On 26/07/12 10:29, Davidlohr Bueso wrote:
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.
Thanks, yes I was able to deduce this from the source but I wasn't
confident I had it right.
Reading this confirms nicely what I thought was going on.
(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.
I've just seen the upcoming changes, removing the messages when
not in dos mode.
Cheers,
Davidlohr
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