Date: 13 Jan 2003 16:45:14 -0800 From: swear@attbi.com (Gary W. Swearingen) To: freebsd-questions@freebsd.org Subject: Dangerously dedicated vs. fully dedicated, etc. Message-ID: <r0ptr0d0f9.tr0@localhost.localdomain>
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I'm having trouble understanding a couple parts of the "disklabel"
manpage related to dangerously/fully dedicated disks.
The "BUGS" section has this paragraph:
For the i386 architecture, the primary bootstrap sector contains an
embedded fdisk table. The disklabel utility takes care to not
clobber it when installing a bootstrap only (-B), or when editing an
existing label (-e), but it unconditionally writes the primary
bootstrap program onto the disk for -w or -R, thus replacing the
fdisk table by the dummy one in the bootstrap program. This is only
of concern if the disk is fully dedicated, so that the BSD disklabel
starts at absolute block 0 on the disk.
1) Can anyone explain that last sentence to me? Shouldn't that "is only
of concern" instead be "is not of concern", reversing the sense?
2) Is a "fully dedicated" disk exactly the same as a "dangerously
dedicated" disk? If not, what's the difference? Can one use a disk
(or a slice) which has no space reserved for stage 1 & 2 boot records,
say, for a non-bootable disk? I know no way to disklabel one that way.
3) Is "block 0" exactly the same as "sector 0"?
4) Shouldn't "BSD disklabel starts at absolute block 0" be "FreeBSD
stage 1 boot record starts at sector 0"? The disklabel immediately
follows the stage 1 boot record, right? So it would be "block 1"?
Under "Writing a standard label", the manpage says:
PC-based systems have special requirements in order for the BIOS to
properly recognize a FreeBSD disklabel. Older systems may require
what is known as a ''dangerously dedicated'' disklabel, which
creates a fake DOS partition to work around problems older BIOSes
have with modern disk geometries.
5) What's "fake DOS partition"? DD disks don't have partitions. Is it
just trying to not bother saying that it fills in the MBR partition
table as if there was at least one slice which the DD MBR will ignore.
6) Why is the problem limited to older systems? Don't new BIOSes
check for a normal-looking partition table?
7) Isn't the first sentence wrong? BIOS don't look for disklabels, do
they? The special requirements are in order for the BIOS to find the
FreeBSD stage 1 boot record, no?. (And the FreeBSD installer doesn't
satisfy the requirements for non-DD disks (as I read the FAQ's DD
answer). I can't imagine why not. Older systems with a standard
MBR boot any active slice (even FreeBSD) without problems, no?)
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