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Disk geometry stopped being a problem around 1997.

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Noted by:	scottl
Approved by:	bcr (mentor)
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Eitan Adler 2012-11-20 18:25:04 +00:00
parent 2df2fea664
commit ec1a479590
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=40110
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79
en_US.ISO8859-1/books/faq/book.xml
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@ -1475,81 +1475,6 @@
</answer>
</qandaentry>
<qandaentry>
<question id="geometry">
<para>Which geometry should I use for a disk drive?</para>
</question>
<answer>
<note>
<para>By the <quote>geometry</quote> of a disk, we mean
the number of cylinders, heads and sectors/track on a
disk. We will refer to this as C/H/S for convenience.
This is how the PC's BIOS works out which area on a disk
to read/write from.</para>
</note>
<para>This causes a lot of confusion among new system
administrators. First of all, the
<emphasis>physical</emphasis> geometry of a SCSI drive is
totally irrelevant, as &os; works in term of disk blocks.
In fact, there is no such thing as <quote>the</quote>
physical geometry, as the sector density varies across the
disk. What manufacturers claim is the <quote>physical
geometry</quote> is usually the geometry that they have
determined wastes the least space. For IDE disks, &os; does
work in terms of C/H/S, but all modern drives internally
convert this into block references.</para>
<para>All that matters is the <emphasis>logical</emphasis>
geometry. This is the answer that the BIOS gets when it
asks the drive <quote>what is your geometry?</quote> It then
uses this geometry to access the disk. As &os; uses the
BIOS when booting, it is very important to get this right.
In particular, if you have more than one operating system on
a disk, they must all agree on the geometry. Otherwise you
will have serious problems booting!</para>
<para>For SCSI disks, the geometry to use depends on whether
extended translation support is turned on in your controller
(this is often referred to as <quote>support for DOS disks
&gt;1GB</quote> or something similar). If it is turned off,
then use <replaceable>N</replaceable> cylinders, 64 heads
and 32 sectors/track, where <replaceable>N</replaceable> is
the capacity of the disk in MB. For example, a 2&nbsp;GB disk
should pretend to have 2048 cylinders, 64 heads and 32
sectors/track.</para>
<para>If it <emphasis>is</emphasis> turned on (it is often
supplied this way to get around certain limitations in
&ms-dos;) and the disk capacity is more than 1&nbsp;GB, use
<replaceable>M</replaceable> cylinders, 63 sectors per track
(<emphasis>not</emphasis> 64), and 255 heads, where
<replaceable>M</replaceable> is the disk capacity in MB
divided by 7.844238 (!). So our example 2&nbsp;GB drive
would have 261 cylinders, 63 sectors per track and 255
heads.</para>
<para>If you are not sure about this, or &os; fails to detect
the geometry correctly during installation, the simplest way
around this is usually to create a small DOS partition on
the disk. The BIOS should then detect the correct geometry,
and you can always remove the DOS partition in the partition
editor if you do not want to keep it. You might want to
leave it around for programming network cards and the like,
however.</para>
<para>Alternatively, there is a freely available utility
distributed with &os; called
<filename>pfdisk.exe</filename>. You can find it in the
<filename class="directory">tools</filename> subdirectory on
the &os; CD-ROM or on the various &os; FTP sites. This
program can be used to work out what geometry the other
operating systems on the disk are using. You can then enter
this geometry in the partition editor.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="disk-divide-restrictions">
<para>Are there any restrictions on how I divide the disk
@ -1606,8 +1531,8 @@
<answer>
<para>This is classically a case of &os; and some other
OS conflicting over their ideas of disk <link
linkend="geometry">geometry</link>. You will have to
OS conflicting over their ideas of disk
geometry. You will have to
reinstall &os;, but obeying the instructions given above
will almost always get you going.</para>
</answer>