Re: [gentoo-user] Normal disk speed?

[Florian Philipp <lists@f_philipp.fastmail.net>]

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Am 30.09.2010 18:00, schrieb Peter Humphrey:
> On Thursday 30 September 2010 14:10:42 Florian Philipp wrote:
> 
>> An HDD gets slower when you read the inner tracks.  The angular
>> velocity is constant (5400 RPM) while the tangential velocity gets
>> lower with the radius.
> 
> Are you telling us that the length of a stored bit is constant? I'd have 
> thought it was the time needed to read or write a bit that was constant; 
> otherwise the electronics would get extremely complex. In that case it's 
> the angular velocity that counts, not the linear velocity, and it 
> matters not which track your data are on. (If a block goes past the head 
> twice as fast, it also occupies twice the space, so you're back where 
> you were.)

Yes, the length of a block is constant. If the innermost "ring" (track)
contains 4 blocks, the next ring contains maybe 5 blocks.[1]

Put another way: If you could pack your bits more densely on innermost
tracks, why wouldn't you pack them that densely on the whole disk and
thereby increase the overall capacity?

> 
> That's the way it was with our imposing new 2MB disks in 1974, anyway. 
> They occupied boxes four feet tall and six feet long, and had external 
> air systems; I was one of those responsible for the maintenance; we were 
> sent on a training course specifically for the disks. I can't remember 
> who made them, but they were part of a Ferranti Argus 500 system at the 
> then national grid control centre.
> 
> Maybe technology has changed since then.
> 

Well, we are talking about devices employing the GMR effect while also
doing error correction and remapping of defect sectors on-the-fly. I
guess a little lookup table from track number to time-per-block doesn't
add too much complexity.

You can easily test this if you have various partitions on your HDD.
Just compare dd throughput for your first partition versus your last one.

[1] The numbers are arbitrary. The number increases linearly. C = 2*pi*r


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