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<BR> <P>
<H1><A NAME="SECTION00560000000000000000">Formatting</A></H1>
<P>
<b>Formatting</b> is the process of writing marks on the
magnetic media that are used to mark tracks and sectors.
Before a disk is formatted, its magnetic surface is a complete
mess of magnetic signals. When it is formatted, some order is
brought into the chaos by essentially drawing lines where the
tracks go, and where they are divided into sectors. The
actual details are not quite exactly like this, but that is
irrelevant. What is important is that a disk cannot be used
unless it has been formatted.
<P>
The terminology is a bit confusing here: in MS-DOS, the word
formatting is used to cover also the process of creating a
filesystem (which will be discussed below). There, the two
processes are often combined, especially for floppies. When
the distinction needs to be made, the real formatting is
called <b>low-level formatting</b>, while making the filesystem
is called <b>high-level formatting</b>. In UNIX circles,
the two are called formatting and making a filesystem, so
that's what is used in this book as well.
<P>
For IDE and some SCSI disks the formatting is actually
done at the factory and doesn't need to be repeated; hence
most people rarely need to worry about it. In fact,
formatting a hard disk can cause it to work less well, for
example because a disk might need to be formatted in some very
special way to allow automatic bad sector replacement to work.
<P>
Disks that need to be or can be formatted often require a special
program anyway, because the interface to the formatting logic
inside the drive is different from drive to drive. The
formatting program is often either on the controller BIOS, or
is supplied as an MS-DOS program; neither of these can easily be used
from within Linux.
<P>
During formatting one might encounter bad spots on the disk,
called <b>bad blocks</b> or <b>bad sectors</b>. These are sometimes
handled by the drive itself,
but even then, if more of them develop, something needs to be
done to avoid using those parts of the disk. The logic to do
this is built into the filesystem; how to add the information
into the filesystem is described below. Alternatively, one
might create a small partition that covers just the bad part
of the disk; this approach might be a good idea if the bad
spot is very large, since filesystems can sometimes have
trouble with very large bad areas.
<P>
Floppies are formatted with <tt>fdformat</tt><A NAME="1489"> </A>. The floppy device
file to use is given as the parameter. For example, the
following command would format a high density,
3.5 inch floppy in the first floppy drive:
<BLOCKQUOTE> <TT>
<code>$</code> <I>fdformat /dev/fd0H1440</I> <BR>
<code>Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.</code> <BR>
<code>Formatting ... done</code> <BR>
<code>Verifying ... done</code> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
Note that if you want to use an autodetecting device (e.g.,
<tt>/dev/fd0</tt><A NAME="1491"> </A>), you <EM>must</EM> set the parameters of the device
with <tt>setfdprm</tt><A NAME="1493"> </A> first. To achieve the same effect as
above, one would have to do the following:
<BLOCKQUOTE> <TT>
<code>$</code> <I>setfdprm /dev/fd0 1440/1440</I> <BR>
<code>$</code> <I>fdformat /dev/fd0</I> <BR>
<code>Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.</code> <BR>
<code>Formatting ... done</code> <BR>
<code>Verifying ... done</code> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
It is usually more convenient to choose the correct device file
that matches the type of the floppy. Note that it is unwise to
format floppies to contain more information than what they are
designed for.
<P>
<tt>fdformat</tt><A NAME="1495"> </A> will also validate the floppy, i.e., check it
for bad blocks. It will try a bad block several times (you
can usually hear this, the drive noise changes dramatically).
If the floppy is only marginally bad (due to dirt on the
read/write head, some errors are false signals), <tt>fdformat</tt><A NAME="1497"> </A> won't
complain, but a real error will abort the validation process.
The kernel will print log messages for each I/O error it
finds; these will go to the console or, if <tt>syslog</tt><A NAME="1499"> </A>
is being used, to the file <tt>/usr/adm/messages</tt><A NAME="1501"> </A>. <tt>fdformat</tt><A NAME="1503"> </A>
itself won't tell where the error is (one usually doesn't care,
floppies are cheap enough that a bad one is automatically thrown
away).
<BLOCKQUOTE> <TT>
<code>$</code> <I>fdformat /dev/fd0H1440</I> <BR>
<code>Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.</code> <BR>
<code>Formatting ... done</code> <BR>
<code>Verifying ... read: Unknown error</code> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
<P>
The <tt>badblocks</tt><A NAME="1505"> </A> command can be used to search any disk or
partition for bad blocks (including a floppy). It does not
format the disk, so it can be used to check even existing
filesystems. The example below checks a 3.5 inch
floppy with two bad blocks.
<BLOCKQUOTE> <TT>
<code>$</code> <I>badblocks /dev/fd0H1440 1440</I> <BR>
<code>718</code> <BR>
<code>719</code> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
<tt>badblocks</tt><A NAME="1507"> </A> outputs the block numbers of the bad blocks it finds.
Most filesystems can avoid such bad blocks. They maintain a list
of known bad blocks, which is initialized when the filesystem is
made, and can be modified later. The initial search for bad
blocks can be done by the <tt>mkfs</tt><A NAME="1509"> </A> command (which initializes
the filesystem), but later checks should be done with
<tt>badblocks</tt><A NAME="1511"> </A> and the new blocks should be added with
<tt>fsck</tt><A NAME="1513"> </A>. We'll describe <tt>mkfs</tt><A NAME="1515"> </A> and <tt>fsck</tt><A NAME="1517"> </A> later.
<P>
Many modern disks automatically notice bad blocks, and attempt
to fix them by using a special, reserved good block instead.
This is invisible to the operating system. This feature should
be documented in the disk's manual, if you're curious if it
is happening. Even such disks can fail, if the number of bad
blocks grows too large, although chances are that by then the disk
will be so rotten as to be unusable.
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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