Date: Thu, 23 Aug 2007 11:59:54 +0100 From: Alex Zbyslaw <xfb52@dial.pipex.com> To: Christopher Key <cjk32@cam.ac.uk> Cc: Jerry McAllister <jerrymc@msu.edu>, freebsd-questions@freebsd.org Subject: Re: FreeBSD MBRs Message-ID: <46CD68AA.4020709@dial.pipex.com> In-Reply-To: <46CD625E.7090508@cam.ac.uk> References: <46CC1DB7.7040506@cam.ac.uk> <20070822152326.GF56142@gizmo.acns.msu.edu> <46CD58BB.9080001@dial.pipex.com> <46CD625E.7090508@cam.ac.uk>
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Christopher Key wrote:
> Sorry the original post wasn't clear, I'll have a go at rexpressing my
> original questions using the above for context.
It was a complicated series of events, so it's easy to end up with a
confusing description (and the fault might lie with us being too dumb to
understand :-))
>
> Firstly, when you hit F5, does it, a) Load the partition table from
> the next disk and update the displayed list of slices, or b) Execute
> the MBR from the next disk? I'll assume the latter.
b). If there is no MBR on the next disk then F5 will beep (might depend
on MBR version) and do nothing.
>
> Secondly, does boot0 'remember' that you pressed F5, and hence do the
> same the next time you boot, even after a power cycle? In this case,
> having done,
>
> Disk1: F5 -> disk2
> Disk2: F5 -> disk3
> Disk3: F1 -> boots FreeBSD
>
> the next time, it will appear as,
>
> Disk3: F1 -> boots FreeBSD
Everything is remembered. However, the sequence will always start with
disk1 - whatever the BIOS thinks is the boot disk - and then default to
exactly what you pressed the last time *for that disk*.
So you would see
Disk1: F5 -> disk2
Disk2: F5 -> disk3
Disk3: F1 -> boots FreeBSD
exactly as above.
(There is an option when writing the MBR to *not* remember - man
bsdlabel would tell you. And you can change the default for a
particular disk, again with bsdlabel, but there are issues with doing
this on mounted disks: IIRC you can set "sysctl kern.geom.debugflags=16"
before writing MBR and set it back to 0 afterwards, if you are having
problems. It's just a magic incantation which I have no deep
understanding of :-().
> The behaviour that I was experiencing was as follows:
>
> Disk1: F1 -> boots FresBSD
>
> reboot
>
> Disk1: F5 -> disk2
> Disk2: Has /boot/mbr on it, and hence attempts to boot the active
> slice. As there is no active slice on the disk, simply fails with the
> message 'Missing operating system'
The missing OS message makes sense for a disk with no active slice, but
the rest simply doesn't accord with my expectations unless perhaps you
somehow wrote an MBR which does not remember.
For the record, I have 4 SATA disk in my workstation and regularly boot
off disk 3 (because disk 1 has Windows). If I boot windows (F1 on
disk1) then the next time I reboot I have to remember to press F5 to get
to disk 2 and on to FreeBSD, because it does remember the F1 from the
previous boot. DIsk 2 will remember it's F5 and disk 3 it's F1,
irrespective of what I do on disk 1. (And yes, if every disk were set
to F5, you'd go round in a loop forever, never booting anything :-))
> Now, subsequent attempts to boot simply display the message 'Missing
> operating system'. Hence, I concluded that either, a) boot0 was
> rembering the F5 keystroke, and passing me on to disk 2 automatically,
> or b) That the BIOS was rememering something and booting straight from
> disk2 despite the boot order having disk1 first.
The memory for the key you pressed is *per disk* because (IIUC) it is
actually stored on the the disk somewhere in the MBR. It's not unheard
of for a BIOS to completely change the boot order when the devices
attached to the machine are changed. So, if you set your BIOS to boot
from a USB stick, then disk X; if you then removed the USB stick I would
not be at all surprised if the BIOS reverted to some apparently random
boot order where disk X was *not* the first disk. I've had immense
trouble with this in th past trying to boot from CD where the BIOS would
not reliably spot that the CD was actually attached. If the boot order
with a CD was e.g.
CD
Disk X
Disk Y
The boot order when the CD was *not* recognised could easily end up as:
Disk Y
Disk X
It might have been controller order or somesuch, but was *not* the same
order just minus the CD.
Could this have happened? If your "disk2" from the original boot order
had become "disk1", would that explain what you saw?
>
> The only was that I found to rectify this was use a boot from a USB
> device with boot0 on it:
>
> USB: F5 -> disk1
> Disk1: F1 -> boots FreeBSD
>
> And now, subsequent reboots work fine:
>
> Disk1: F1 -> boots FresBSD
--Alex
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