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Add information about BIOS renumbering of disk drives, and why it might

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be a bad idea from a FreeBSD perspective.

Submitted by:   bill@2ez.com
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Nik Clayton 2000-04-30 22:10:06 +00:00
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commit e8e0d365eb
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=7119
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101
en_US.ISO8859-1/books/handbook/disks/chapter.sgml
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The FreeBSD Documentation Project
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/disks/chapter.sgml,v 1.16 2000/03/25 20:05:40 nbm Exp $
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/disks/chapter.sgml,v 1.17 2000/04/03 02:15:38 chris Exp $
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<chapter id="disks">
@ -14,6 +14,105 @@
memory, or networked, on FreeBSD.</para>
</sect1>
<sect1 id="disks-bios-numbering">
<title>BIOS Drive Numbering</title>
<para>Before you install and configure FreeBSD on your system, there is an
important subject that you should be aware of if, especially if you have
multiple hard drives.</para>
<para>In a PC running DOS or any of the BIOS-dependent operating systems
(WINxxx), the BIOS is able to abstract the normal disk drive order, and
the operating system goes along with the change. This allows the user
to boot from a disk drive other than the so-called <quote>primary
master</quote>. This is especially convenient for some users who have
found that the simplest and cheapest way to keep a system backup is to
buy an identical second hard drive, and perform routine copies of the
first drive to the second drive using Ghost or XCOPY. Then, if the
first drive fails, or is attacked by a virus, or is scribbled upon by an
operating system defect, he can easily recover by instructing the BIOS
to logically swap the drives. It's like switching the cables on the
drives, but without having to open the case.</para>
<para>More expensive systems with SCSI controllers often include BIOS
extensions which allow the SCSI drives to be re-ordered in a similar
fashion for up to seven drives.</para>
<para>A user who is accustomed to taking advantage of these features may
become surprised when the results with FreeBSD are not as expected.
FreeBSD does not use the BIOS, and does not know the <quote>logical BIOS
drive mapping</quote>. This can lead to very perplexing situations,
especially when drives are physically identical in geometry, and have
also been made as data clones of one another.</para>
<para>When using FreeBSD, always restore the BIOS to natural drive
numbering before installing FreeBSD, and then leave it that way. If you
need to switch drives around, then do so, but do it the hard way, and
open the case and move the jumpers and cables.</para>
<sidebar>
<title>An illustration from the files of Bill and Fred's Exceptional
Adventures:</title>
<para>Bill breaks-down an older WIntel box to make another FreeBSD box
for Fred. Bill installs a single SCSI drive as SCSI unit zero, and
installs FreeBSD on it.</para>
<para>Fred begins using the system, but after several days notices that
the older SCSI drive is reporting numerous soft errors, and reports
this fact to Bill.</para>
<para>After several more days, Bill decides it's time to address the
situation, so he grabs an identical SCSI drive from the disk drive
"archive" in the back room. An initial surface scan indicates that
this drive is functioning well, so Bill installs this drive as SCSI
unit four, and makes an image copy from drive zero to drive four. Now
that the new drive is installed and functioning nicely, Bill decides
that it's a good idea to start using it, so he uses features in the
SCSI BIOS to re-order the disk drives so that the system boots from
SCSI unit four. FreeBSD boots and runs just fine.</para>
<para>Fred continues his work for several days, and soon Bill and Fred
decide that it's time for a new adventure -- time to upgrade to a
newer version of FreeBSD. Bill removes SCSI unit zero because it was
a bit flakey, and replaces it with another identical disk drive from
the "archive." Bill then installs the new version of FreeBSD onto the
new SCSI unit zero using Fred's magic internet FTP floppies. The
installation goes well.</para>
<para>Fred uses the new version of FreeBSD for a few days, and certifies
that it is good enough for use in the engineering department...it's
time to copy all of his work from the old version. So Fred mounts
SCSI unit four (the latest copy of the older FreeBSD version). Fred
is dismayed to find that none of his precious work is present on SCSI
unit four.</para>
<para>Where did the data go?</para>
<para>When Bill made an image copy of the original SCSI unit zero onto
SCSI unit four, unit four became the "new clone," When Bill
re-ordered the SCSI BIOS so that he could boot from SCSI unit four, he
was only fooling himself. FreeBSD was still running on SCSI unit zero.
Making this kind of BIOS change will cause some or all of the Boot and
Loader code to be fetched from the selected BIOS drive, but when the
FreeBSD kernel drivers take-over, the BIOS drive numbering will be
ignored, and FreeBSD will transition back to normal drive numbering.
In the illustration at hand, the system continued to operate on the
original SCSI unit zero, and all of Fred's data was there, not on SCSI
unit four. The fact that the system appeared to be running on SCSI
unit four was simply an artifact of human expectations.</para>
<para>We are delighted to mention that no data bytes were killed or
harmed in any way by our discovery of this phenomenon. The older SCSI
unit zero was retrieved from the bonepile, and all of Fred's work was
returned to him, (and now Bill knows that he can count as high as
zero).</para>
<para>Although SCSI drives were used in this illustration, the concepts
apply equally to IDE drives.</para>
</sidebar>
</sect1>
<sect1 id="disks-naming">
<title>Disk Naming</title>

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The FreeBSD Documentation Project
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/install/chapter.sgml,v 1.36 2000/03/10 22:32:34 jim Exp $
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/install/chapter.sgml,v 1.37 2000/04/05 12:25:23 nik Exp $
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<chapter id="install">
@ -486,6 +486,15 @@ ftp:*:99:99::0:0:FTP:/cdrom:/nonexistent</programlisting>
installation requests them.</para>
</sect4>
</sect3>
<sect3>
<title>Check your BIOS drive numbering</title>
<para>If you have used features in your BIOS to renumber your disk
drives without recabling them then you should read <xref
linkend="disks-bios-numbering"> first to ensure you do not
confused.</para>
</sect3>
</sect2>
<sect2 id="install-freebsd">
@ -1615,3 +1624,14 @@ ftp:*:99:99::0:0:FTP:/cdrom:/nonexistent</programlisting>
</sect2>
</sect1>
</chapter>
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