Re: [Linux-NTFS-Dev] Geometry determination

From: Andries Brouwer
Date: Mon Mar 29 2004 - 09:42:20 EST

On Mon, Mar 29, 2004 at 03:48:01PM +0200, Szakacsits Szabolcs wrote:

> On Mon, 29 Mar 2004, Anton Altaparmakov wrote:
> > I have been experimenting a little with what Windows / Linux 2.4 / Linux
> > 2.6 think the geometry of a couple of HDs is and the results are not
> > very promising. )-:
> >
> > Using Linux 2.4, HDIO_GETGEO ioctl, I get the same Heads and Sectors per
> > Track as Windows on both HDs I tried it on. This is the good news.
> > I.e. at least on those two disks mkntfs as it stands now would create
> > Windows bootable partitions.
> >
> > The bad news is that Linux 2.4, HDIO_GETGEO ioctl returns wrong values
> You mean 2.6? That's what I'm saying also for a while. It's a known issue
> and people are complaining about it because kernel breaks things (e.g.
> they can't boot anymore and think Linux thrashed their systems).
> I don't know who is the kernel maintainer today but apparently nobody.
> Old maintainer, Andries Brouwer, only repeates that the geometry doesn't
> exists even if he was/is proved and pointed out to be wrong many times.
> The issue was discussed long, several times on linux-kernel without a
> satisfying solution.

Well, to be more precise, my point of view is that roughly speaking
(I know of some obscure exceptions) "geometry" does not play a role
for Linux. Something entirely different is the use of other operating

What I also claim is that Linux has no way of knowing what geometry
other operating systems will assign to a disk. Different operating
systems invent different translations.

Moreover, there is the matter of the correspondence of BIOS disk numbers
and Linux disk names. Especially when both IDE and SCSI disks are present,
and when more than two disks are present, it may be impossible to get
this correspondence right. Details depend on the type of BIOS.

So, I claim, until now the kernel attempted to do something, and it often
worked, and it often failed.

However, there is really no good reason why the kernel would try to guess
at the geometry other systems like to see. For the past years, the main
thing the kernel did was inferring the desired geometry from the partition
table. But fdisk or LILO or whatever can do that as well. So, really
no kernel help is needed.

I say this every few months and have not ever seen a good reason why
this code would have to be in the kernel.

(In other words: please, never use HDIO_GETGEO, the kernel does not
have an opinion anymore, and HDIO_GETGEO produces random results.
Do as *fdisk does: get a geometry, if one is needed, from the
partition table.)

(And, you say, what if I am partitioning an empty disk to be used
by DOS/Windows? There my stock answer "look at the partition table"
fails, but there is some EDD stuff that could be used instead.
Of course one should always use the FDISK of some operating system
to create partitions for that operating system.)

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