linux-kernel - Re: Disk geometry from /sys

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Date:	Fri, 18 Apr 2008 15:37:13 +0200
From:	Seewer Philippe <philippe.seewer@....ch>
To:	Mark Lord <lkml@....ca>
CC:	Francis Moreau <francis.moro@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Disk geometry from /sys


Mark Lord wrote:
> That can sound a bit misleading.  The complete story, for ATA/SATA drives,
> is that the disk has two geometries:  an internal physical one, with a 
> fixed number of heads and cylinders, but variable sectors/track
> (which normally varies by cylinder zone).
> 
> Software *never* sees or knows about that geometry, so ignore it.
> 
> The second geometry, is the one that the drive reports to software
> as its "native" geometry.  This is what you see from "hdparm -I"
> and friends, and this geometry is what has to be used by software
> when using cylinder/head/sector (CHS) addressing for I/O operations.
> The hardware interface has a limit of 4-bits for the head value,
> so the maximum number of heads can never be more than 16.
> 
> Nobody uses CHS addressing for I/O operations, at least not on
> any hardware newer than at least ten years old, so this geometry
> is also unimportant for most uses.
> 
> That's what the drive knows about.
> 
> Software, for compatibility with the MS-DOS partition table scheme,
> sometimes uses a "logical" geometry, where we "pretend" that a drive
> can have up to 255 heads, which then allows more of the disk to be
> described within the limitations of the partition table data layout.
> That's where one frequently sees "255 heads", even though the drive
> underneath uses 16 at the interface level, and probably as only 2
> or 4 real heads inside the shell.

Aye. Though I prefer the term virtual geometry. But thats cosmetics. 
Sorry for beeing unclear, and many thanks for untangling my post.

If anyones interested in even more Details about C/H/S adressing and so 
on, there's a very good document about that to be found here:
http://www.mossywell.com/boot-sequence/
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