sysinstall/fdisk drive geometry problem

Jerry McAllister jerrymc at
Wed Mar 16 06:58:46 PST 2005

> Hello
> I finally got another hdd for my home box, decided to do a fresh fbsd 
> install and I ended up with a disk problem :( :
> ide0: 40GB [FreeBSD wannabe (ufs2) 10GB, storage2 (fat32) 30GB]
> ide2: 40GB [WinXP (ntfs) 10GB, storage1 (fat32) 30GB]
> Now when I begin the installation, and go to the partitioning section, 
> fdisk complains about drive geometry and rearranges the drive. Here are 
> the settings:
> cyls/heads/sectors
> 19xxx/16/255 (BIOS)
> 7xxx/16/63 (dmesg "this is what fbsd complains to")
> 4xxx/255/63 (the new fdisk rearrangement)
> and FreeBSD's fdisk seems to get off on setting the head count to 255!!! 
> I tried setting the bios value and fdisk complains again, but for some 
> reason when I swap the heads and sectors values it does not. Same goes 
> for the dmesg values (drive geometry from the dmesg output).
> Since I have no experience with changing drive geometry, nor do I 
> understand the bloody thing, I have several issues:
> - Will I loose the data on 'storage2' if I change the drive geometry?
> - How should I set up the new drive geometry? What setting is 
> faster/better? more cyls or more sectors (since fdisk is set to use 255 
> heads)?
> - If I backup 'storage2', change drive geometry as fbsd/fdisk suggests 
> (or whatever), create a 10g slice and leave the other space 
> unpartitioned, will I be able to partition and format the drive to fat32 
> from WinXP?

Generally, I don't tink with drive geometry.  I just let the system take 
care of it.   I haven't done much with IDE - only a couple of systems - but 
it seemed to work the same on those.  The geometry values you see are 
mostly fiction and are there for some historical (hysterical??) reasons.
What fdisk sees will not match what is really on the disk.

There is a FAQ and lots of stuff in the archives on this.   Some of
the entries, especially the FAQ explain it fairly well - better than
I can.   But, the sum of it is just go ahead and try the new disk.
Don't change BIOS or anything about the gometry.  Ignore its whining.   
If it seems to work, then the system knows what it is doing.  If it 
doesn't, then take the details from what goes wrong and ask some 
more questions.   

In the meantime, don't do anything with your old disk, just to
be careful.

> That's about it... any examples would be great!
> thanks ahead!
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