Formatting 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.
The terminology is a bit confusing here: in MS-DOS and MS
Windows, 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
low-level formatting, while making the
filesystem is called high-level formatting.
In UNIX circles, the two are called formatting and making a
filesystem, so that's what is used in this book as well.
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.
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.
During formatting one might encounter bad spots on the
disk, called bad blocks or bad
sectors. 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.
Floppies are formatted with fdformat. 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:
$ fdformat /dev/fd0H1440
Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 kB.
Formatting ... done
Verifying ... done
$
|
Note that if you want to use an autodetecting device (e.g.,
/dev/fd0), you
must set
the parameters of the device with
setfdprm first.
To achieve the same effect as above, one would have to do the
following:
$ setfdprm /dev/fd0 1440/1440
$ fdformat /dev/fd0
Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 kB.
Formatting ... done
Verifying ... done
$
|
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.
fdformat 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),
fdformat 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
syslog is being used, to the file
/usr/log/messages. fdformat
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).
$ fdformat /dev/fd0H1440
Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 kB.
Formatting ... done
Verifying ... read: Unknown error
$
|
The
badblocks 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.
$ badblocks /dev/fd0H1440 1440
718
719
$
|
badblocks 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 initialised when the
filesystem is made, and can be modified later. The initial search
for bad blocks can be done by the
mkfs command
(which initialises the filesystem), but later checks should be done
with
badblocks and the new blocks should be added
with
fsck. We'll describe
mkfs
and
fsck later.
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.
