From: Davidlohr Bueso <dave@gnu.org>
To: John Lane <util-linux@jelmail.com>
Cc: util-linux@vger.kernel.org
Subject: Re: fdisk geometry
Date: Thu, 26 Jul 2012 11:29:40 +0200 [thread overview]
Message-ID: <1343294980.2609.4.camel@offbook> (raw)
In-Reply-To: <50109E8D.2000702@jelmail.com>
On Thu, 2012-07-26 at 02:34 +0100, John Lane wrote:
> Hello, I'm in the middle of studying disk organisation to enhance my
> understanding in this area. I have a few questions about fdisk (trying
> to fill a gap in my knowledge if that's ok)...
use the source, Luke :)
>
> In the below output of fdisk it shows (correctly) a geometry of 224
> heads, 56 sectors/track. I'm unclear about where this information comes
> from. Normally a disk's geometry would be reported as 255 heads and 63
> sectors. I explicitly partitioned this disk using a 224/56 geometry, so
> the report is correct. I'd just like to understand:
>
> (a) where fdisk gets the information about the geometry from in this
> specific case?
fdisk gets the information from (i) user input, (ii) what the
kernel/bios thinks the geometry is, with the HDIO_GETGEO ioctl and (iii)
by inferring it from the partition table geometry.
> (b) when fdisk defaults to 255/63 is that because it's hard coded that
> way or is there another reason?
Correct.
>
> I understand why it's 255/63 by default but haven't been able to
> understand how the geometry is determined. I do know that it's all kind
> of moot these days anyway because of sector based addressing but that
> leads to one other question: given sector based addressing why does
> fdisk even bother displaying a geometry any more ? Doesn't it just serve
> to confuse?
That's because fdisk is/was mostly legacy code, we are currently
updating the program.
Cheers,
Davidlohr
>
> # fdisk /dev/sda
> Welcome to fdisk (util-linux 2.21.2).
>
> Changes will remain in memory only, until you decide to write them.
> Be careful before using the write command.
>
>
> Command (m for help): p
>
> Disk /dev/sda: 80.0 GB, 80026361856 bytes
> 224 heads, 56 sectors/track, 12460 cylinders, total 156301488 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disk identifier: 0xc4c4122e
>
> Device Boot Start End Blocks Id System
> /dev/sda1 * 56 501759 250852 83 Linux
> /dev/sda2 501760 156298239 77898240 8e Linux LVM
>
> Command (m for help): q
>
> Many thanks for your time,
> John Lane.
>
>
>
> --
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>
next prev parent reply other threads:[~2012-07-26 9:29 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-26 1:34 fdisk geometry John Lane
2012-07-26 9:29 ` Davidlohr Bueso [this message]
2012-07-26 9:57 ` Karel Zak
2012-07-26 10:29 ` Davidlohr Bueso
2012-07-26 10:38 ` Karel Zak
2012-07-28 0:20 ` John Lane
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