Loader, MBR and the boot process

[Mark Andrews]

In message <cf9b1ee01001240759j2476cf3es2babd8b32a90f9aa at mail.gmail.com>, Dan N
aumov writes:
> On Sun, Jan 24, 2010 at 5:29 PM, John <john at starfire.mn.org> wrote:
> > On Fri, Jan 22, 2010 at 07:02:53AM +0200, Dan Naumov wrote:
> >> On Fri, Jan 22, 2010 at 6:49 AM, Dan Naumov <dan.naumov at gmail.com> wrote=
> :
> >> > On Fri, Jan 22, 2010 at 6:12 AM, Thomas K. <fwd at gothschlampen.com> wro=
> te:
> >> >> On Fri, Jan 22, 2010 at 05:57:23AM +0200, Dan Naumov wrote:
> >> >>
> >> >> Hi,
> >> >>
> >> >>> I recently found a nifty "FreeBSD ZFS root installation script" and
> >> >>> been reworking it a bit to suit my needs better, including changing =
> it
> >> >>> from GPT to MBR partitioning. However, I was stumped, even though I
> >> >>> had done everything right (or so I thought), the system would get
> >> >>> stuck at Loader and refuse to go anywhere. After trying over a dozen
> >> >>
> >> >> probably this line is the cause:
> >> >>
> >> >> dd if=3D/mnt2/boot/zfsboot of=3D/dev/"${TARGETDISK}"s1a skip=3D1 seek=
> =3D1024
> >> >>
> >> >> Unless by "swap first" you meant the on-disk location, and not the
> >> >> partition letter. If swap is partition "a", you're writing the loader
> >> >> into swapspace.
> >> >>
> >> >>
> >> >> Regards,
> >> >> Thomas
> >> >
> >> > At first you made me feel silly, but then I decided to double-check, I
> >> > uncommented the swap line in the partitioning part again, ensured I
> >> > was writing the bootloader to "${TARGETDISK}"s1b and ran the script.
> >> > Same problem, hangs at loader. Again, if I comment out the swap,
> >> > giving the entire slice to ZFS and then write the bootloader to
> >> > "${TARGETDISK}"s1a, run the script, everything works.
> >>
> >> I have also just tested creating 2 slices, like this:
> >>
> >> gpart create -s mbr "${TARGETDISK}"
> >> gpart add -s 3G -t freebsd "${TARGETDISK}"
> >> gpart create -s BSD "${TARGETDISK}"s1
> >> gpart add -t freebsd-swap "${TARGETDISK}"s1
> >>
> >> gpart add -t freebsd "${TARGETDISK}"
> >> gpart create -s BSD "${TARGETDISK}"s2
> >> gpart add -t freebsd-zfs "${TARGETDISK}"s2
> >>
> >> gpart set -a active -i 2 "${TARGETDISK}"
> >> gpart bootcode -b /mnt2/boot/boot0 "${TARGETDISK}"
> >>
> >>
> >> and later:
> >>
> >> dd if=3D/mnt2/boot/zfsboot of=3D/dev/"${TARGETDISK}"s2 count=3D1
> >> dd if=3D/mnt2/boot/zfsboot of=3D/dev/"${TARGETDISK}"s2a skip=3D1 seek=3D=
> 1024
> >>
> >>
> >> Putting the swap into it's own slice and then putting FreeBSD into
> >> it's own slice worked fine. So why the hell can't they both coexist in
> >> 1 slice if the swap comes first?
> >
> > I know what the answer to this USED to be, but I don't know if it is
> > still true (obviously, I think so, I or wouldn't waste your time).
> >
> > The filesystem code is all carefully written to avoid the very
> > first few sector of the partition. =A0That's because the partition
> > table is there for the first filesystem of the slice (or disk).
> > That's a tiny amout of space wasted, because it's also skipped on
> > all the other filesystems even though there's not actually anything
> > there, but it was a small inefficency, even in the 70's.
> >
> > Swap does not behave that way. =A0SWAP will begin right at the slice
> > boundry, with 0 offset. =A0As long as it's not the first partition, no
> > harm, no foul. =A0If it IS the first partition, you just nuked your parti=
> tion
> > table. =A0As long as SWAP owns the slice, again, no harm, no foul, but
> > if there were filesystems BEHIND it, you just lost 'em.
> >
> > That's the way it always used to be, and I think it still is. =A0SWAP can
> > only be first if it is the ONLY thing using that slice (disk), otherwise,
> > you need a filesystem first to protect the partition table.
> > --
> >
> > John Lind
> > john at starfire.MN.ORG
> 
> This explanation does sound logical, but holy crap, if this is the
> case, you'd think there would be bells, whistles and huge red label
> warnings in EVERY FreeBSD installation / partitioning guide out there
> warning people to not put swap first (unless given a dedicated slice)
> under any circumstances. The warnings were nowhere to be seen and lots
> of pointy hair first greyed and were then lost during the process of
> me trying to figure out why my system would install but wouldn't boot.

>From "man bsdlabel".

     offset  The offset of the start of the partition from the beginning of
             the drive in sectors, or * to have bsdlabel calculate the correct
             offset to use (the end of the previous partition plus one, ignor-
             ing partition `c'.  For partition `c', * will be interpreted as
             an offset of 0.  The first partition should start at offset 16,
             because the first 16 sectors are reserved for metadata.

> - Sincerely,
> Dan Naumov
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org