Date: Mon, 1 Feb 1999 14:01:14 -0500 (EST) From: Charles Owens <owensc@enc.edu> To: Robert Nordier <rnordier@nordier.com> Cc: root@jules.res.cmu.edu, stable@FreeBSD.ORG, kingsled@enc.edu Subject: Re: disk problem at bootup... (me too!) Message-ID: <Pine.BSF.3.96.990201135100.24908f-100000@itsdsv2.enc.edu> In-Reply-To: <199901311749.TAA04694@ceia.nordier.com>
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On Sun, 31 Jan 1999, Robert Nordier wrote:
> Charles Owens wrote:
>
> > Greg,
> >
> > I believe I'm having the same problem that you are. I had my system
> > running 2.2.7 just fine. I did a clean reinstall of 3.0-RELEASE:
> > during the "fdisk" phase I said to my buddy, "I'm pretty sure that it
> > will _not_ work to make a bootable drive dangerously dedicated." Rather
> > than just go with what I has worked for years I decided to test my
> > assertion by attempting to dangerously dedicate the disk (duh :-). As
> > I'd expected I got an error during the commit phase. So I went back to
> > the fdisk screen, redid things normally, and completed the install.
>
> You can boot from an unsliced ("dangerously dedicated") disk; but
> it's probably best to slice your disks unless you've got really good
> reasons not to.
Just for the sake of arguement, why is dangerously dedicated so
"dangerous?" What does either approach buy you? (Other than the normal
mode's provision for multi-OS configurations.) It seemed that the install
disk tools got confused with my attempt to do an bootable-disk install on
a dangerously dedicated disk (though maybe it was a fluke). Sysinstall
has no problem, though, with just fdisk'ing and disklabel'ing non-bootable
disks in dangerously dedicated mode (I've done that many times).
> In particular, unless you're starting with a completely blank disk
> (where sysinstall detects no slices and no partitions whatever), you
> don't want to go changing between sliced and unsliced. (However, a
> blank disk may be what you meant by a "clean reinstall".)
No, it had 2.2.6 (or maybe 2.2.7) on it before I began. I didn't do an
upgrade install, though. I re-fdisk'ed and re-disklablel'ed it and
installed 3.0-RELEASE from scratch.
> >
> > When attempting the first boot I get a "missing operating system"
> > error. I can get into the system by booting from the install floppy and
> > typing "0:da(0,a)/boot/loader" at the first boot prompt.
>
> A "Missing operating system" error generally means that the OS-
> specific bootstrap can't be found by the mbr code.
>
> Anyway, the error at least indicates you have working mbr code
> installed (see below).
>
> >
> > So I'm thinking that my messing around with dangerously dedicated mode
> > must have screwed something up. Thus far I've done this to attempt to
> > fix it:
> >
> > -- Booted the install disk. When to fdisk screen and did the normal
> > thing I'd do during and install: Use all disk (_not_ dangerously
> > dedicated), Set FBSD partition bootable. Then I hit 'W' to write it
> > out. Didn't help.
> >
> > -- From the booted system (getting in via the on-floppy boot sector) I
> > typed both:
> > disklabel -B da0s1
> > and disklabel -B da0
> > I'm not quite sure if this was right, but it helped me as much as it
> > helped you.
>
> Both of those were reasonable things to try.
>
> > In digging through the mail archives I've found some folks that seem to
> > have had this problem in the past. Several people suggested that the
> > installing DOS MBR via the DOS "fdisk /mbr" command would fix things.
> > Isn't this what the above disklabel commands are supposed to do? What
> > _is_ the correct way to do this from FreeBSD? Not that I'm _entirely_
> > opposed to doing it with the DOS utility. ;-) (I'm going to give this
> > a try Monday.)
>
> An "fdisk /mbr" will rewrite the mbr code (rather than data). The
> easiest way to do the same thing from FreeBSD is to use the recent
> fdisk -b option, but this is available only in -current. You can
> do the same thing using sysinstall, by instructing it to use a
> "Standard MBR", but the process is more involved and error-prone.
>
> I think the fdisk -b option should probably be back-ported to -stable.
Sounds like a fine idea!
> However, an "fdisk /mbr" or equivalent is unlikely to help you at
> this stage (see above).
>
> >
> > Someone else pointed out that some motherboards have a BIOS setting by
> > which the motherboard will prevent the writing of the boot sector...
> > which can mess up FreeBSD install. Good point... I don't think that's
> > my problem here though.
>
> Yes, apparently some BIOSes do this. Unless one's using Microsoft
> OSes exclusively, though, this is probably more trouble than it's
> worth.
>
> >
> > I must admit I'm a bit unclear on how this is all arranged. Are there
> > multiple "boot sectors" that get processed during boot up? One "MBR"
> > for the whole disk and then another boot record that lives on each
> > bootable slice?
>
> The mbr will load sector 0 of the slice. If this has the correct
> signature (the last two bytes are 0x55, 0xaa), control will be
> passed to it. What happens after that is OS-specific (and maybe
> bootstrap-specific). The usual FreeBSD behavior is to load another
> 14 or 15 consecutive sectors following sector 0 of the slice.
> Together, these 15 or 16 sectors comprise boot1, the disk label,
> and boot2. Control is passed (blindly) from boot1 to boot2. And
> boot2 looks in the root directory for /boot/loader, and loads and
> passes control to that, if found.
>
> As far as your problem is concerned, I don't think there's a
> substitute for actually looking at the disk to find out what has
> ended up there.
>
> If you want to send me the output of
>
> dd if=/dev/rda0 size=512 count=80 | hexdump -C
>
> together with fdisk and disklabel output, I can probably tell you
> where things are going wrong.
Thanks! I'll send this in another message in a minute or two.
---
-------------------------------------------------------------------------
Charles N. Owens Email: owensc@enc.edu
http://www.enc.edu/~owensc
Network & Systems Administrator
Information Technology Services "Outside of a dog, a book is a man's
Eastern Nazarene College best friend. Inside of a dog it's
too dark to read." - Groucho Marx
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