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diff --git a/sag-0.6.1-www/Invisible/sag-0.6/node30.html b/sag-0.6.1-www/Invisible/sag-0.6/node30.html new file mode 100644 index 0000000..49dc7ab --- /dev/null +++ b/sag-0.6.1-www/Invisible/sag-0.6/node30.html @@ -0,0 +1,150 @@ +<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> +<!--Converted with LaTeX2HTML 96.1-h (September 30, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --> +<HTML> +<HEAD> +<TITLE>Hard disks</TITLE> +<META NAME="description" CONTENT="Hard disks"> +<META NAME="keywords" CONTENT="sag"> +<META NAME="resource-type" CONTENT="document"> +<META NAME="distribution" CONTENT="global"> +<LINK REL=STYLESHEET HREF="sag.css"> +</HEAD> +<BODY LANG="EN" > + <A NAME="tex2html692" HREF="node31.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html690" HREF="node28.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html684" HREF="node29.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html694" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html695" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html693" HREF="node31.html">Floppies</A> +<B>Up:</B> <A NAME="tex2html691" HREF="node28.html">Using Disks and Other </A> +<B> Previous:</B> <A NAME="tex2html685" HREF="node29.html">Two kinds of devices</A> +<BR> <P> +<H1><A NAME="SECTION00520000000000000000">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> + See figure <A HREF="node30.html#fighdschematic">4.1</A> for a schematic picture of the + important parts in a hard disk. A hard disk consists of one or + more circular <b>platters</b>,<A NAME="tex2html11" HREF="footnode.html#1028"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> of which either or both <b>surfaces</b> are coated with a + magnetic substance used for recording the data. For each + surface, there is a <b>read-write head</b> 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> + The processor (CPU) and the actual disk communicate through + a <b>disk controller</b>. 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> + 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> + The surfaces are usually divided into concentric rings, called + <b>tracks</b>, and these in turn are divided into <b>sectors</b>. + 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> + <P><A NAME="1038"> </A><A NAME="fighdschematic"> </A><IMG WIDTH=367 HEIGHT=410 ALIGN=BOTTOM ALT="figure1034" SRC="img4.gif"><BR> +<STRONG>Figure 4.1:</STRONG> A schematic picture of a hard disk.<BR> +<P> +<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 <b>cylinder</b>. 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 <b>fragmented</b>. +<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 <b>geometry</b> of a hard disk. The + geometry is usually stored in a special, battery-powered memory + location called the <b>CMOS RAM</b>, from where the operating + system can fetch it during bootup or driver initialization. +<P> + Unfortunately, the BIOS<A NAME="tex2html12" HREF="footnode.html#1045"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></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 + <b>translates the addresses</b> 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="tex2html13" HREF="footnode.html#1047"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></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> + 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> + 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> + 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>/dev/hda</tt><A NAME="1460"> </A>, <tt>/dev/hdb</tt><A NAME="1462"> </A>, + <tt>/dev/hdc</tt><A NAME="1464"> </A>, and <tt>/dev/hdd</tt><A NAME="1466"> </A>, respectively. SCSI + hard disks are known as <tt>/dev/sda</tt><A NAME="1468"> </A>, <tt>/dev/sdb</tt><A NAME="1470"> </A>, and + so on. Similar naming conventions exist for other hard + disk types; see [<A HREF="node113.html#device-list">Anv</A>] 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> +<HR><A NAME="tex2html692" HREF="node31.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html690" HREF="node28.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html684" HREF="node29.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html694" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html695" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html693" HREF="node31.html">Floppies</A> +<B>Up:</B> <A NAME="tex2html691" HREF="node28.html">Using Disks and Other </A> +<B> Previous:</B> <A NAME="tex2html685" HREF="node29.html">Two kinds of devices</A> +<P><ADDRESS> +<I>Lars Wirzenius <BR> +Sat Nov 15 02:32:11 EET 1997</I> +</ADDRESS> +</BODY> +</HTML> |