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Am 30.09.2010 18:00, schrieb Peter Humphrey: |
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> On Thursday 30 September 2010 14:10:42 Florian Philipp wrote: |
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> |
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>> An HDD gets slower when you read the inner tracks. The angular |
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>> velocity is constant (5400 RPM) while the tangential velocity gets |
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>> lower with the radius. |
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> |
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> Are you telling us that the length of a stored bit is constant? I'd have |
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> thought it was the time needed to read or write a bit that was constant; |
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> otherwise the electronics would get extremely complex. In that case it's |
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> the angular velocity that counts, not the linear velocity, and it |
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> matters not which track your data are on. (If a block goes past the head |
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> twice as fast, it also occupies twice the space, so you're back where |
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> you were.) |
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Yes, the length of a block is constant. If the innermost "ring" (track) |
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contains 4 blocks, the next ring contains maybe 5 blocks.[1] |
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Put another way: If you could pack your bits more densely on innermost |
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tracks, why wouldn't you pack them that densely on the whole disk and |
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thereby increase the overall capacity? |
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|
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> |
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> That's the way it was with our imposing new 2MB disks in 1974, anyway. |
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> They occupied boxes four feet tall and six feet long, and had external |
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> air systems; I was one of those responsible for the maintenance; we were |
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> sent on a training course specifically for the disks. I can't remember |
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> who made them, but they were part of a Ferranti Argus 500 system at the |
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> then national grid control centre. |
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> |
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> Maybe technology has changed since then. |
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> |
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Well, we are talking about devices employing the GMR effect while also |
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doing error correction and remapping of defect sectors on-the-fly. I |
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guess a little lookup table from track number to time-per-block doesn't |
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add too much complexity. |
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You can easily test this if you have various partitions on your HDD. |
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Just compare dd throughput for your first partition versus your last one. |
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[1] The numbers are arbitrary. The number increases linearly. C = 2*pi*r |