linux-kernel - Re: Disk geometry from /sys

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Message-ID: <38b2ab8a0804221311u12fd2209l1fe71c99779eb6fa@mail.gmail.com>
Date:	Tue, 22 Apr 2008 22:11:13 +0200
From:	"Francis Moreau" <francis.moro@...il.com>
To:	"Seewer Philippe" <philippe.seewer@....ch>
Cc:	"Mark Lord" <lkml@....ca>, linux-kernel@...r.kernel.org
Subject: Re: Disk geometry from /sys

On Fri, Apr 18, 2008 at 3:37 PM, Seewer Philippe <philippe.seewer@....ch> wrote:
>
>  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/
>

unfortunately this link doesn't work for now...

-- 
Francis
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