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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> <answer>
<note> <note>
<para>By the <quote>geometry</quote> of a disk, we mean the <para>By the <quote>geometry</quote> of a disk, we mean
number of cylinders, heads and sectors/track on a disk. We will the number of cylinders, heads and sectors/track on a
refer to this as C/H/S for convenience. This is how the PC's disk. We will refer to this as C/H/S for
BIOS works out which area on a disk to read/write from.</para> convenience. This is how the PC's BIOS works out which
area on a disk to read/write from.</para>
</note> </note>
<para>This causes a lot of confusion among new system administrators. <para>This causes a lot of confusion among new system
First of all, the <emphasis>physical</emphasis> geometry of a administrators. First of all, the
SCSI drive is totally irrelevant, as FreeBSD works in term of <emphasis>physical</emphasis> geometry of a SCSI drive is
disk blocks. In fact, there is no such thing as 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 <quote>the</quote> physical geometry, as the sector
density varies across the disk. What manufacturers claim is density varies across the disk. What manufacturers claim
the <quote>physical geometry</quote> is usually the is the <quote>physical geometry</quote> is usually the
geometry that they have determined wastes the least geometry that they have determined wastes the least
space. For IDE disks, FreeBSD does work in terms of C/H/S, but space. For IDE disks, FreeBSD does work in terms of C/H/S,
all modern drives internally convert this into block references. but all modern drives internally convert this into block
references.
</para> </para>
<para>All that matters is the <emphasis>logical</emphasis> <para>All that matters is the <emphasis>logical</emphasis>
geometry. This is the answer that the BIOS gets when it asks geometry. This is the answer that the BIOS gets when it
the drive <quote>what is your geometry?</quote> It then asks the drive <quote>what is your geometry?</quote> It
uses this geometry to access then uses this geometry to access the disk. As FreeBSD
the disk. As FreeBSD uses the BIOS when booting, it is very uses the BIOS when booting, it is very important to get
important to get this right. In particular, if you have more this right. In particular, if you have more than one
than one operating system on a disk, they must all agree on the operating system on a disk, they must all agree on the
geometry. Otherwise you will have serious problems geometry. Otherwise you will have serious problems
booting!</para> booting!</para>
<para>For SCSI disks, the geometry to use depends on whether <para>For SCSI disks, the geometry to use depends on whether
extended translation support is turned on in your controller extended translation support is turned on in your
(this is often referred to as <quote>support for DOS disks controller (this is often referred to as <quote>support for
&gt;1GB</quote> or something similar). If it is turned off, then DOS disks &gt;1GB</quote> or something similar). If it is
use <replaceable>N</replaceable> cylinders, 64 heads and 32 turned off, then use <replaceable>N</replaceable>
sectors/track, where <replaceable>N</replaceable> is the cylinders, 64 heads and 32 sectors/track, where
capacity of the disk in MB. For example, a 2GB disk should <replaceable>N</replaceable> is the capacity of the disk in
pretend to have 2048 cylinders, 64 heads and 32 MB. For example, a 2GB disk should pretend to have 2048
sectors/track.</para> cylinders, 64 heads and 32 sectors/track.</para>
<para>If it <emphasis>is</emphasis> turned on (it is often supplied <para>If it <emphasis>is</emphasis> turned on (it is often
this way to get around certain limitations in MSDOS) and the supplied this way to get around certain limitations in
disk capacity is more than 1GB, use M cylinders, 63 sectors per MSDOS) and the disk capacity is more than 1GB, use M
track (<emphasis>not</emphasis> 64), and 255 heads, where 'M' is the disk capacity cylinders, 63 sectors per track (<emphasis>not</emphasis>
in MB divided by 7.844238 (!). So our example 2GB drive would 64), and 255 heads, where 'M' is the disk capacity in MB
have 261 cylinders, 63 sectors per track and 255 heads.</para> 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 <para>If you are not sure about this, or FreeBSD fails to
the geometry correctly during installation, the simplest way detect the geometry correctly during installation, the
around this is usually to create a small DOS partition on the simplest way around this is usually to create a small DOS
disk. The BIOS should then detect the correct geometry, partition on the disk. The BIOS should then detect the
and you can correct geometry, and you can always remove the DOS
always remove the DOS partition in the partition editor if you partition in the partition editor if you do not want to
do not want to keep it. You might want to leave it around for programming keep it. You might want to leave it around for
network cards and the like, however.</para> programming network cards and the like, however.</para>
<para>Alternatively, there is a freely available utility <para>Alternatively, there is a freely available utility
distributed with FreeBSD called <filename>pfdisk.exe</filename>. distributed with FreeBSD called
You can find it in the <filename>tools</filename> subdirectory on the <filename>pfdisk.exe</filename>. You can find it in the
FreeBSD CDROM or on the various FreeBSD FTP sites. This program can be <filename>tools</filename> subdirectory on the FreeBSD
used to work out what geometry the other operating systems on CDROM or on the various FreeBSD FTP sites. This program
the disk are using. You can then enter this geometry in the can be used to work out what geometry the other operating
partition editor.</para> systems on the disk are using. You can then enter this
geometry in the partition editor.</para>
</answer> </answer>
</qandaentry> </qandaentry>