the disk, right? The highest logical addresses? The slowest, right?

The outer edge of the disk is moving fastest with respect to the read head of the drive. It's just simple math: Say the drive is spinning at 7200 RPM. That's 120 revolutions per second. Let's make the math simple...let's say it's a 3.5" drive, and the disk platter itself is 3" in diameter. On the innermost track (let's say it's 1" from the center), the amount of data the read head gets the opportunity to "see" is:

distance/revolution = 2*radius(pi)

At 1" from the center (i.e. the innermost track), the distance the read head "moves" (with respect to the platter) is 2*1*3.14, or 6.28 inches per revolution. At 1.5" from the center (i.e. the outermost track), the distance the read head "moves" (with respect to the platter) is 2*1.5*3.14, or 9.42 inches per revolution. There's nothing magical about it...it's simple math.

Now, let's say each inch of platter contains 512 bytes of data (this is an intentional approximation...in reality the density changes based on where the track is within each "zone"...the start of a zone (outer edge) will be less data-dense and the end of a zone (inner edge) will be more data-dense...but each *zone* (i.e. group of tracks) has approximately the same "bytes per inch", so-to-speak). This means that the amount of data that can be read each second = (revolutions/sec)*(distance/revolution)*(bytes/inch). At 1.5" from center, this equates to 120*9.42*512 bytes = 578764.8 bytes per second, or approximately 0.55 MB/sec. At 1" from center, this equates to 385843.2, or approximately 0.37 MB/sec. Again, nothing magical about it...it's simple math...the OUTER tracks are faster than the INNER tracks. This happens because the speed the drive rotates at is constant (whereas a CD drive alters how fast it spins to compensate for this nonsense).

It's true that the convention is not standard, but the argument is that the OUTERMOST edge of the platter is the BEGINNING of the disk, and the INNERMOST edge of the platter is the END of the disk. It's not standard, but everyone seems to do it that way. If you find a hard drive that defies this convention, please let me know!

I think a lot of the frustration in this thread is that you hang on to this idea that the innermost track is the start. Sure, it's easier to make a pizza from the inside out, but we aren't talking pizza here.
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Last edited by rocket357; 16th June 2011 at 01:44 AM.
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