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FreeBSD Boot Details

From: Werner Scheinast
Subject: FreeBSD Boot Details
Date: Tue, 12 Jun 2012 17:06:35 +0200 (CEST)
User-agent: Alpine 2.00 (BSF 1167 2008-08-23)

Hi all,

in the attempt to get a somewhat unusual FreeBSD installation booted by
GRUB, I got stuck. Before I describe this, I ask a general question and
would like either an acknowledgement or a correction:

FreeBSD has its own partition scheme that is used either for the full hard
disk ("dangerously dedicated") or nested in a primary partition of the MBR
partitioning scheme. (Let's not consider GPT here.)
The boot sector in the beginning contains stage 1 of the boot code, then
comes a "disklabel" without file system that includes some reserved space
for stage 2 of the boot loader, this is followed by a series of (sub-)
partitions a,b,d,... whose 'a' has stage 3 of the boot loader as a normal
file in its file system.
If you install GRUB here, it wants - it must! - follow the same scheme,
especially concerning stage 2. If there is no disklabel, GRUB can't be

My questions:
 Is this correct?
 Can I circumvent this, i.e. install GRUB to work like in Linux?

I made an experimental installation of FreeBSD in a primary partition
(slice) ada0s3. This slice contains a "naked" FreeBSD, without
disklabel and sub-partitions, just a file system directly in ada0s3.
I have it booted by means of a small dedicated /boot partition ada0s2a.
Now I copied the /boot branch directly into the local file system,
installed the GRUB port and wanted to make it bootable itself, without
ada0s2. But I can't install GRUB:

/boot# grub-install --root-directory=/boot/grub /dev/ada0s3
/usr/local/sbin/grub-setup: error: /dev/ada0,3 appears to contain a ufs2
filesystem which isn't known to reserve space for DOS-style boot.
Installing GRUB there could result in FILESYSTEM DESTRUCTION if valuable
data is overwritten by grub-setup (--skip-fs-probe disables this check,
use at your own risk).

>From this error message I derived what I wrote above.
So: can I cope with this?


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