mirror of
https://github.com/RsyncProject/rsync.git
synced 2026-05-31 18:26:12 -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 The standalone repo at github.com/RsyncProject/rsync-web is retained for historical reference and will be archived once the in-tree copy proves itself.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.
146 lines
5.2 KiB
HTML
146 lines
5.2 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>Checksum searching</TITLE>
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<META NAME="description" CONTENT="Checksum searching">
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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="node5.html">
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<LINK REL="previous" HREF="node3.html">
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<LINK REL="up" HREF="tech_report.html">
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<LINK REL="next" HREF="node5.html">
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</HEAD>
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<BODY >
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<!--Navigation Panel-->
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<A NAME="tex2html52"
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HREF="node5.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="tex2html50"
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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="tex2html44"
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HREF="node3.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="tex2html53"
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HREF="node5.html">Pipelining</A>
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<B> Up:</B> <A NAME="tex2html51"
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HREF="tech_report.html">The rsync algorithm</A>
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<B> Previous:</B> <A NAME="tex2html45"
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HREF="node3.html">Rolling checksum</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="SECTION00040000000000000000">
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Checksum searching</A>
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</H1>
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<P>
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Once <IMG
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WIDTH="15" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img1.gif"
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ALT="$\alpha$">
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has received the list of checksums of the blocks of <I>B</I>,
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it must search <I>A</I> for any blocks at any offset that match the
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checksum of some block of <I>B</I>. The basic strategy is to compute the
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32-bit rolling checksum for a block of length <I>S</I> starting at each
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byte of <I>A</I> in turn, and for each checksum, search the list for a
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match. To do this our implementation uses a
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simple 3 level searching scheme.
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<P>
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The first level uses a 16-bit hash of the 32-bit rolling checksum and
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a 2<SUP>16</SUP> entry hash table. The list of checksum values (i.e., the
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checksums from the blocks of <I>B</I>) is sorted according to the 16-bit
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hash of the 32-bit rolling checksum. Each entry in the hash table
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points to the first element of the list for that hash value, or
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contains a null value if no element of the list has that hash value.
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<P>
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At each offset in the file the 32-bit rolling checksum and its 16-bit
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hash are calculated. If the hash table entry for that hash value is
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not a null value, the second level check is invoked.
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<P>
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The second level check involves scanning the sorted checksum list
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starting with the entry pointed to by the hash table entry, looking
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for an entry whose 32-bit rolling checksum matches the current value.
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The scan terminates when it reaches an entry whose 16-bit hash
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differs. If this search finds a match, the third level check is
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invoked.
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<P>
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The third level check involves calculating the strong checksum for the
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current offset in the file and comparing it with the strong checksum
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value in the current list entry. If the two strong checksums match,
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we assume that we have found a block of <I>A</I> which matches a block of
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<I>B</I>. In fact the blocks could be different, but the probability of
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this is microscopic, and in practice this is a reasonable assumption.
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<P>
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When a match is found, <IMG
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WIDTH="15" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img1.gif"
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ALT="$\alpha$">
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sends <IMG
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WIDTH="14" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img2.gif"
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ALT="$\beta$">
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the data in <I>A</I> between
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the current file offset and the end of the previous match, followed by
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the index of the block in <I>B</I> that matched. This data is sent
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immediately a match is found, which allows us to overlap the
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communication with further computation.
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<P>
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If no match is found at a given offset in the file, the rolling
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checksum is updated to the next offset and the search proceeds. If a
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match is found, the search is restarted at the end of the matched
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block. This strategy saves a considerable amount of computation for
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the common case where the two files are nearly identical. In
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addition, it would be a simple matter to encode the block indexes as
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runs, for the common case where a portion of <I>A</I> matches a series of
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blocks of <I>B</I> in order.
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<P>
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<HR>
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<!--Navigation Panel-->
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<A NAME="tex2html52"
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HREF="node5.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="tex2html50"
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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="tex2html44"
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HREF="node3.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="tex2html53"
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HREF="node5.html">Pipelining</A>
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<B> Up:</B> <A NAME="tex2html51"
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HREF="tech_report.html">The rsync algorithm</A>
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<B> Previous:</B> <A NAME="tex2html45"
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HREF="node3.html">Rolling checksum</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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