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diff --git a/sag-0.6.1-www/Invisible/sag-0.6/node42.html b/sag-0.6.1-www/Invisible/sag-0.6/node42.html new file mode 100644 index 0000000..3a78689 --- /dev/null +++ b/sag-0.6.1-www/Invisible/sag-0.6/node42.html @@ -0,0 +1,91 @@ +<!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>What are filesystems?</TITLE> +<META NAME="description" CONTENT="What are filesystems?"> +<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="tex2html849" HREF="node43.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html847" HREF="node41.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html841" HREF="node41.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html851" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html852" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html850" HREF="node43.html">Filesystems galore</A> +<B>Up:</B> <A NAME="tex2html848" HREF="node41.html">Filesystems</A> +<B> Previous:</B> <A NAME="tex2html842" HREF="node41.html">Filesystems</A> +<BR> <P> +<H2><A NAME="SECTION00581000000000000000">What are filesystems?</A></H2> +<P> + A <b>filesystem</b> is the methods and data structures that an + operating system uses to keep track of files on a disk or partition; + that is, the way the + files are organized on the disk. The word is also used to + refer to a partition or disk that is used to store the files + or the type of the filesystem. Thus, one might say ``I have + two filesystems'' meaning one has two partitions on which one + stores files, or that one is using the ``extended + filesystem'', meaning the type of the filesystem. +<P> + The difference between a disk or partition and the filesystem + it contains is important. A few programs (including, + reasonably enough, programs that create filesystems) operate + directly on the raw sectors of a disk or partition; if there + is an existing file system there it will be destroyed or + seriously corrupted. Most programs operate on a filesystem, + and therefore won't work on a partition that doesn't contain + one (or that contains one of the wrong type). +<P> + Before a partition or disk can be used as a filesystem, it + needs to be initialized, and the bookkeeping data structures need + to be written to the disk. This process is called + <b>making a filesystem</b>. +<P> + Most UNIX filesystem types have a similar general structure, + although the exact details vary quite a bit. The central + concepts are <b>superblock</b>, <b>inode</b>, <b>data block</b>, + <b>directory block</b>, and <b>indirection block</b>. The + superblock contains information about the filesystem as a + whole, such as its size (the exact information here depends + on the filesystem). An inode contains all information about + a file, except its name. The name is stored in the directory, + together with the number of the inode. A directory entry consists + of a filename and the number of the inode which represents the + file. The inode contains the numbers of several data blocks, + which are used to store the data in the file. There is space + only for a few data block numbers in the inode, however, and if more + are needed, more space for pointers to the data blocks is allocated + dynamically. These dynamically allocated blocks are indirect + blocks; the name indicates that in order to find the data block, + one has to find its number in the indirect block first. +<P> + UNIX filesystems usually allow one to create a <b>hole</b> + in a file (this is done with <tt>lseek</tt><A NAME="1557"> </A>; check the manual + page), which means that the filesystem just pretends that at a + particular place in the file there is just zero bytes, but no + actual disk sectors are reserved for that place in the file + (this means that the file will use a bit less disk + space). This happens especially often for small binaries, Linux + shared libraries, some databases, and a few other special cases. + (Holes are + implemented by storing a special value as the address of the + data block in the indirect block or inode. This special address + means that no data block is allocated for that part of the file, + ergo, there is a hole in the file.) +<P> + Holes are moderately useful. On the author's system, a simple + measurement showed a potential for about 4 MB of savings through + holes of about 200 MB total used disk space. That system, however, + contains relatively few programs and no database files. The + measurement tool is described in appendix <A HREF="node111.html#chapmeasureholes">A</A>. +<P> +<HR><A NAME="tex2html849" HREF="node43.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html847" HREF="node41.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html841" HREF="node41.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html851" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html852" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html850" HREF="node43.html">Filesystems galore</A> +<B>Up:</B> <A NAME="tex2html848" HREF="node41.html">Filesystems</A> +<B> Previous:</B> <A NAME="tex2html842" HREF="node41.html">Filesystems</A> +<P><ADDRESS> +<I>Lars Wirzenius <BR> +Sat Nov 15 02:32:11 EET 1997</I> +</ADDRESS> +</BODY> +</HTML> |