Subject: Re: disk basics (i386)
To: None <>
From: Manuel Bouyer <>
List: netbsd-users
Date: 01/06/2002 15:57:05
On Sun, Jan 06, 2002 at 09:16:15AM +0000, Jim Breton wrote:
> Howdy, I just installed 1.5.2 on an x86 system with a ~405 MB IDE disk.
> The installer handled all the drive geometry for me so that was not an
> issue.
> This is not my first installation of NetBSD, but it was my first time
> _adding_ a disk to an already-installed BSD system, and I was muddling my
> way through it.  I seem to have learned a few things about disks in the
> process but a few questions remain.
> The IDE disk is wd0, the disk I added was a scsi disk, sd0.  At the end
> of this message I will paste a bunch of (slightly snipped) info to which
> I refer in my questions.
> The questions are:
> Why does the installer appear to use the last "DOS" partition on the
> drive, rather than the first?  (Partition 3 rather than 0.)

This is the default, but you can change it if you want.

> Why does the NetBSD geometry disagree with the BIOS geometry?  Is this
> to deal with BIOS limitations (with which I'm familiar, but I wasn't
> aware of _how_ OSes "worked-around" them)?

Yes, it is. NetBSD uses the real disk geometry (as reported by the disk),
the bios use a fake one to work around cylinders and sectots limits.

> When I fdisked sd0, I wasn't sure at first what to specify as the
> "size."  After a while I figured out it's the cyls * sec/cyl (- offset
> if you need one for an MBR IIUC).  But, I wasn't sure which idea of the
> geometry I should use -- the NetBSD one, or the BIOS one.  So to be on
> the safe side I went with the NetBSD geometry since it was a bit smaller
> in total sectors (better to guess on the small side than on the large
> side); however the installer seems to have gone with the BIOS geometry:
> 838*92-62=831234.  How do I know which one to use?

If you don't want to boot from this drive it doesn't really matter.
Otherwise you have to use the BIOS geometry.

Manuel Bouyer <>