mirror of
https://github.com/RsyncProject/rsync.git
synced 2026-05-31 02:09:48 -04:00
0af88421dc
Fold the standalone rsync-web repo into the rsync source tree as rsync-web/, eliminating the sibling-checkout convention and the drift it causes between the release-time HTML snapshot in ../release/rsync-html and the source of truth in ../rsync-web. Flat-copy import (no git history merge). The standalone repo at github.com/RsyncProject/rsync-web is retained for historical reference and will be archived once the in-tree copy proves itself. Add /rsync-web/ to .gitattributes with export-ignore so the website content does not bloat the release source tarball produced by 'git archive' in packaging/release.py step_7_tarball. A follow-up commit repoints HTML_SRC in packaging/release.py at the new in-tree location.
120 lines
4.2 KiB
HTML
120 lines
4.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<!--Converted with LaTeX2HTML 98.1p1 release (March 2nd, 1998)
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originally by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds
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* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan
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* with significant contributions from:
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Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
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<HTML>
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<HEAD>
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<TITLE>The problem</TITLE>
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<META NAME="description" CONTENT="The problem">
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<META NAME="keywords" CONTENT="tech_report">
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<META NAME="resource-type" CONTENT="document">
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<META NAME="distribution" CONTENT="global">
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<LINK REL="STYLESHEET" HREF="tech_report.css">
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<LINK REL="next" HREF="node2.html">
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<LINK REL="previous" HREF="tech_report.html">
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<LINK REL="up" HREF="tech_report.html">
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<LINK REL="next" HREF="node2.html">
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</HEAD>
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<BODY >
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<!--Navigation Panel-->
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<A NAME="tex2html22"
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HREF="node2.html">
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<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
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SRC="next.gif"></A>
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<A NAME="tex2html20"
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HREF="tech_report.html">
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<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
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SRC="up.gif"></A>
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<A NAME="tex2html14"
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HREF="tech_report.html">
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<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
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SRC="previous.gif"></A>
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<BR>
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<B> Next:</B> <A NAME="tex2html23"
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HREF="node2.html">The rsync algorithm</A>
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<B> Up:</B> <A NAME="tex2html21"
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HREF="tech_report.html">The rsync algorithm</A>
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<B> Previous:</B> <A NAME="tex2html15"
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HREF="tech_report.html">The rsync algorithm</A>
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<BR>
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<BR>
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<!--End of Navigation Panel-->
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<H1><A NAME="SECTION00010000000000000000">
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The problem</A>
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</H1>
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<P>
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Imagine you have two files, <I>A</I> and <I>B</I>, and you wish to update <I>B</I> to be
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the same as <I>A</I>. The obvious method is to copy <I>A</I> onto <I>B</I>.
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<P>
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Now imagine that the two files are on machines connected by a slow
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communications link, for example a dial up IP link. If <I>A</I> is large,
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copying <I>A</I> onto <I>B</I> will be slow. To make it faster you could
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compress <I>A</I> before sending it, but that will usually only gain a
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factor of 2 to 4.
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<P>
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Now assume that <I>A</I> and <I>B</I> are quite similar, perhaps both derived
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from the same original file. To really speed things up you would need
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to take advantage of this similarity. A common method is to send just
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the differences between <I>A</I> and <I>B</I> down the link and then use this
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list of differences to reconstruct the file.
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<P>
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The problem is that the normal methods for creating a set of
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differences between two files rely on being able to read both files.
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Thus they require that both files are available beforehand at one end
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of the link. If they are not both available on the same machine,
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these algorithms cannot be used (once you had copied the file over,
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you wouldn't need the differences). This is the problem that rsync
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addresses.
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<P>
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The rsync algorithm efficiently computes which parts of a source file
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match some part of an existing destination file. These parts need not
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be sent across the link; all that is needed is a reference to the part
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of the destination file. Only parts of the source file which are not
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matched in this way need to be sent verbatim. The receiver can then
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construct a copy of the source file using the references to parts of
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the existing destination file and the verbatim material.
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<P>
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Trivially, the data sent to the receiver can be compressed using any
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of a range of common compression algorithms, for further speed
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improvements.
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<P>
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<HR>
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<!--Navigation Panel-->
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<A NAME="tex2html22"
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HREF="node2.html">
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<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
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SRC="next.gif"></A>
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<A NAME="tex2html20"
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HREF="tech_report.html">
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<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
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SRC="up.gif"></A>
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<A NAME="tex2html14"
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HREF="tech_report.html">
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<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
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SRC="previous.gif"></A>
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<BR>
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<B> Next:</B> <A NAME="tex2html23"
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HREF="node2.html">The rsync algorithm</A>
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<B> Up:</B> <A NAME="tex2html21"
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HREF="tech_report.html">The rsync algorithm</A>
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<B> Previous:</B> <A NAME="tex2html15"
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HREF="tech_report.html">The rsync algorithm</A>
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<!--End of Navigation Panel-->
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<ADDRESS>
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<I>Andrew Tridgell</I>
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<BR><I>1998-11-09</I>
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</ADDRESS>
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</BODY>
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</HTML>
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