FreeBSD slices and the Boot Manager

Polytropon freebsd at
Sun Jul 28 06:09:12 UTC 2013

On Sat, 27 Jul 2013 19:39:30 +0200 (CEST), Conny Andersson wrote:
> Hi,
> I have a workstation with two factory installed hard disks. The first disk, 
> ada0, is occupied by a Windows 7 Pro OS (mainly kept for the three year 
> warranty of the workstation as Dell techs mostly speak the Microsoft 
> language).

It's just a series of pictures, not a language. ;-)

> Instead I have configured the BIOS to boot from the MBR on the second disk 
> as I most of the time (99%) use FreeBSD. The MBR on ada1 was installed with 
> sysinstall's option "Install the FreeBSD Boot Manager", when I installed 
> the FreeBSD 8.3-RELEASE.
> (The latest BIOS version 2.4.0 for Dell T1500 does not support 
> The second disk ada1, now has three FreeBSD slices:
> 1) ada1s1 with FreeBSD 8.1-RELEASE
> 2) ada1s2 with FreeBSD 8.2-RELEASE
> 3) ada1s3 with FreeBSD 8.3-RELEASE
> I want to install the new FreeBSD 8.4-RELEASE on ada1s1 by overwriting the 
> now existing two first slices. This means that ada1s3, must become ada1s2 
> instead. Is this possible to do?

Why do you want to do this? If you keep the s1 slice, you can
easily install FreeBSD 8.4 into that slice, leading to this

1) ada1s1 with FreeBSD 8.4-RELEASE
2) ada1s2 with FreeBSD 8.2-RELEASE
3) ada1s3 with FreeBSD 8.3-RELEASE

Or is the numbering order important to you?

You could even keep the partitioning inside s1, but there is
no problem re-partitioning inside s1.

> A very important question is if sysinstall's option "Install the FreeBSD 
> Boot Manager" detects that I have a FreeBSD 8.3 and detect it as slice 2 on 
> disk 1?

I'm not sure I'm following you correctly. The sysinstall program
is considered obsolete, the new system installer is bsdinstall.

> So it becomes a boot option when I am rebooting? (Maybe the slice 
> may come up as ad6s2, because AHCI in FreeBSD 8.4 isn't enabled at the time 
> of the install.)

That is a _good_ consideration! To make sure things work independently
from "boot-time recognition", use labels for the file system and then
mount them by using the labels. Encode the OS version number in the
labels, so it's even easier to deal with them. Use "newfs -L" on
un-mounted partitions (you can do that from the install media).

>From the install media, you can easily go to the CLI and use the
bsdlabel program to re-write the boot blocks and boot manager if

> Can I mount ada1s2a (FreeBSD 8.3) from the newly installed FreeBSD 8.4 and 
> edit my FreeBSD's 8.3-R /etc/fstab according to the new disk layout, and 
> occasionally run FreeBSD 8.3 without problems? Or do I have to do more to 
> get it to work?

Yes, that should be possible. I don't see any problem because this
is a UFS partition. As I mentioned earlier, if you apply labels to
the partitions on the slices, it's even easier to determine _which_
'a' partition (root partition) you are currently dealing with. And
if you continue your installation scheme in further versions, you
will be freed from remembering what OS version resides on what slice.
You then simply do "mount /dev/ufs/root83 /mnt; vi /mnt/etc/fstab"
and you _immediately_ know which installation you're currently
dealing with.

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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