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Clarify the situation with "dangerously dedicated" disks some more.

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Remove a lot of patronising twaddle and try to help avoid use of DD mode
wherever possible.
This commit is contained in:
Mike Smith 1999-02-05 01:48:47 +00:00
parent 924e31bcdf
commit 8c670928e2
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=4216
1 changed files with 55 additions and 44 deletions

99
FAQ/admin.sgml
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@ -1,4 +1,4 @@
<!-- $Id: admin.sgml,v 1.16 1999-01-28 00:00:50 nik Exp $ -->
<!-- $Id: admin.sgml,v 1.17 1999-02-05 01:48:47 msmith Exp $ -->
<!-- The FreeBSD Documentation Project -->
<sect>
@ -318,22 +318,8 @@
with a FreeBSD slice that employs partitions of its own.
Optionally, one can chose to install a boot-selector to switch
between the possible operating systems on the disk(s).
<p>Now, while this is certainly the common case for people
coming from a PC background, those people coming more from a
Unix background and who are going to setup a machine just to
run FreeBSD and only FreeBSD, are more used to the classic
Unix way where the operating system owns the entire disks,
from the very first sector through the end. A true fdisk
table isn't of any use in this case, the machine is running
FreeBSD 24 hours per day, 7 days per week, no other operating
system should ever be booted on it. So, if you select
``A)ll FreeBSD'' in sysinstall's fdisk editor, and answer the
next question with ``No'', you'll get this mode. Note that
this means the BSD bootstrap also forms the MBR for this drive,
so there's no space left for anything like a boot manager.
Don't ever try to install one, or you'll damage the BSD
bootstrap.
The alternative uses the entire disk for FreeBSD, and makes
no attempt to be compatible with other operating systems.
<p>So why it is called ``dangerous''? A disk in this mode
doesn't contain what normal PC utilities would consider a
@ -341,34 +327,59 @@
designed, they might complain at you once they are getting
in contact with such a disk, or even worse, they might
damage the BSD bootstrap without even asking or notifying
you. Some kind of operating system that is in rather
widespread use on PCs is known for this kind of
user-unfriendliness (of course, it does this in the name of
``user-friendliness''). At least one Award BIOS that is for
example used in HP Netservers (but not only there) is known
to ignore any harddisk that doesn't have what it believes to
be a valid fdisk table. When it comes to booting, it simply
ignores such a disk drive, advances to the floppy drive, and
barfs at you with just ``Read error''. Very impressive, eh?
They probably also call this ``user-friendly'', who knows?
you. In addition, the ``dangerously dedicated'' disk's layout
is known to confuse many BIOSsen, including those from AWARD
(eg. as found in HP Netserver and Micronics systems as well as
many others) and Symbios/NCR (for the popular 53C8xx range of
SCSI controllers). This isn't a complete list, there are more.
Symptoms of this confusion include the "read error" message
printed by the FreeBSD bootstrap when it can't find itself,
as well as system lockups when booting.
<p>The advantages of this mode are: FreeBSD owns the entire
disk, no need to waste several fictitious `tracks' for just
nothing but a 1980-aged simplistic partitioning model
enforcing some artificial and now rather nonsensical
constraints on how this partitioning needs to be done.
These constraints often lead to what might be the biggest
headaches for OS installations on PCs, geometry mismatch
hassles resulting out of two different, redundant ways how
to store the partitioning information in the fdisk table.
See the chapter about <ref id="missing_os" name="Missing
Operating System">. In ``dangerously dedicated'' mode, the
BSD bootstrap starts at sector 0, and this one is the only
sector that always translates into the same C/H/S values,
regardless of which `translation' your BIOS is using for
your disk. Thus, you can also swap disks between
systems/controllers that use a different translation scheme,
without risking that they won't boot anymore.
<p>Why have this mode at all then? It only saves a few kbytes
of disk space, and it can cause real problems for a new
installation. ``Dangerously dedicated'' mode's origins lie
in a desire to avoid one of the most common problems plaguing
new FreeBSD installers - matching the BIOS ``geometry'' numbers
for a disk to the disk itself.
<p>``Geometry'' is an outdated concept, but one still at the
heart of the PC's BIOS and its interaction with disks. When
the FreeBSD installer creates slices, it has to recoord the
location of these slices on the disk in a fashion that
corresponds with the way the BIOS expects to find them. If
it gets it wrong, you won't be able to boot.
<p>``Dangerously dedicated'' mode tries to work around this
by making the problem simpler. In some cases, it gets it right.
But it's meant to be used as a last-ditch alternative - there
are better ways to solve the problem 99 times out of 100.
<p>So, how do you avoid the need for ``DD'' mode when you're
installing? Start by making a note of the geometry that your
BIOS claims to be using for your disks. You can arrange to have
the kernel print this as it boots by specifying ``-v'' at the
``boot:'' prompt, or using ``boot -v'' in the loader. Just
before the installer starts, the kernel will print a list of
BIOS geometries. Don't panic - wait for the installer to start
and then use scrollback to read the numbers. Typically the BIOS
disk units will be in the same order that FreeBSD lists your
disks, first IDE, then SCSI.
<p>When you're slicing up your disk, check that the disk geometry
displayed in the FDISK screen is correct (ie. it matches the BIOS
numbers); if it's wrong, use the ``g'' key to fix it. You may have
to do this if there's absolutely nothing on the disk, or if the
disk has been moved from another system. Note that this is only
an issue with the disk that you're going to boot from; FreeBSD
will sort itself out just fine with any other disks you may have.
<p>Once you've got the BIOS and FreeBSD agreeing about the
geometry of the disk, your problems are almost guaranteed to be
over, and with no need for ``DD'' mode at all. If, however,
you are still greeted with the dreaded ``read error'' message
when you try to boot, it's time to cross your fingers and
go for it - there's nothing left to lose.
<p>To return a ``dangerously dedicated'' disk for normal PC
use, there are basically two options. The first is, you