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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter>
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4 <title>Behind the scenes</title>
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5 <para>\label{chap:concepts}</para>
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6
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7 <para>Unlike many revision control systems, the concepts upon which
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8 Mercurial is built are simple enough that it's easy to understand how
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9 the software really works. Knowing this certainly isn't necessary,
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10 but I find it useful to have a <quote>mental model</quote> of what's going on.</para>
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11
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12 <para>This understanding gives me confidence that Mercurial has been
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13 carefully designed to be both <emphasis>safe</emphasis> and <emphasis>efficient</emphasis>. And
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14 just as importantly, if it's easy for me to retain a good idea of what
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15 the software is doing when I perform a revision control task, I'm less
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16 likely to be surprised by its behaviour.</para>
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17
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18 <para>In this chapter, we'll initially cover the core concepts behind
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19 Mercurial's design, then continue to discuss some of the interesting
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20 details of its implementation.</para>
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21
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22 <sect1>
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23 <title>Mercurial's historical record</title>
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24
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25 <sect2>
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26 <title>Tracking the history of a single file</title>
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27
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28 <para>When Mercurial tracks modifications to a file, it stores the history
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29 of that file in a metadata object called a <emphasis>filelog</emphasis>. Each entry
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30 in the filelog contains enough information to reconstruct one revision
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31 of the file that is being tracked. Filelogs are stored as files in
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32 the <filename role="special" class="directory">.hg/store/data</filename> directory. A filelog contains two kinds
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33 of information: revision data, and an index to help Mercurial to find
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34 a revision efficiently.</para>
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35
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36 <para>A file that is large, or has a lot of history, has its filelog stored
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37 in separate data (<quote><literal>.d</literal></quote> suffix) and index (<quote><literal>.i</literal></quote>
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38 suffix) files. For small files without much history, the revision
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39 data and index are combined in a single <quote><literal>.i</literal></quote> file. The
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40 correspondence between a file in the working directory and the filelog
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41 that tracks its history in the repository is illustrated in
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42 figure <xref linkend="fig:concepts:filelog"/>.</para>
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43
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44 <informalfigure>
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45
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46 <para> <mediaobject><imageobject><imagedata fileref="filelog"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
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47 \caption{Relationships between files in working directory and
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48 filelogs in repository}
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49 \label{fig:concepts:filelog}</para>
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50 </informalfigure>
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51
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52 </sect2>
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53 <sect2>
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54 <title>Managing tracked files</title>
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55
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56 <para>Mercurial uses a structure called a <emphasis>manifest</emphasis> to collect
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57 together information about the files that it tracks. Each entry in
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58 the manifest contains information about the files present in a single
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59 changeset. An entry records which files are present in the changeset,
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60 the revision of each file, and a few other pieces of file metadata.</para>
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61
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62 </sect2>
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63 <sect2>
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64 <title>Recording changeset information</title>
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65
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66 <para>The <emphasis>changelog</emphasis> contains information about each changeset. Each
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67 revision records who committed a change, the changeset comment, other
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68 pieces of changeset-related information, and the revision of the
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69 manifest to use.
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70 </para>
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71
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72 </sect2>
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73 <sect2>
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74 <title>Relationships between revisions</title>
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75
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76 <para>Within a changelog, a manifest, or a filelog, each revision stores a
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77 pointer to its immediate parent (or to its two parents, if it's a
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78 merge revision). As I mentioned above, there are also relationships
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79 between revisions <emphasis>across</emphasis> these structures, and they are
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80 hierarchical in nature.
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81 </para>
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82
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83 <para>For every changeset in a repository, there is exactly one revision
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84 stored in the changelog. Each revision of the changelog contains a
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85 pointer to a single revision of the manifest. A revision of the
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86 manifest stores a pointer to a single revision of each filelog tracked
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87 when that changeset was created. These relationships are illustrated
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88 in figure <xref linkend="fig:concepts:metadata"/>.
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89 </para>
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90
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91 <informalfigure>
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92
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93 <para> <mediaobject><imageobject><imagedata fileref="metadata"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
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94 <caption><para>Metadata relationships</para></caption>
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95 \label{fig:concepts:metadata}
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96 </para>
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97 </informalfigure>
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98
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99 <para>As the illustration shows, there is <emphasis>not</emphasis> a <quote>one to one</quote>
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100 relationship between revisions in the changelog, manifest, or filelog.
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101 If the manifest hasn't changed between two changesets, the changelog
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102 entries for those changesets will point to the same revision of the
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103 manifest. If a file that Mercurial tracks hasn't changed between two
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104 changesets, the entry for that file in the two revisions of the
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105 manifest will point to the same revision of its filelog.
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106 </para>
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107
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108 </sect2>
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109 </sect1>
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110 <sect1>
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111 <title>Safe, efficient storage</title>
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112
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113 <para>The underpinnings of changelogs, manifests, and filelogs are provided
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114 by a single structure called the <emphasis>revlog</emphasis>.
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115 </para>
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116
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117 <sect2>
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118 <title>Efficient storage</title>
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119
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120 <para>The revlog provides efficient storage of revisions using a
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121 <emphasis>delta</emphasis> mechanism. Instead of storing a complete copy of a file
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122 for each revision, it stores the changes needed to transform an older
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123 revision into the new revision. For many kinds of file data, these
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124 deltas are typically a fraction of a percent of the size of a full
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125 copy of a file.
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126 </para>
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127
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128 <para>Some obsolete revision control systems can only work with deltas of
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129 text files. They must either store binary files as complete snapshots
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130 or encoded into a text representation, both of which are wasteful
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131 approaches. Mercurial can efficiently handle deltas of files with
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132 arbitrary binary contents; it doesn't need to treat text as special.
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133 </para>
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134
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135 </sect2>
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136 <sect2>
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137 <title>Safe operation</title>
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138 <para>\label{sec:concepts:txn}
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139 </para>
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140
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141 <para>Mercurial only ever <emphasis>appends</emphasis> data to the end of a revlog file.
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142 It never modifies a section of a file after it has written it. This
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143 is both more robust and efficient than schemes that need to modify or
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144 rewrite data.
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145 </para>
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146
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147 <para>In addition, Mercurial treats every write as part of a
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148 <emphasis>transaction</emphasis> that can span a number of files. A transaction is
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149 <emphasis>atomic</emphasis>: either the entire transaction succeeds and its effects
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150 are all visible to readers in one go, or the whole thing is undone.
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151 This guarantee of atomicity means that if you're running two copies of
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152 Mercurial, where one is reading data and one is writing it, the reader
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153 will never see a partially written result that might confuse it.
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154 </para>
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155
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156 <para>The fact that Mercurial only appends to files makes it easier to
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157 provide this transactional guarantee. The easier it is to do stuff
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158 like this, the more confident you should be that it's done correctly.
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159 </para>
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160
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161 </sect2>
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162 <sect2>
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163 <title>Fast retrieval</title>
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164
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165 <para>Mercurial cleverly avoids a pitfall common to all earlier
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166 revision control systems: the problem of <emphasis>inefficient retrieval</emphasis>.
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167 Most revision control systems store the contents of a revision as an
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168 incremental series of modifications against a <quote>snapshot</quote>. To
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169 reconstruct a specific revision, you must first read the snapshot, and
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170 then every one of the revisions between the snapshot and your target
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171 revision. The more history that a file accumulates, the more
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172 revisions you must read, hence the longer it takes to reconstruct a
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173 particular revision.
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174 </para>
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175
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176 <informalfigure>
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177
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178 <para> <mediaobject><imageobject><imagedata fileref="snapshot"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
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179 <caption><para>Snapshot of a revlog, with incremental deltas</para></caption>
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180 \label{fig:concepts:snapshot}
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181 </para>
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182 </informalfigure>
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183
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184 <para>The innovation that Mercurial applies to this problem is simple but
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185 effective. Once the cumulative amount of delta information stored
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186 since the last snapshot exceeds a fixed threshold, it stores a new
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187 snapshot (compressed, of course), instead of another delta. This
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188 makes it possible to reconstruct <emphasis>any</emphasis> revision of a file
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189 quickly. This approach works so well that it has since been copied by
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190 several other revision control systems.
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191 </para>
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192
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193 <para>Figure <xref linkend="fig:concepts:snapshot"/> illustrates the idea. In an entry
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194 in a revlog's index file, Mercurial stores the range of entries from
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195 the data file that it must read to reconstruct a particular revision.
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196 </para>
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197
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198 <sect3>
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199 <title>Aside: the influence of video compression</title>
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200
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201 <para>If you're familiar with video compression or have ever watched a TV
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202 feed through a digital cable or satellite service, you may know that
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203 most video compression schemes store each frame of video as a delta
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204 against its predecessor frame. In addition, these schemes use
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205 <quote>lossy</quote> compression techniques to increase the compression ratio, so
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206 visual errors accumulate over the course of a number of inter-frame
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207 deltas.
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208 </para>
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209
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210 <para>Because it's possible for a video stream to <quote>drop out</quote> occasionally
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211 due to signal glitches, and to limit the accumulation of artefacts
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212 introduced by the lossy compression process, video encoders
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213 periodically insert a complete frame (called a <quote>key frame</quote>) into the
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214 video stream; the next delta is generated against that frame. This
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215 means that if the video signal gets interrupted, it will resume once
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216 the next key frame is received. Also, the accumulation of encoding
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217 errors restarts anew with each key frame.
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218 </para>
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219
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220 </sect3>
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221 </sect2>
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222 <sect2>
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223 <title>Identification and strong integrity</title>
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224
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225 <para>Along with delta or snapshot information, a revlog entry contains a
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226 cryptographic hash of the data that it represents. This makes it
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227 difficult to forge the contents of a revision, and easy to detect
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228 accidental corruption.
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229 </para>
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230
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231 <para>Hashes provide more than a mere check against corruption; they are
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232 used as the identifiers for revisions. The changeset identification
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233 hashes that you see as an end user are from revisions of the
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234 changelog. Although filelogs and the manifest also use hashes,
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235 Mercurial only uses these behind the scenes.
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236 </para>
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237
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238 <para>Mercurial verifies that hashes are correct when it retrieves file
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239 revisions and when it pulls changes from another repository. If it
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240 encounters an integrity problem, it will complain and stop whatever
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241 it's doing.
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242 </para>
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243
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244 <para>In addition to the effect it has on retrieval efficiency, Mercurial's
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245 use of periodic snapshots makes it more robust against partial data
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246 corruption. If a revlog becomes partly corrupted due to a hardware
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247 error or system bug, it's often possible to reconstruct some or most
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248 revisions from the uncorrupted sections of the revlog, both before and
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249 after the corrupted section. This would not be possible with a
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250 delta-only storage model.
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251 </para>
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252
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253 <para>\section{Revision history, branching,
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254 and merging}
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255 </para>
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256
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257 <para>Every entry in a Mercurial revlog knows the identity of its immediate
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258 ancestor revision, usually referred to as its <emphasis>parent</emphasis>. In fact,
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259 a revision contains room for not one parent, but two. Mercurial uses
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260 a special hash, called the <quote>null ID</quote>, to represent the idea <quote>there
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261 is no parent here</quote>. This hash is simply a string of zeroes.
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262 </para>
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263
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264 <para>In figure <xref linkend="fig:concepts:revlog"/>, you can see an example of the
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265 conceptual structure of a revlog. Filelogs, manifests, and changelogs
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266 all have this same structure; they differ only in the kind of data
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267 stored in each delta or snapshot.
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268 </para>
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269
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270 <para>The first revision in a revlog (at the bottom of the image) has the
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271 null ID in both of its parent slots. For a <quote>normal</quote> revision, its
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272 first parent slot contains the ID of its parent revision, and its
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273 second contains the null ID, indicating that the revision has only one
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274 real parent. Any two revisions that have the same parent ID are
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275 branches. A revision that represents a merge between branches has two
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276 normal revision IDs in its parent slots.
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277 </para>
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278
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279 <informalfigure>
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280
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281 <para> <mediaobject><imageobject><imagedata fileref="revlog"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
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282 \caption{}
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283 \label{fig:concepts:revlog}
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284 </para>
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285 </informalfigure>
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286
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287 </sect2>
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288 </sect1>
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289 <sect1>
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290 <title>The working directory</title>
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291
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292 <para>In the working directory, Mercurial stores a snapshot of the files
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293 from the repository as of a particular changeset.
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294 </para>
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295
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296 <para>The working directory <quote>knows</quote> which changeset it contains. When you
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297 update the working directory to contain a particular changeset,
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298 Mercurial looks up the appropriate revision of the manifest to find
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299 out which files it was tracking at the time that changeset was
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300 committed, and which revision of each file was then current. It then
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301 recreates a copy of each of those files, with the same contents it had
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302 when the changeset was committed.
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303 </para>
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304
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305 <para>The <emphasis>dirstate</emphasis> contains Mercurial's knowledge of the working
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306 directory. This details which changeset the working directory is
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307 updated to, and all of the files that Mercurial is tracking in the
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308 working directory.
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309 </para>
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310
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311 <para>Just as a revision of a revlog has room for two parents, so that it
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312 can represent either a normal revision (with one parent) or a merge of
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313 two earlier revisions, the dirstate has slots for two parents. When
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314 you use the <command role="hg-cmd">hg update</command> command, the changeset that you update to
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315 is stored in the <quote>first parent</quote> slot, and the null ID in the second.
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316 When you <command role="hg-cmd">hg merge</command> with another changeset, the first parent
|
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317 remains unchanged, and the second parent is filled in with the
|
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318 changeset you're merging with. The <command role="hg-cmd">hg parents</command> command tells you
|
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319 what the parents of the dirstate are.
|
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320 </para>
|
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321
|
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322 <sect2>
|
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323 <title>What happens when you commit</title>
|
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324
|
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325 <para>The dirstate stores parent information for more than just book-keeping
|
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326 purposes. Mercurial uses the parents of the dirstate as \emph{the
|
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327 parents of a new changeset} when you perform a commit.
|
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328 </para>
|
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329
|
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330 <informalfigure>
|
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331
|
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332 <para> <mediaobject><imageobject><imagedata fileref="wdir"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
|
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333 <caption><para>The working directory can have two parents</para></caption>
|
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334 \label{fig:concepts:wdir}
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335 </para>
|
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336 </informalfigure>
|
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337
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338 <para>Figure <xref linkend="fig:concepts:wdir"/> shows the normal state of the working
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339 directory, where it has a single changeset as parent. That changeset
|
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340 is the <emphasis>tip</emphasis>, the newest changeset in the repository that has no
|
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341 children.
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342 </para>
|
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343
|
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344 <informalfigure>
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345
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346 <para> <mediaobject><imageobject><imagedata fileref="wdir-after-commit"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
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347 <caption><para>The working directory gains new parents after a commit</para></caption>
|
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348 \label{fig:concepts:wdir-after-commit}
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349 </para>
|
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350 </informalfigure>
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351
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352 <para>It's useful to think of the working directory as <quote>the changeset I'm
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353 about to commit</quote>. Any files that you tell Mercurial that you've
|
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354 added, removed, renamed, or copied will be reflected in that
|
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355 changeset, as will modifications to any files that Mercurial is
|
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356 already tracking; the new changeset will have the parents of the
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357 working directory as its parents.
|
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358 </para>
|
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359
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360 <para>After a commit, Mercurial will update the parents of the working
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361 directory, so that the first parent is the ID of the new changeset,
|
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362 and the second is the null ID. This is shown in
|
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363 figure <xref linkend="fig:concepts:wdir-after-commit"/>. Mercurial doesn't touch
|
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364 any of the files in the working directory when you commit; it just
|
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365 modifies the dirstate to note its new parents.
|
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366 </para>
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367
|
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368 </sect2>
|
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369 <sect2>
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370 <title>Creating a new head</title>
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371
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372 <para>It's perfectly normal to update the working directory to a changeset
|
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373 other than the current tip. For example, you might want to know what
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374 your project looked like last Tuesday, or you could be looking through
|
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375 changesets to see which one introduced a bug. In cases like this, the
|
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376 natural thing to do is update the working directory to the changeset
|
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377 you're interested in, and then examine the files in the working
|
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378 directory directly to see their contents as they were when you
|
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379 committed that changeset. The effect of this is shown in
|
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380 figure <xref linkend="fig:concepts:wdir-pre-branch"/>.
|
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381 </para>
|
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382
|
belaran@964
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383 <informalfigure>
|
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384
|
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|
385 <para> <mediaobject><imageobject><imagedata fileref="wdir-pre-branch"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
|
belaran@964
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386 <caption><para>The working directory, updated to an older changeset</para></caption>
|
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387 \label{fig:concepts:wdir-pre-branch}
|
belaran@964
|
388 </para>
|
belaran@964
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389 </informalfigure>
|
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|
390
|
belaran@964
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391 <para>Having updated the working directory to an older changeset, what
|
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392 happens if you make some changes, and then commit? Mercurial behaves
|
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393 in the same way as I outlined above. The parents of the working
|
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394 directory become the parents of the new changeset. This new changeset
|
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|
395 has no children, so it becomes the new tip. And the repository now
|
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396 contains two changesets that have no children; we call these
|
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397 <emphasis>heads</emphasis>. You can see the structure that this creates in
|
belaran@964
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398 figure <xref linkend="fig:concepts:wdir-branch"/>.
|
belaran@964
|
399 </para>
|
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400
|
belaran@964
|
401 <informalfigure>
|
belaran@964
|
402
|
belaran@964
|
403 <para> <mediaobject><imageobject><imagedata fileref="wdir-branch"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
|
belaran@964
|
404 <caption><para>After a commit made while synced to an older changeset</para></caption>
|
belaran@964
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405 \label{fig:concepts:wdir-branch}
|
belaran@964
|
406 </para>
|
belaran@964
|
407 </informalfigure>
|
belaran@964
|
408
|
belaran@964
|
409 <note>
|
belaran@964
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410 <para> If you're new to Mercurial, you should keep in mind a common
|
belaran@964
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411 <quote>error</quote>, which is to use the <command role="hg-cmd">hg pull</command> command without any
|
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|
412 options. By default, the <command role="hg-cmd">hg pull</command> command <emphasis>does not</emphasis>
|
belaran@964
|
413 update the working directory, so you'll bring new changesets into
|
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|
414 your repository, but the working directory will stay synced at the
|
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|
415 same changeset as before the pull. If you make some changes and
|
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416 commit afterwards, you'll thus create a new head, because your
|
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417 working directory isn't synced to whatever the current tip is.
|
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418 </para>
|
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419
|
belaran@964
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420 <para> I put the word <quote>error</quote> in quotes because all that you need to do
|
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421 to rectify this situation is <command role="hg-cmd">hg merge</command>, then <command role="hg-cmd">hg commit</command>. In
|
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|
422 other words, this almost never has negative consequences; it just
|
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423 surprises people. I'll discuss other ways to avoid this behaviour,
|
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424 and why Mercurial behaves in this initially surprising way, later
|
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425 on.
|
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|
426 </para>
|
belaran@964
|
427 </note>
|
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|
428
|
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|
429 </sect2>
|
belaran@964
|
430 <sect2>
|
belaran@964
|
431 <title>Merging heads</title>
|
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|
432
|
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433 <para>When you run the <command role="hg-cmd">hg merge</command> command, Mercurial leaves the first
|
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|
434 parent of the working directory unchanged, and sets the second parent
|
belaran@964
|
435 to the changeset you're merging with, as shown in
|
belaran@964
|
436 figure <xref linkend="fig:concepts:wdir-merge"/>.
|
belaran@964
|
437 </para>
|
belaran@964
|
438
|
belaran@964
|
439 <informalfigure>
|
belaran@964
|
440
|
belaran@964
|
441 <para> <mediaobject><imageobject><imagedata fileref="wdir-merge"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
|
belaran@964
|
442 <caption><para>Merging two heads</para></caption>
|
belaran@964
|
443 \label{fig:concepts:wdir-merge}
|
belaran@964
|
444 </para>
|
belaran@964
|
445 </informalfigure>
|
belaran@964
|
446
|
belaran@964
|
447 <para>Mercurial also has to modify the working directory, to merge the files
|
belaran@964
|
448 managed in the two changesets. Simplified a little, the merging
|
belaran@964
|
449 process goes like this, for every file in the manifests of both
|
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|
450 changesets.
|
belaran@964
|
451 </para>
|
belaran@964
|
452 <itemizedlist>
|
belaran@964
|
453 <listitem><para>If neither changeset has modified a file, do nothing with that
|
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|
454 file.
|
belaran@964
|
455 </para>
|
belaran@964
|
456 </listitem>
|
belaran@964
|
457 <listitem><para>If one changeset has modified a file, and the other hasn't,
|
belaran@964
|
458 create the modified copy of the file in the working directory.
|
belaran@964
|
459 </para>
|
belaran@964
|
460 </listitem>
|
belaran@964
|
461 <listitem><para>If one changeset has removed a file, and the other hasn't (or
|
belaran@964
|
462 has also deleted it), delete the file from the working directory.
|
belaran@964
|
463 </para>
|
belaran@964
|
464 </listitem>
|
belaran@964
|
465 <listitem><para>If one changeset has removed a file, but the other has modified
|
belaran@964
|
466 the file, ask the user what to do: keep the modified file, or remove
|
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|
467 it?
|
belaran@964
|
468 </para>
|
belaran@964
|
469 </listitem>
|
belaran@964
|
470 <listitem><para>If both changesets have modified a file, invoke an external
|
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|
471 merge program to choose the new contents for the merged file. This
|
belaran@964
|
472 may require input from the user.
|
belaran@964
|
473 </para>
|
belaran@964
|
474 </listitem>
|
belaran@964
|
475 <listitem><para>If one changeset has modified a file, and the other has renamed
|
belaran@964
|
476 or copied the file, make sure that the changes follow the new name
|
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|
477 of the file.
|
belaran@964
|
478 </para>
|
belaran@964
|
479 </listitem></itemizedlist>
|
belaran@964
|
480 <para>There are more details&emdash;merging has plenty of corner cases&emdash;but
|
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|
481 these are the most common choices that are involved in a merge. As
|
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|
482 you can see, most cases are completely automatic, and indeed most
|
belaran@964
|
483 merges finish automatically, without requiring your input to resolve
|
belaran@964
|
484 any conflicts.
|
belaran@964
|
485 </para>
|
belaran@964
|
486
|
belaran@964
|
487 <para>When you're thinking about what happens when you commit after a merge,
|
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|
488 once again the working directory is <quote>the changeset I'm about to
|
belaran@964
|
489 commit</quote>. After the <command role="hg-cmd">hg merge</command> command completes, the working
|
belaran@964
|
490 directory has two parents; these will become the parents of the new
|
belaran@964
|
491 changeset.
|
belaran@964
|
492 </para>
|
belaran@964
|
493
|
belaran@964
|
494 <para>Mercurial lets you perform multiple merges, but you must commit the
|
belaran@964
|
495 results of each individual merge as you go. This is necessary because
|
belaran@964
|
496 Mercurial only tracks two parents for both revisions and the working
|
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|
497 directory. While it would be technically possible to merge multiple
|
belaran@964
|
498 changesets at once, the prospect of user confusion and making a
|
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|
499 terrible mess of a merge immediately becomes overwhelming.
|
belaran@964
|
500 </para>
|
belaran@964
|
501
|
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|
502 </sect2>
|
belaran@964
|
503 </sect1>
|
belaran@964
|
504 <sect1>
|
belaran@964
|
505 <title>Other interesting design features</title>
|
belaran@964
|
506
|
belaran@964
|
507 <para>In the sections above, I've tried to highlight some of the most
|
belaran@964
|
508 important aspects of Mercurial's design, to illustrate that it pays
|
belaran@964
|
509 careful attention to reliability and performance. However, the
|
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|
510 attention to detail doesn't stop there. There are a number of other
|
belaran@964
|
511 aspects of Mercurial's construction that I personally find
|
belaran@964
|
512 interesting. I'll detail a few of them here, separate from the <quote>big
|
belaran@964
|
513 ticket</quote> items above, so that if you're interested, you can gain a
|
belaran@964
|
514 better idea of the amount of thinking that goes into a well-designed
|
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|
515 system.
|
belaran@964
|
516 </para>
|
belaran@964
|
517
|
belaran@964
|
518 <sect2>
|
belaran@964
|
519 <title>Clever compression</title>
|
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|
520
|
belaran@964
|
521 <para>When appropriate, Mercurial will store both snapshots and deltas in
|
belaran@964
|
522 compressed form. It does this by always <emphasis>trying to</emphasis> compress a
|
belaran@964
|
523 snapshot or delta, but only storing the compressed version if it's
|
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|
524 smaller than the uncompressed version.
|
belaran@964
|
525 </para>
|
belaran@964
|
526
|
belaran@964
|
527 <para>This means that Mercurial does <quote>the right thing</quote> when storing a file
|
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|
528 whose native form is compressed, such as a <literal>zip</literal> archive or a
|
belaran@964
|
529 JPEG image. When these types of files are compressed a second time,
|
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|
530 the resulting file is usually bigger than the once-compressed form,
|
belaran@964
|
531 and so Mercurial will store the plain <literal>zip</literal> or JPEG.
|
belaran@964
|
532 </para>
|
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|
533
|
belaran@964
|
534 <para>Deltas between revisions of a compressed file are usually larger than
|
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|
535 snapshots of the file, and Mercurial again does <quote>the right thing</quote> in
|
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|
536 these cases. It finds that such a delta exceeds the threshold at
|
belaran@964
|
537 which it should store a complete snapshot of the file, so it stores
|
belaran@964
|
538 the snapshot, again saving space compared to a naive delta-only
|
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|
539 approach.
|
belaran@964
|
540 </para>
|
belaran@964
|
541
|
belaran@964
|
542 <sect3>
|
belaran@964
|
543 <title>Network recompression</title>
|
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|
544
|
belaran@964
|
545 <para>When storing revisions on disk, Mercurial uses the <quote>deflate</quote>
|
belaran@964
|
546 compression algorithm (the same one used by the popular <literal>zip</literal>
|
belaran@964
|
547 archive format), which balances good speed with a respectable
|
belaran@964
|
548 compression ratio. However, when transmitting revision data over a
|
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|
549 network connection, Mercurial uncompresses the compressed revision
|
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|
550 data.
|
belaran@964
|
551 </para>
|
belaran@964
|
552
|
belaran@964
|
553 <para>If the connection is over HTTP, Mercurial recompresses the entire
|
belaran@964
|
554 stream of data using a compression algorithm that gives a better
|
belaran@964
|
555 compression ratio (the Burrows-Wheeler algorithm from the widely used
|
belaran@964
|
556 <literal>bzip2</literal> compression package). This combination of algorithm
|
belaran@964
|
557 and compression of the entire stream (instead of a revision at a time)
|
belaran@964
|
558 substantially reduces the number of bytes to be transferred, yielding
|
belaran@964
|
559 better network performance over almost all kinds of network.
|
belaran@964
|
560 </para>
|
belaran@964
|
561
|
belaran@964
|
562 <para>(If the connection is over <command>ssh</command>, Mercurial <emphasis>doesn't</emphasis>
|
belaran@964
|
563 recompress the stream, because <command>ssh</command> can already do this
|
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|
564 itself.)
|
belaran@964
|
565 </para>
|
belaran@964
|
566
|
belaran@964
|
567 </sect3>
|
belaran@964
|
568 </sect2>
|
belaran@964
|
569 <sect2>
|
belaran@964
|
570 <title>Read/write ordering and atomicity</title>
|
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|
571
|
belaran@964
|
572 <para>Appending to files isn't the whole story when it comes to guaranteeing
|
belaran@964
|
573 that a reader won't see a partial write. If you recall
|
belaran@964
|
574 figure <xref linkend="fig:concepts:metadata"/>, revisions in the changelog point to
|
belaran@964
|
575 revisions in the manifest, and revisions in the manifest point to
|
belaran@964
|
576 revisions in filelogs. This hierarchy is deliberate.
|
belaran@964
|
577 </para>
|
belaran@964
|
578
|
belaran@964
|
579 <para>A writer starts a transaction by writing filelog and manifest data,
|
belaran@964
|
580 and doesn't write any changelog data until those are finished. A
|
belaran@964
|
581 reader starts by reading changelog data, then manifest data, followed
|
belaran@964
|
582 by filelog data.
|
belaran@964
|
583 </para>
|
belaran@964
|
584
|
belaran@964
|
585 <para>Since the writer has always finished writing filelog and manifest data
|
belaran@964
|
586 before it writes to the changelog, a reader will never read a pointer
|
belaran@964
|
587 to a partially written manifest revision from the changelog, and it will
|
belaran@964
|
588 never read a pointer to a partially written filelog revision from the
|
belaran@964
|
589 manifest.
|
belaran@964
|
590 </para>
|
belaran@964
|
591
|
belaran@964
|
592 </sect2>
|
belaran@964
|
593 <sect2>
|
belaran@964
|
594 <title>Concurrent access</title>
|
belaran@964
|
595
|
belaran@964
|
596 <para>The read/write ordering and atomicity guarantees mean that Mercurial
|
belaran@964
|
597 never needs to <emphasis>lock</emphasis> a repository when it's reading data, even
|
belaran@964
|
598 if the repository is being written to while the read is occurring.
|
belaran@964
|
599 This has a big effect on scalability; you can have an arbitrary number
|
belaran@964
|
600 of Mercurial processes safely reading data from a repository safely
|
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|
601 all at once, no matter whether it's being written to or not.
|
belaran@964
|
602 </para>
|
belaran@964
|
603
|
belaran@964
|
604 <para>The lockless nature of reading means that if you're sharing a
|
belaran@964
|
605 repository on a multi-user system, you don't need to grant other local
|
belaran@964
|
606 users permission to <emphasis>write</emphasis> to your repository in order for them
|
belaran@964
|
607 to be able to clone it or pull changes from it; they only need
|
belaran@964
|
608 <emphasis>read</emphasis> permission. (This is <emphasis>not</emphasis> a common feature among
|
belaran@964
|
609 revision control systems, so don't take it for granted! Most require
|
belaran@964
|
610 readers to be able to lock a repository to access it safely, and this
|
belaran@964
|
611 requires write permission on at least one directory, which of course
|
belaran@964
|
612 makes for all kinds of nasty and annoying security and administrative
|
belaran@964
|
613 problems.)
|
belaran@964
|
614 </para>
|
belaran@964
|
615
|
belaran@964
|
616 <para>Mercurial uses locks to ensure that only one process can write to a
|
belaran@964
|
617 repository at a time (the locking mechanism is safe even over
|
belaran@964
|
618 filesystems that are notoriously hostile to locking, such as NFS). If
|
belaran@964
|
619 a repository is locked, a writer will wait for a while to retry if the
|
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|
620 repository becomes unlocked, but if the repository remains locked for
|
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|
621 too long, the process attempting to write will time out after a while.
|
belaran@964
|
622 This means that your daily automated scripts won't get stuck forever
|
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|
623 and pile up if a system crashes unnoticed, for example. (Yes, the
|
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|
624 timeout is configurable, from zero to infinity.)
|
belaran@964
|
625 </para>
|
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626
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627 <sect3>
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628 <title>Safe dirstate access</title>
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629
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630 <para>As with revision data, Mercurial doesn't take a lock to read the
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631 dirstate file; it does acquire a lock to write it. To avoid the
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632 possibility of reading a partially written copy of the dirstate file,
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633 Mercurial writes to a file with a unique name in the same directory as
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634 the dirstate file, then renames the temporary file atomically to
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635 <filename>dirstate</filename>. The file named <filename>dirstate</filename> is thus
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636 guaranteed to be complete, not partially written.
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637 </para>
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638
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639 </sect3>
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640 </sect2>
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641 <sect2>
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642 <title>Avoiding seeks</title>
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643
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644 <para>Critical to Mercurial's performance is the avoidance of seeks of the
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645 disk head, since any seek is far more expensive than even a
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646 comparatively large read operation.
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647 </para>
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648
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649 <para>This is why, for example, the dirstate is stored in a single file. If
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650 there were a dirstate file per directory that Mercurial tracked, the
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651 disk would seek once per directory. Instead, Mercurial reads the
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652 entire single dirstate file in one step.
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653 </para>
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654
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655 <para>Mercurial also uses a <quote>copy on write</quote> scheme when cloning a
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656 repository on local storage. Instead of copying every revlog file
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657 from the old repository into the new repository, it makes a <quote>hard
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658 link</quote>, which is a shorthand way to say <quote>these two names point to the
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659 same file</quote>. When Mercurial is about to write to one of a revlog's
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660 files, it checks to see if the number of names pointing at the file is
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661 greater than one. If it is, more than one repository is using the
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662 file, so Mercurial makes a new copy of the file that is private to
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663 this repository.
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664 </para>
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665
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666 <para>A few revision control developers have pointed out that this idea of
|
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667 making a complete private copy of a file is not very efficient in its
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668 use of storage. While this is true, storage is cheap, and this method
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669 gives the highest performance while deferring most book-keeping to the
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670 operating system. An alternative scheme would most likely reduce
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671 performance and increase the complexity of the software, each of which
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672 is much more important to the <quote>feel</quote> of day-to-day use.
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673 </para>
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674
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675 </sect2>
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676 <sect2>
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677 <title>Other contents of the dirstate</title>
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678
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679 <para>Because Mercurial doesn't force you to tell it when you're modifying a
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680 file, it uses the dirstate to store some extra information so it can
|
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681 determine efficiently whether you have modified a file. For each file
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682 in the working directory, it stores the time that it last modified the
|
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683 file itself, and the size of the file at that time.
|
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684 </para>
|
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685
|
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686 <para>When you explicitly <command role="hg-cmd">hg add</command>, <command role="hg-cmd">hg remove</command>, <command role="hg-cmd">hg rename</command> or
|
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687 <command role="hg-cmd">hg copy</command> files, Mercurial updates the dirstate so that it knows
|
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688 what to do with those files when you commit.
|
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689 </para>
|
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690
|
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691 <para>When Mercurial is checking the states of files in the working
|
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692 directory, it first checks a file's modification time. If that has
|
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693 not changed, the file must not have been modified. If the file's size
|
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694 has changed, the file must have been modified. If the modification
|
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695 time has changed, but the size has not, only then does Mercurial need
|
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696 to read the actual contents of the file to see if they've changed.
|
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697 Storing these few extra pieces of information dramatically reduces the
|
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698 amount of data that Mercurial needs to read, which yields large
|
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699 performance improvements compared to other revision control systems.
|
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700 </para>
|
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701
|
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702 </sect2>
|
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703 </sect1>
|
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704 </chapter>
|
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705
|
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706 <!--
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707 local variables:
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708 sgml-parent-document: ("00book.xml" "book" "chapter")
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709 end:
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710 --> |