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Whitespace cleanup of disk-geometry question. No content changes.

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Approved by:	bmah
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Michael Lucas 2001-12-12 12:49:31 +00:00
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@ -1749,67 +1749,73 @@ BUSY</literallayout></entry>
<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>
<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 FreeBSD works in term of
disk blocks. In fact, there is no such thing as
<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 FreeBSD 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
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, FreeBSD does work in terms of C/H/S, but
all modern drives internally convert this into block references.
space. For IDE disks, FreeBSD 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 FreeBSD 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. 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 FreeBSD
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 2GB disk should
pretend to have 2048 cylinders, 64 heads and 32
sectors/track.</para>
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 2GB 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 MSDOS) and the
disk capacity is more than 1GB, use M cylinders, 63 sectors per
track (<emphasis>not</emphasis> 64), and 255 heads, where 'M' is the disk capacity
in MB divided by 7.844238 (!). So our example 2GB drive would
have 261 cylinders, 63 sectors per track and 255 heads.</para>
<para>If it <emphasis>is</emphasis> turned on (it is often
supplied this way to get around certain limitations in
MSDOS) and the disk capacity is more than 1GB, use M
cylinders, 63 sectors per track (<emphasis>not</emphasis>
64), and 255 heads, where 'M' is the disk capacity in MB
divided by 7.844238 (!). So our example 2GB drive would
have 261 cylinders, 63 sectors per track and 255
heads.</para>
<para>If you are not sure about this, or FreeBSD 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>If you are not sure about this, or FreeBSD 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 FreeBSD called <filename>pfdisk.exe</filename>.
You can find it in the <filename>tools</filename> subdirectory on the
FreeBSD CDROM or on the various FreeBSD 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>
distributed with FreeBSD called
<filename>pfdisk.exe</filename>. You can find it in the
<filename>tools</filename> subdirectory on the FreeBSD
CDROM or on the various FreeBSD 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>