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Subject: AW: Need to force IDE geometry
Date: Sat, 16 Feb 2002 10:13:20 +0100
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Thread-Topic: Need to force IDE geometry
Thread-Index: AcG2yi0vJFzBy0/fTj6sO91tYdMYTg==
From: "Martin Bene" <martin.bene@icomedias.com>
To: <Andries.Brouwer@cwi.nl>, <john@mwk.co.nz>, <linux-kernel@vger.kernel.org>
Cc: <andre@linux-ide.org>, <hugo@firstlinux.net>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Andries,

> Your question is based on your assumptions about geometry
> and LBA. But your assumptions are incorrect, and therefore
> your questions do not make sense. Please tell what you do
> and what error messages you get.

For some reasons the c/h/s settings reported for LBA disks depend on ide device number: /hda uses 255 heads, 63 sectors while /dev/hdc and above use 16 head, 63 sectors.

hda: 150136560 sectors (76870 MB) w/1916KiB Cache, CHS=9345/255/63, UDMA(33)
hdb: 150136560 sectors (76870 MB) w/1916KiB Cache, CHS=148945/16/63, UDMA(33)
hdc: 150136560 sectors (76870 MB) w/1916KiB Cache, CHS=148945/16/63, UDMA(33)

As you can see, this means you end up with different reported drive geometries for identical disks. Esp. if you want to use software raid this is a major nuisance. The usual workaround is to change head/cylinder settings when first partitioning the drive and let linux change the geometry during partition table check.

Partition check:
 hda: hda1 hda2 < hda5 hda6 > hda3
 hdb: [PTBL] [9345/255/63] hdb1 hdb2 < hdb5 hdb6 > hdb3
 hdc: [PTBL] [9345/255/63] hdc1 hdc2 < hdc5 hdc6 > hdc3

While this works, it's quite unintuitive and confusing; correct behaviour would be to treat all disks identicaly regardless of device number.

Bye, Martin
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