option bootable in setup-storage

Holger Parplies wfai at parplies.de
Thu May 7 13:08:29 CEST 2009


Michael Tautschnig wrote on 2009-05-07 06:17:28 +0200 [Re: option bootable in setup-storage]:
> > [...]
> > I'm just wondering ... what happens if / is on a logical partition or
> > LVM LV?
> I do test for / being on a real disk and only set the bootable flag (or
> actually the value in the hash that will later cause the bootable flag to
> be set) only in such cases, so we should be fine. But anyway, thanks for
> looking closely!

I wasn't really worried about the implementation. I was wondering about the
concept. I don't fully understand the implications of the matter, so maybe my
point is, err, pointless.

Ideally, the config file specifies which partition (if any) is to be marked
bootable. If none is marked, we have the root partition as default, if (and
only if) it is on a primary partition. Otherwise we have no default.

The historical purpose of the bootable flag (as I understand it, on x86
systems) is to tell DOS MBR which partition to boot. With lilo, grub et al.
this is obsolete. I take it some BIOSes make a misguided attempt of
plausibility checking and only boot the MBR if a partition is marked bootable
(this assumption is - possibly incorrectly - deduced from the initial issue
of this thread). Sure, DOS MBR *can* boot a grub stage1 or lilo boot sector
off a primary partition (which would, thus, need to be marked bootable), but
this can just as well be /boot or even /usr/local/foo if someone were inclined
to set it up that way.

My point is: the default of / can be wrong, there remain cases without a
default, and there are cases where no partition needs to be marked bootable,
so what is the point of providing a default in the first place (which can
really just be a convenience to save typing, but not thinking)?
Put differently: the current solution is complicated to define, which might
mean it is not optimal.

Then again, I might be missing important facts (especially on non-x86-
platforms) ...


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