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1 \chapter{Behind the scenes}
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2 \label{chap:concepts}
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3
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4 Unlike many revision control systems, the concepts upon which
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5 Mercurial is built are simple enough that it's easy to understand how
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6 the software really works. Knowing this certainly isn't necessary,
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7 but I find it useful to have a ``mental model'' of what's going on.
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8
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9 This understanding gives me confidence that Mercurial has been
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10 carefully designed to be both \emph{safe} and \emph{efficient}. And
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11 just as importantly, if it's easy for me to retain a good idea of what
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12 the software is doing when I perform a revision control task, I'm less
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13 likely to be surprised by its behaviour.
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14
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15 \section{Mercurial's historical record}
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16
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17 \subsection{Tracking the history of a single file}
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18
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19 When Mercurial tracks modifications to a file, it stores the history
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20 of that file in a metadata object called a \emph{filelog}. Each entry
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21 in the filelog contains enough information to reconstruct one revision
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22 of the file that is being tracked. Filelogs are stored as files in
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23 the \sdirname{.hg/data} directory. A filelog contains two kinds of
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24 information: revision data, and an index to help Mercurial to find a
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25 revision efficiently.
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26
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27 A file that is large, or has a lot of history, has its filelog stored
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28 in separate data (``\texttt{.d}'' suffix) and index (``\texttt{.i}''
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29 suffix) files. For small files without much history, the revision
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30 data and index are combined in a single ``\texttt{.i}'' file. The
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31 correspondence between a file in the working directory and the filelog
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32 that tracks its history in the repository is illustrated in
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33 figure~\ref{fig:concepts:filelog}.
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34
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35 \begin{figure}[ht]
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36 \centering
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37 \grafix{filelog}
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38 \caption{Relationships between files in working directory and
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39 filelogs in repository}
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40 \label{fig:concepts:filelog}
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41 \end{figure}
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42
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43 \subsection{Managing tracked files}
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44
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45 Mercurial uses a structure called a \emph{manifest} to collect
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46 together information about the files that it tracks. Each entry in
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47 the manifest contains information about the files present in a single
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48 changeset. An entry records which files are present in the changeset,
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49 the revision of each file, and a few other pieces of file metadata.
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50
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51 \subsection{Recording changeset information}
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52
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53 The \emph{changelog} contains information about each changeset. Each
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54 revision records who committed a change, the changeset comment, other
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55 pieces of changeset-related information, and the revision of the
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56 manifest to use.
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57
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58 \subsection{Relationships between revisions}
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59
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60 Within a changelog, a manifest, or a filelog, each revision stores a
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61 pointer to its immediate parent (or to its two parents, if it's a
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62 merge revision). As I mentioned above, there are also relationships
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63 between revisions \emph{across} these structures, and they are
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64 hierarchical in nature.
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65
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66 For every changeset in a repository, there is exactly one revision
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67 stored in the changelog. Each revision of the changelog contains a
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68 pointer to a single revision of the manifest. A revision of the
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69 manifest stores a pointer to a single revision of each filelog tracked
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70 when that changeset was created. These relationships are illustrated
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71 in figure~\ref{fig:concepts:metadata}.
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72
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73 \begin{figure}[ht]
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74 \centering
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75 \grafix{metadata}
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76 \caption{Metadata relationships}
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77 \label{fig:concepts:metadata}
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78 \end{figure}
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79
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80 As the illustration shows, there is \emph{not} a ``one to one''
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81 relationship between revisions in the changelog, manifest, or filelog.
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82 If the manifest hasn't changed between two changesets, the changelog
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83 entries for those changesets will point to the same revision of the
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84 manifest. If a file that Mercurial tracks hasn't changed between two
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85 changesets, the entry for that file in the two revisions of the
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86 manifest will point to the same revision of its filelog.
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87
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88 \section{Safe, efficient storage}
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89
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90 The underpinnings of changelogs, manifests, and filelogs are provided
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91 by a single structure called the \emph{revlog}.
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92
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93 \subsection{Efficient storage}
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94
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95 The revlog provides efficient storage of revisions using a
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96 \emph{delta} mechanism. Instead of storing a complete copy of a file
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97 for each revision, it stores the changes needed to transform an older
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98 revision into the new revision. For many kinds of file data, these
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99 deltas are typically a fraction of a percent of the size of a full
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100 copy of a file.
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101
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102 Some obsolete revision control systems can only work with deltas of
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103 text files. They must either store binary files as complete snapshots
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104 or encoded into a text representation, both of which are wasteful
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105 approaches. Mercurial can efficiently handle deltas of files with
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106 arbitrary binary contents; it doesn't need to treat text as special.
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107
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108 \subsection{Safe operation}
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109
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110 Mercurial only ever \emph{appends} data to the end of a revlog file.
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111 It never modifies a section of a file after it has written it. This
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112 is both more robust and efficient than schemes that need to modify or
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113 rewrite data.
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114
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115 In addition, Mercurial treats every write as part of a
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116 \emph{transaction} that can span a number of files. A transaction is
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117 \emph{atomic}: either the entire transaction succeeds and its effects
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118 are all visible to readers in one go, or the whole thing is undone.
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119 This guarantee of atomicity means that if you're running two copies of
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120 Mercurial, where one is reading data and one is writing it, the reader
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121 will never see a partially written result that might confuse it.
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122
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123 The fact that Mercurial only appends to files makes it easier to
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124 provide this transactional guarantee. The easier it is to do stuff
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125 like this, the more confident you should be that it's done correctly.
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126
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127 \subsection{Fast retrieval}
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128
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129 Mercurial cleverly avoids a pitfall common to all earlier
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130 revision control systems: the problem of \emph{inefficient retrieval}.
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131 Most revision control systems store the contents of a revision as an
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132 incremental series of modifications against a ``snapshot''. To
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133 reconstruct a specific revision, you must first read the snapshot, and
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134 then every one of the revisions between the snapshot and your target
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135 revision. The more history that a file accumulates, the more
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136 revisions you must read, hence the longer it takes to reconstruct a
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137 particular revision.
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138
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139 \begin{figure}[ht]
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140 \centering
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141 \grafix{snapshot}
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142 \caption{Snapshot of a revlog, with incremental deltas}
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143 \label{fig:concepts:snapshot}
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144 \end{figure}
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145
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146 The innovation that Mercurial applies to this problem is simple but
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147 effective. Once the cumulative amount of delta information stored
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148 since the last snapshot exceeds a fixed threshold, it stores a new
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149 snapshot (compressed, of course), instead of another delta. This
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150 makes it possible to reconstruct \emph{any} revision of a file
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151 quickly. This approach works so well that it has since been copied by
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152 several other revision control systems.
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153
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154 Figure~\ref{fig:concepts:snapshot} illustrates the idea. In an entry
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155 in a revlog's index file, Mercurial stores the range of entries from
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156 the data file that it must read to reconstruct a particular revision.
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157
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158 \subsubsection{Aside: the influence of video compression}
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159
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160 If you're familiar with video compression or have ever watched a TV
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161 feed through a digital cable or satellite service, you may know that
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162 most video compression schemes store each frame of video as a delta
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163 against its predecessor frame. In addition, these schemes use
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164 ``lossy'' compression techniques to increase the compression ratio, so
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165 visual errors accumulate over the course of a number of inter-frame
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166 deltas.
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167
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168 Because it's possible for a video stream to ``drop out'' occasionally
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169 due to signal glitches, and to limit the accumulation of artefacts
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170 introduced by the lossy compression process, video encoders
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171 periodically insert a complete frame (called a ``key frame'') into the
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172 video stream; the next delta is generated against that frame. This
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173 means that if the video signal gets interrupted, it will resume once
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174 the next key frame is received. Also, the accumulation of encoding
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175 errors restarts anew with each key frame.
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176
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177 \subsection{Strong integrity}
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178
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179 Along with delta or snapshot information, a revlog entry contains a
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180 cryptographic hash of the data that it represents. This makes it
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181 difficult to forge the contents of a revision, and easy to detect
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182 accidental corruption. The hash that Mercurial uses is SHA-1, which
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183 is 160 bits long. Although all revision data is hashed, the changeset
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184 hashes that you see as an end user are from revisions of the
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185 changelog. Manifest and file hashes are only used behind the scenes.
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186
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187 Mercurial checks these hashes when retrieving file revisions and when
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188 pulling changes from a repository. If it encounters an integrity
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189 problem, it will complain and stop whatever it's doing.
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190
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191 In addition to the effect it has on retrieval efficiency, Mercurial's
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192 use of periodic snapshots makes it more robust against partial data
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193 corruption. If a revlog becomes partly corrupted due to a hardware
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194 error or system bug, it's often possible to reconstruct some or most
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195 revisions from the uncorrupted sections of the revlog, both before and
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196 after the corrupted section. This would not be possible with a
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197 delta-only storage model.
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198
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199 \section{The working directory}
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200
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201 Mercurial's good ideas are not confined to the repository; it also
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202 needs to manage the working directory. The \emph{dirstate} contains
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203 Mercurial's knowledge of the working directory. This details which
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204 revision(s) the working directory is updated to, and all files that
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205 Mercurial is tracking in the working directory.
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206
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207 Because Mercurial doesn't force you to tell it when you're modifying a
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208 file, it uses the dirstate to store some extra information so it can
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209 determine efficiently whether you have modified a file. For each file
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210 in the working directory, it stores the time that it last modified the
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211 file itself, and the size of the file at that time.
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212
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213 When Mercurial is checking the states of files in the working
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214 directory, it first checks a file's modification time. If that has
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215 not changed, the file must not have been modified. If the file's size
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216 has changed, the file must have been modified. If the modification
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217 time has changed, but the size has not, only then does Mercurial need
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218 to read the actual contents of the file to see if they've changed.
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219 Storing these few extra pieces of information dramatically reduces the
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220 amount of data that Mercurial needs to read, which yields large
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221 performance improvements compared to other revision control systems.
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222
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223 \section{Other interesting design features}
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224
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225 In the sections above, I've tried to highlight some of the most
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226 important aspects of Mercurial's design, to illustrate that it pays
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227 careful attention to reliability and performance. However, the
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228 attention to detail doesn't stop there. There are a number of other
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229 aspects of Mercurial's construction that I personally find
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230 interesting. I'll detail a few of them here, separate from the ``big
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231 ticket'' items above, so that if you're interested, you can gain a
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232 better idea of the amount of thinking that goes into a well-designed
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233 system.
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234
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235 \subsection{Clever compression}
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236
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237 When appropriate, Mercurial will store both snapshots and deltas in
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238 compressed form. It does this by always \emph{trying to} compress a
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239 snapshot or delta, but only storing the compressed version if it's
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240 smaller than the uncompressed version.
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241
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242 This means that Mercurial does ``the right thing'' when storing a file
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243 whose native form is compressed, such as a \texttt{zip} archive or a
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244 JPEG image. When these types of files are compressed a second time,
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245 the resulting file is usually bigger than the once-compressed form,
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246 and so Mercurial will store the plain \texttt{zip} or JPEG.
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247
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248 Deltas between revisions of a compressed file are usually larger than
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249 snapshots of the file, and Mercurial again does ``the right thing'' in
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250 these cases. It finds that such a delta exceeds the threshold at
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251 which it should store a complete snapshot of the file, so it stores
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252 the snapshot, again saving space compared to a naive delta-only
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253 approach.
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254
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255 \subsubsection{Network recompression}
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256
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257 When storing revisions on disk, Mercurial uses the ``deflate''
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258 compression algorithm (the same one used by the popular \texttt{zip}
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259 archive format), which balances good speed with a respectable
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260 compression ratio. However, when transmitting revision data over a
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261 network connection, Mercurial uncompresses the compressed revision
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262 data.
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263
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264 If the connection is over HTTP, Mercurial recompresses the entire
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265 stream of data using a compression algorithm that gives a etter
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266 compression ratio (the Burrows-Wheeler algorithm from the widely used
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267 \texttt{bzip2} compression package). This combination of algorithm
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268 and compression of the entire stream (instead of a revision at a time)
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269 substantially reduces the number of bytes to be transferred, yielding
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270 better network performance over almost all kinds of network.
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271
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272 (If the connection is over \command{ssh}, Mercurial \emph{doesn't}
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273 recompress the stream, because \command{ssh} can already do this
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274 itself.)
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275
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276 \subsection{Read/write ordering and atomicity}
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277
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278 Appending to files isn't the whole story when it comes to guaranteeing
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279 that a reader won't see a partial write. If you recall
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280 figure~\ref{fig:concepts:metadata}, revisions in the changelog point to
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281 revisions in the manifest, and revisions in the manifest point to
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282 revisions in filelogs. This hierarchy is deliberate.
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283
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284 A writer starts a transaction by writing filelog and manifest data,
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285 and doesn't write any changelog data until those are finished. A
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286 reader starts by reading changelog data, then manifest data, followed
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287 by filelog data.
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288
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289 Since the writer has always finished writing filelog and manifest data
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290 before it writes to the changelog, a reader will never read a pointer
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291 to a partially written manifest revision from the changelog, and it will
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292 never read a pointer to a partially written filelog revision from the
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293 manifest.
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294
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295 \subsection{Concurrent access}
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296
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297 The read/write ordering and atomicity guarantees mean that Mercurial
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298 never needs to \emph{lock} a repository when it's reading data, even
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299 if the repository is being written to while the read is occurring.
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300 This has a big effect on scalability; you can have an arbitrary number
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301 of Mercurial processes safely reading data from a repository safely
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302 all at once, no matter whether it's being written to or not.
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303
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304 The lockless nature of reading means that if you're sharing a
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305 repository on a multi-user system, you don't need to grant other local
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306 users permission to \emph{write} to your repository in order for them
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307 to be able to clone it or pull changes from it; they only need
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308 \emph{read} permission. (This is \emph{not} a common feature among
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309 revision control systems, so don't take it for granted! Most require
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310 readers to be able to lock a repository to access it safely, and this
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311 requires write permission on at least one directory, which of course
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312 makes for all kinds of nasty and annoying security and administrative
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313 problems.)
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314
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315 Mercurial uses locks to ensure that only one process can write to a
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316 repository at a time (the locking mechanism is safe even over
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317 filesystems that are notoriously hostile to locking, such as NFS). If
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318 a repository is locked, a writer will wait for a while to retry if the
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319 repository becomes unlocked, but if the repository remains locked for
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320 too long, the process attempting to write will time out after a while.
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321 This means that your daily automated scripts won't get stuck forever
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322 and pile up if a system crashes unnoticed, for example. (Yes, the
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323 timeout is configurable, from zero to infinity.)
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324
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325 \subsubsection{Safe dirstate access}
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326
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327 As with revision data, Mercurial doesn't take a lock to read the
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328 dirstate file; it does acquire a lock to write it. To avoid the
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329 possibility of reading a partially written copy of the dirstate file,
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330 Mercurial writes to a file with a unique name in the same directory as
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331 the dirstate file, then renames the temporary file atomically to
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332 \filename{dirstate}. The file named \filename{dirstate} is thus
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333 guaranteed to be complete, not partially written.
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334
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335 \subsection{Avoiding seeks}
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336
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337 Critical to Mercurial's performance is the avoidance of seeks of the
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338 disk head, since any seek is far more expensive than even a
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339 comparatively large read operation.
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340
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341 This is why, for example, the dirstate is stored in a single file. If
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342 there were a dirstate file per directory that Mercurial tracked, the
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343 disk would seek once per directory. Instead, Mercurial reads the
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344 entire single dirstate file in one step.
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345
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346 Mercurial also uses a ``copy on write'' scheme when cloning a
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347 repository on local storage. Instead of copying every revlog file
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348 from the old repository into the new repository, it makes a ``hard
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349 link'', which is a shorthand way to say ``these two names point to the
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350 same file''. When Mercurial is about to write to one of a revlog's
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351 files, it checks to see if the number of names pointing at the file is
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352 greater than one. If it is, more than one repository is using the
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353 file, so Mercurial makes a new copy of the file that is private to
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354 this repository.
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355
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356 A few revision control developers have pointed out that this idea of
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357 making a complete private copy of a file is not very efficient in its
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358 use of storage. While this is true, storage is cheap, and this method
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359 gives the highest performance while deferring most book-keeping to the
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360 operating system. An alternative scheme would most likely reduce
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361 performance and increase the complexity of the software, each of which
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362 is much more important to the ``feel'' of day-to-day use.
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363
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364 %%% Local Variables:
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365 %%% mode: latex
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366 %%% TeX-master: "00book"
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367 %%% End:
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