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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Hard disks</TITLE
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN747"
>Hard disks</A
></H1
><P
>This subsection introduces terminology related to hard
disks. If you already know the terms and concepts, you can skip
this subsection.</P
><P
>See <A
HREF="x747.html#HD-SCHEMATIC"
>Figure 4-1</A
> for a schematic picture
of the important parts in a hard disk. A hard disk consists of
one or more circular <I
CLASS="GLOSSTERM"
>platters</I
>,
<A
NAME="AEN753"
HREF="#FTN.AEN753"
>[1]</A
>
of which either or both <I
CLASS="GLOSSTERM"
>surfaces</I
> are coated
with a magnetic substance used for recording the data. For each
surface, there is a <I
CLASS="GLOSSTERM"
>read-write head</I
> that
examines or alters the recorded data. The platters rotate on
a common axis; a typical rotation speed is 3600 rotations per
minute, although high-performance hard disks have higher speeds.
The heads move along the radius of the platters; this movement
combined with the rotation of the platters allows the head to
access all parts of the surfaces.</P
><P
>The processor (CPU) and the actual disk communicate through
a <I
CLASS="GLOSSTERM"
>disk controller</I
>. This relieves the rest of the computer
from knowing how to use the drive, since the controllers for
different types of disks can be made to use the same interface
towards the rest of the computer. Therefore, the computer can
say just ``hey disk, gimme what I want'', instead of a long and
complex series of electric signals to move the head to the proper
location and waiting for the correct position to come under
the head and doing all the other unpleasant stuff necessary.
(In reality, the interface to the controller is still complex,
but much less so than it would otherwise be.) The controller
can also do some other stuff, such as caching, or automatic bad
sector replacement.</P
><P
>The above is usually all one needs to understand about the
hardware. There is also a bunch of other stuff, such as the
motor that rotates the platters and moves the heads, and the
electronics that control the operation of the mechanical
parts, but that is mostly not relevant for understanding the
working principle of a hard disk.</P
><P
>The surfaces are usually divided into concentric rings,
called <I
CLASS="GLOSSTERM"
>tracks</I
>, and these in turn are
divided into <I
CLASS="GLOSSTERM"
>sectors</I
>. This division
is used to specify locations on the hard disk and to allocate
disk space to files. To find a given place on the hard disk,
one might say ``surface 3, track 5, sector 7''. Usually the
number of sectors is the same for all tracks, but some hard disks
put more sectors in outer tracks (all sectors are of the same
physical size, so more of them fit in the longer outer tracks).
Typically, a sector will hold 512 bytes of data. The disk itself
can't handle smaller amounts of data than one sector.</P
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="HD-SCHEMATIC"
>Figure 4-1. A schematic picture of a hard disk.</A
></B
></P
><P
><IMG
SRC="hd-schematic.gif"></P
></DIV
><P
>Each surface is divided into tracks (and sectors) in
the same way. This means that when the head for one surface
is on a track, the heads for the other surfaces are also on
the corresponding tracks. All the corresponding tracks taken
together are called a <I
CLASS="GLOSSTERM"
>cylinder</I
>. It takes
time to move the heads from one track (cylinder) to another,
so by placing the data that is often accessed together (say, a
file) so that it is within one cylinder, it is not necessary to
move the heads to read all of it. This improves performance.
It is not always possible to place files like this; files
that are stored in several places on the disk are called
<I
CLASS="GLOSSTERM"
>fragmented</I
>.</P
><P
>The number of surfaces (or heads, which is the same thing),
cylinders, and sectors vary a lot; the specification of the
number of each is called the <I
CLASS="GLOSSTERM"
>geometry</I
> of a hard disk. The
geometry is usually stored in a special, battery-powered memory
location called the <I
CLASS="GLOSSTERM"
>CMOS RAM</I
>, from where the operating
system can fetch it during bootup or driver initialization.</P
><P
>Unfortunately, the BIOS
<A
NAME="AEN773"
HREF="#FTN.AEN773"
>[2]</A
>
has a design limitation, which makes it
impossible to specify a track number that is larger than 1024 in
the CMOS RAM,
which is too little for a large hard disk. To overcome this,
the hard disk controller lies about the geometry, and
<I
CLASS="GLOSSTERM"
>translates the addresses</I
> given by the computer into something
that fits reality. For example, a hard disk might have 8 heads,
2048 tracks, and 35 sectors per track.
<A
NAME="AEN776"
HREF="#FTN.AEN776"
>[3]</A
>
Its controller could lie to the computer and claim that it
has 16 heads, 1024 tracks, and 35 sectors per track, thus not
exceeding the limit on tracks, and translates the address that
the computer gives it by halving the head number, and doubling
the track number. The math can be more complicated in reality,
because the numbers are not as nice as here (but again, the
details are not relevant for understanding the principle).
This translation distorts the operating system's view of how
the disk is organized, thus making it impractical to use the
all-data-on-one-cylinder trick to boost performance.</P
><P
>The translation is only a problem for IDE disks. SCSI disks
use a sequential sector number (i.e., the controller translates
a sequential sector number to a head, cylinder, and sector
triplet), and a completely different method for the CPU to talk
with the controller, so they are insulated from the problem.
Note, however, that the computer might not know the real geometry
of an SCSI disk either.</P
><P
>Since Linux often will not know the real geometry of a disk,
its filesystems don't even try to keep files within a single
cylinder. Instead, it tries to assign sequentially numbered
sectors to files, which almost always gives similar performance.
The issue is further complicated by on-controller caches, and
automatic prefetches done by the controller.</P
><P
>Each hard disk is represented by a separate device
file. There can (usually) be only two or four IDE hard
disks. These are known as <TT
CLASS="FILENAME"
>/dev/hda</TT
>,
<TT
CLASS="FILENAME"
>/dev/hdb</TT
>, <TT
CLASS="FILENAME"
>/dev/hdc</TT
>,
and <TT
CLASS="FILENAME"
>/dev/hdd</TT
>, respectively. SCSI
hard disks are known as <TT
CLASS="FILENAME"
>/dev/sda</TT
>,
<TT
CLASS="FILENAME"
>/dev/sdb</TT
>, and so on. Similar naming
conventions exist for other hard disk types; see XXX (device
list) for more information. Note that the device files for
the hard disks give access to the entire disk, with no regard
to partitions (which will be discussed below), and it's easy to
mess up the partitions or the data in them if you aren't careful.
The disks' device files are usually used only to get access to the
master boot record (which will also be discussed below).</P
></DIV
><H3
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN753"
HREF="x747.html#AEN753"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The platters are made of a hard
substance, e.g., aluminium, which gives the hard disk
its name.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN773"
HREF="x747.html#AEN773"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The BIOS is some built-in software stored on
ROM chips. It takes care, among other things, of the
initial stages of booting.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN776"
HREF="x747.html#AEN776"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The numbers are completely
imaginary.</P
></TD
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