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Re: FreeBSD Boot Details

From: Jordan Uggla
Subject: Re: FreeBSD Boot Details
Date: Mon, 18 Jun 2012 12:14:24 -0700

On Sun, Jun 17, 2012 at 9:25 AM, Werner Scheinast <address@hidden> wrote:
> Am Freitag schrieb Jordan Uggla:
> I'm not sure what you wanted to point me to. Okay, I should talk about
> core.img rather than stage 1.5 in GRUB 2, but this does not influence my
> question.
>> Blocklists pointing to blocks on an active filesystem are unreliable,
>> and thus should not be used.
> They just have to be administrated thoroughly, i.e. grub-install be called
> after touching anything in /boot/grub.
>> Most filesystems do not provide an
>> embedding area for the bootloader and thus for most configurations the
>> only reliable place to store a boot sector (if you use a boot sector
>> at all, rather than having your preferred boot manager load grub's
>> core.img reliably using multiboot), is in the MBR. Thus, grub's boot
>> sector should always go in the MBR (unless you're installing grub to a
>> partition containing a filesystem like btrfs, but even in that case I
>> would recommend installing grub's boot sector to the mbr for
>> simplicity).
> I agree that installation in the MBR is generally the most simple and
> failsafe method. But GRUB should offer the mere possibility to install
> itself only locally and apply block lists if forced, i.e. in the absence
> of an embedding area.

The problem is that you don't just have an absence of an embedding
area, with UFS it's not even clear that you have a reserved *boot

You can see the mailing list discussion about this here

> It seems that a local GRUB insists in block addressing for Linux (ext234)

That's because extN have the first sector reserved for a bootloader,
even if they don't have a full embedding area. If you use ext2 with
FreeBSD you can use unreliable blocklists from grub as well.

> and refuses it strictly for FreeBSD. The latter is the reason for my error
> message.
> Is there no way to customize this? "Please use a block list here!" as
> opposed to "please don't us a b.l. but assume a reserved space here!"

There is a way to tell grub-install "Clobber the first sector of this
partition even though that might make the entire filesystem unusable,
or clobber something more subtle but also important" but it seems like
such a bad idea to do that that I'm reluctant to state how.

You should either install grub's boot sector to the MBR or nowhere
(using "grub-install --grub-setup=/bin/true /dev/sda" and loading the
core.img from another bootloader using multiboot).

Jordan Uggla (Jordan_U on

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