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From: Andries Brouwer <aebr@win.tue.nl>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Walt H <waltabbyh@comcast.net>,
	arjanv@redhat.com,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	davzaffiro@tasking.nl
Subject: Re: [PATCH] pdcraid and weird IDE geometry
Date: Thu, 17 Jul 2003 17:34:13 +0200	[thread overview]
Message-ID: <20030717173413.A2393@pclin040.win.tue.nl> (raw)
In-Reply-To: <1058453918.9055.12.camel@dhcp22.swansea.linux.org.uk>; from alan@lxorguk.ukuu.org.uk on Thu, Jul 17, 2003 at 03:58:38PM +0100

On Thu, Jul 17, 2003 at 03:58:38PM +0100, Alan Cox wrote:
> On Iau, 2003-07-17 at 15:37, Walt H wrote:
> 
> > On the second drive, it's like this:
> > capacity = 80418240, head=255, sect = 63
> > lba = capacity / (head * sect) = 5005 int or 5005.80 float
> > lba = lba * (head * sect) = 80405325 int or 80418240.01 float
> > lba = lba - sect = 80405262 int or 80418177 float
> 
> Would fixed point solve this. Start from capacity <<= 16 and then
> do the maths. That would put lba in 65536ths of a sector which
> should have the same result as your float maths

Ach Alan - I have not seen these earlier posts, but float or
fixed point here is just nonsense.

The purpose of
	A = B/C;
	A *= C;
can only be to round B down to the largest multiple of C below it.
Using infinite precision just turns this into
	A = B;

He needs the first sector of the last cylinder, in a setup where
cylinders have size 16*63 or so, but the surrounding software
thinks that it is 255*63, a mistake.

I don't know anything about these RAIDs, but possibly it would
help to give boot parameters for this disk.

Maybe he is victim of the completely ridiculous
	drive->head = 255;
in ide-disk.c.
(We have drive->head, the number of physical heads, and
drive->bios_head, the translation presently used by the BIOS -
or at least that is the intention. It is a bug if the former
is larger than 16. The latter may well be 255.)

Andries


  parent reply	other threads:[~2003-07-17 15:20 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-07-17  2:26 Walt H
2003-07-17  8:49 ` Arjan van de Ven
2003-07-17 14:37   ` Walt H
2003-07-17 14:58     ` Alan Cox
2003-07-17 15:34       ` Jeff Garzik
2003-07-17 16:03         ` Andries Brouwer
2003-07-17 15:34       ` Andries Brouwer [this message]
2003-07-18  2:33         ` Walt H
2003-07-18  8:58           ` Andries Brouwer
2003-07-18 13:56             ` Walt H
2003-07-17 12:19 ` David Zaffiro
2003-07-18 14:51 Walt H

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