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diff --git a/sag-0.6.1-www/sag-0.6.1.html/x747.html b/sag-0.6.1-www/sag-0.6.1.html/x747.html new file mode 100644 index 0000000..1efe4a4 --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/x747.html @@ -0,0 +1,406 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>Hard disks</TITLE +><META +NAME="GENERATOR" +CONTENT="Modular DocBook HTML Stylesheet"><LINK +REL="HOME" +TITLE="The Linux System Administrators' Guide" +HREF="book1.html"><LINK +REL="UP" +TITLE="Using Disks and Other Storage Media" +HREF="c701.html"><LINK +REL="PREVIOUS" +TITLE="Using Disks and Other Storage Media" +HREF="c701.html"><LINK +REL="NEXT" +TITLE="Floppies" +HREF="x787.html"></HEAD +><BODY +BGCOLOR="#FFFFFF" +TEXT="#000000" +><DIV +CLASS="NAVHEADER" +><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TH +COLSPAN="3" +ALIGN="center" +>The Linux System Administrators' Guide</TH +></TR +><TR +><TD +WIDTH="10%" +ALIGN="left" +VALIGN="bottom" +><A +HREF="c701.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +>Chapter 4. Using Disks and Other Storage Media</TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="x787.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><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 +></TR +></TABLE +><DIV +CLASS="NAVFOOTER" +><HR +ALIGN="LEFT" +WIDTH="100%"><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +><A +HREF="c701.html" +>Prev</A +></TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="book1.html" +>Home</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +><A +HREF="x787.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>Using Disks and Other Storage Media</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="c701.html" +>Up</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>Floppies</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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