Add a chapter about ``dangerously dedicated'' disks, their pro's and con's.

svn path=/head/; revision=1930

FAQ/FAQ.sgml

@@ -1,12 +1,12 @@
 <!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN">
-<!-- $Id: FAQ.sgml,v 1.70 1997-09-07 12:17:46 jkh Exp $ -->
+<!-- $Id: FAQ.sgml,v 1.71 1997-09-07 14:04:39 joerg Exp $ -->
 
   <article>
 
 <title>Frequently Asked Questions for FreeBSD 2.X
 <author>Maintainer: Peter da Silva <tt><htmlurl url='mailto:peter@taronga.com'
   name='&lt;peter@taronga.com&gt;'></tt>
-<date>$Date: 1997-09-07 12:17:46 $</date>
+<date>$Date: 1997-09-07 14:04:39 $</date>
 
 <abstract>
 This is the FAQ for FreeBSD systems version 2.X  All entries are
@@ -784,7 +784,7 @@ make release BUILDNAME=3.0-MY-SNAP CHROOTDIR=/some/big/filesystem/release
 	  reasonably careful, a 20 megabyte boot partition should be plenty.
 
       <sect1>
-	<heading>When I boot FreeBSD I get ``Missing Operating System''.</heading>
+	<heading>When I boot FreeBSD I get ``Missing Operating System''.<label id="missing_os"></heading>
         <p>
           This is classically a case of FreeBSD and DOS or some other OS
           conflicting over their ideas of disk <ref id="geometry"
@@ -2399,6 +2399,92 @@ drivedata: 0
 	  to reinstall Windows95 (which is a Jealous Operating System, and
 	  will bear no other Operating Systems in the Master Boot Record).
 
+      <sect1>
+	<heading>Will a ``dangerously dedicated'' disk endagner my health?</heading>
+	<p>
+	  The installation procedure allows you to chose two different
+	  modi how to partition your harddisk(s).  The default way makes
+	  it compatible with other operating systems on the same machine,
+	  by using fdisk table entries (called ``slices'' in FreeBSD),
+	  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.
+
+	<p>
+	  So why it is called ``dangerous''?  A disk in this mode
+	  doesn't contain what normal PC utilities would consider a
+	  valid fdisk table.  Depending on how well they have been
+	  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?
+
+	<p>
+	  The advantages of this mode are: FreeBSD owns the entire
+	  disk, no need to waste several ficticuous `tracks' for just
+	  nothing but a 1980-aged simplicistic 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>
+	  To return a ``dangerously dedicated'' disk for normal PC
+	  use, there are basically two options.  The first is, you
+	  write enough NULL bytes over the MBR to make any subsequent
+	  installation believe this to be a blank disk.  You can do
+	  this for example with
+
+	  <verb>
+	    dd if=/dev/zero of=/dev/rsd0 count=15
+	  </verb>
+
+	  Alternatively, the undocumented DOS command
+
+	  <verb>
+	    fdisk /mbr
+	  </verb>
+
+	  is supposed to install a new master boot record as well,
+	  thus clobbering the BSD bootstrap.
+
+
       <sect1>
         <heading>How can I add more swap space?</heading>