Re: fdisk(8) no longer capable of altering geometry

On 2005.11.12 01:19:52 +0100, Dario Freni wrote:
> My little follow-up to this issue. This bug is quite critical on 
> products using bsdinstaller such as FreeSBIE or pfSense. I heard rumours 
> that PC-BSD also encounter this problem and had to workaround it by 
> sysinstall.
> 
> Can somebody please take a look at it?

Just curious... why do you need to alter the geometry?  I never once
had a problem just ignoring the fact that fdisk/sysinstall warned
about geometry...

> Chris Pressey wrote:
> >[this is a follow-up to / correction of my post to geom_at_ a few days ago,
> >to which there was no reply]
> >
> >Hello,
> >
> >It appears that fdisk(8) is no longer capable of altering the geometry
> >of a disk.  (By which I mean, the kernel's idea of the BIOS'es idea of
> >the geometry, of course.)  I'd find it reassuring to know whether or not
> >anyone else is seeing the same behaviour, before I go the official route
> >and file a PR.
> >
> >Initially I thought that this failure case was only for uninitialized
> >disks, but I have tried further tests and I can't get fdisk(8) to change
> >the geometry in any of the cases.  These cases are:
> >
> >a. uninitialized, totally blank disk (dd if=/dev/zero of=/dev/ad1 ...)
> >b. initialized disk with FreeBSD (or any other OS) installed on it
> >c. same as b, but with its root partition mounted on /mnt
> >d. the disk containing the currently booted FreeBSD system
> >   (mounted on /, of course)
> >
> >In each of these cases, I tried a sequence like the following:
> >
> >  fdisk -BI ad1
> >  fdisk -u ad1
> >  <when asked if I want to change "our idea of what the BIOS thinks",
> >   say yes, and plug in different but compatible values for cylinders,
> >   heads, and sectors/track>
> >  <when asked if I want to write the new partition table, say yes>
> >  fdisk ad1
> >  <observe that the geometry hasn't changed>
> >
> >The behaviour I see is, in summary:
> >
> >a & b: fdisk issues the warning "fdisk: Geom not found" which presumably
> >       refers to the fact that there is no GEOM MBR provider for that
> >       disk. It then falls back to the legacy behaviour of raw-writing
> >       the partition table into sector zero of the disk.  This does not,
> >       however, trigger an update of the kernel's idea of the geometry.
> >
> >c & d: no "Geom not found" warning, but no change in geometry either.
> >
> >I don't see this behaviour on DragonFly; cases a and b work as you would
> >logically expect (as they worked in 4.x, AFAIR, but I have not yet
> >tested this) where the geometry does get changed, and subsequent runs of
> >fdisk report the changed geometry.
> >
> >In cases c & d, the behaviour is the same as FreeBSD - nothing changes. 
> >This is not too surprising, since the disk _is_ in use - but an error
> >message would probably make more sense.
> >
> >This bug is one of the few remaining things standing in the way of
> >porting the BSD Installer to FreeBSD.  Without some way of altering the
> >the geometry, it can't install onto a system whose BIOS misreports the
> >disk geometry.
> >
> >My analysis of the problem can be found in my previous post to geom_at_,
> >but to sum it up: I think fdisk needs to inform GEOM somehow that the
> >geometry should be changed.  It might need to trigger the creation of a
> >GEOM MBR provider for the disk before it does so; but I'm not certain of
> >any of this, since my knowledge of GEOM is slim at best.
> >
> >Hopefully someone more familiar with GEOM and such under -CURRENT is
> >listening and can shed more light on this problem and/or provide a
> >workaround and/or explain how I'm wrong and show me the right way to do
> >what I'm trying to do (change geometry) in -CURRENT.
> >
> >Thanks for your time,

-- 
Simon L. Nielsen