lkml.org 
[lkml]   [2008]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 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/


\
 
 \ /
  Last update: 2008-04-18 15:41    [W:0.110 / U:0.648 seconds]
©2003-2014 Jasper Spaans. Advertise on this site