hgbook / en / ch04-concepts.xml

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<!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->

<chapter id="chap:concepts">
  <?dbhtml filename="behind-the-scenes.html"?>
  <title>Behind the scenes</title>

  <para id="x_2e8">Unlike many revision control systems, the concepts
    upon which Mercurial is built are simple enough that it's easy to
    understand how the software really works.  Knowing these details
    certainly isn't necessary, so it is certainly safe to skip this
    chapter.  However, I think you will get more out of the software
    with a <quote>mental model</quote> of what's going on.</para>

  <para id="x_2e9">Being able to understand what's going on behind the
    scenes gives me confidence that Mercurial has been carefully
    designed to be both <emphasis>safe</emphasis> and
    <emphasis>efficient</emphasis>.  And just as importantly, if it's
    easy for me to retain a good idea of what the software is doing
    when I perform a revision control task, I'm less likely to be
    surprised by its behavior.</para>

  <para id="x_2ea">In this chapter, we'll initially cover the core concepts
    behind Mercurial's design, then continue to discuss some of the
    interesting details of its implementation.</para>

  <sect1>
    <title>Mercurial's historical record</title>

    <sect2>
      <title>Tracking the history of a single file</title>

      <para id="x_2eb">When Mercurial tracks modifications to a file, it stores
	the history of that file in a metadata object called a
	<emphasis>filelog</emphasis>.  Each entry in the filelog
	contains enough information to reconstruct one revision of the
	file that is being tracked.  Filelogs are stored as files in
	the <filename role="special"
	  class="directory">.hg/store/data</filename> directory.  A
	filelog contains two kinds of information: revision data, and
	an index to help Mercurial to find a revision
	efficiently.</para>

      <para id="x_2ec">A file that is large, or has a lot of history, has its
	filelog stored in separate data
	(<quote><literal>.d</literal></quote> suffix) and index
	(<quote><literal>.i</literal></quote> suffix) files.  For
	small files without much history, the revision data and index
	are combined in a single <quote><literal>.i</literal></quote>
	file.  The correspondence between a file in the working
	directory and the filelog that tracks its history in the
	repository is illustrated in <xref
	  linkend="fig:concepts:filelog"/>.</para>

      <figure id="fig:concepts:filelog">
	<title>Relationships between files in working directory and
	  filelogs in repository</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/filelog.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

    </sect2>
    <sect2>
      <title>Managing tracked files</title>

      <para id="x_2ee">Mercurial uses a structure called a
	<emphasis>manifest</emphasis> to collect together information
	about the files that it tracks.  Each entry in the manifest
	contains information about the files present in a single
	changeset.  An entry records which files are present in the
	changeset, the revision of each file, and a few other pieces
	of file metadata.</para>

    </sect2>
    <sect2>
      <title>Recording changeset information</title>

      <para id="x_2ef">The <emphasis>changelog</emphasis> contains information
	about each changeset.  Each revision records who committed a
	change, the changeset comment, other pieces of
	changeset-related information, and the revision of the
	manifest to use.</para>

    </sect2>
    <sect2>
      <title>Relationships between revisions</title>

      <para id="x_2f0">Within a changelog, a manifest, or a filelog, each
	revision stores a pointer to its immediate parent (or to its
	two parents, if it's a merge revision).  As I mentioned above,
	there are also relationships between revisions
	<emphasis>across</emphasis> these structures, and they are
	hierarchical in nature.</para>

      <para id="x_2f1">For every changeset in a repository, there is exactly one
	revision stored in the changelog.  Each revision of the
	changelog contains a pointer to a single revision of the
	manifest.  A revision of the manifest stores a pointer to a
	single revision of each filelog tracked when that changeset
	was created.  These relationships are illustrated in
	<xref linkend="fig:concepts:metadata"/>.</para>

      <figure id="fig:concepts:metadata">
	<title>Metadata relationships</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/metadata.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_2f3">As the illustration shows, there is
	<emphasis>not</emphasis> a <quote>one to one</quote>
	relationship between revisions in the changelog, manifest, or
	filelog. If a file that
	Mercurial tracks hasn't changed between two changesets, the
	entry for that file in the two revisions of the manifest will
	point to the same revision of its filelog<footnote>
	  <para id="x_725">It is possible (though unusual) for the manifest to
	    remain the same between two changesets, in which case the
	    changelog entries for those changesets will point to the
	    same revision of the manifest.</para>
	</footnote>.</para>

    </sect2>
  </sect1>
  <sect1>
    <title>Safe, efficient storage</title>

    <para id="x_2f4">The underpinnings of changelogs, manifests, and filelogs are
      provided by a single structure called the
      <emphasis>revlog</emphasis>.</para>

    <sect2>
      <title>Efficient storage</title>

      <para id="x_2f5">The revlog provides efficient storage of revisions using a
	<emphasis>delta</emphasis> mechanism.  Instead of storing a
	complete copy of a file for each revision, it stores the
	changes needed to transform an older revision into the new
	revision.  For many kinds of file data, these deltas are
	typically a fraction of a percent of the size of a full copy
	of a file.</para>

      <para id="x_2f6">Some obsolete revision control systems can only work with
	deltas of text files.  They must either store binary files as
	complete snapshots or encoded into a text representation, both
	of which are wasteful approaches.  Mercurial can efficiently
	handle deltas of files with arbitrary binary contents; it
	doesn't need to treat text as special.</para>

    </sect2>
    <sect2 id="sec:concepts:txn">
      <title>Safe operation</title>

      <para id="x_2f7">Mercurial only ever <emphasis>appends</emphasis> data to
	the end of a revlog file. It never modifies a section of a
	file after it has written it.  This is both more robust and
	efficient than schemes that need to modify or rewrite
	data.</para>

      <para id="x_2f8">In addition, Mercurial treats every write as part of a
	<emphasis>transaction</emphasis> that can span a number of
	files.  A transaction is <emphasis>atomic</emphasis>: either
	the entire transaction succeeds and its effects are all
	visible to readers in one go, or the whole thing is undone.
	This guarantee of atomicity means that if you're running two
	copies of Mercurial, where one is reading data and one is
	writing it, the reader will never see a partially written
	result that might confuse it.</para>

      <para id="x_2f9">The fact that Mercurial only appends to files makes it
	easier to provide this transactional guarantee.  The easier it
	is to do stuff like this, the more confident you should be
	that it's done correctly.</para>

    </sect2>
    <sect2>
      <title>Fast retrieval</title>

      <para id="x_2fa">Mercurial cleverly avoids a pitfall common to
	all earlier revision control systems: the problem of
	<emphasis>inefficient retrieval</emphasis>. Most revision
	control systems store the contents of a revision as an
	incremental series of modifications against a
	<quote>snapshot</quote>.  (Some base the snapshot on the
	oldest revision, others on the newest.)  To reconstruct a
	specific revision, you must first read the snapshot, and then
	every one of the revisions between the snapshot and your
	target revision.  The more history that a file accumulates,
	the more revisions you must read, hence the longer it takes to
	reconstruct a particular revision.</para>

      <figure id="fig:concepts:snapshot">
	<title>Snapshot of a revlog, with incremental deltas</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/snapshot.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_2fc">The innovation that Mercurial applies to this problem is
	simple but effective.  Once the cumulative amount of delta
	information stored since the last snapshot exceeds a fixed
	threshold, it stores a new snapshot (compressed, of course),
	instead of another delta.  This makes it possible to
	reconstruct <emphasis>any</emphasis> revision of a file
	quickly.  This approach works so well that it has since been
	copied by several other revision control systems.</para>

      <para id="x_2fd"><xref linkend="fig:concepts:snapshot"/> illustrates
	the idea.  In an entry in a revlog's index file, Mercurial
	stores the range of entries from the data file that it must
	read to reconstruct a particular revision.</para>

      <sect3>
	<title>Aside: the influence of video compression</title>

	<para id="x_2fe">If you're familiar with video compression or
	  have ever watched a TV feed through a digital cable or
	  satellite service, you may know that most video compression
	  schemes store each frame of video as a delta against its
	  predecessor frame.</para>

	<para id="x_2ff">Mercurial borrows this idea to make it
	  possible to reconstruct a revision from a snapshot and a
	  small number of deltas.</para>

      </sect3>
    </sect2>
    <sect2>
      <title>Identification and strong integrity</title>

      <para id="x_300">Along with delta or snapshot information, a revlog entry
	contains a cryptographic hash of the data that it represents.
	This makes it difficult to forge the contents of a revision,
	and easy to detect accidental corruption.</para>

      <para id="x_301">Hashes provide more than a mere check against corruption;
	they are used as the identifiers for revisions.  The changeset
	identification hashes that you see as an end user are from
	revisions of the changelog.  Although filelogs and the
	manifest also use hashes, Mercurial only uses these behind the
	scenes.</para>

      <para id="x_302">Mercurial verifies that hashes are correct when it
	retrieves file revisions and when it pulls changes from
	another repository.  If it encounters an integrity problem, it
	will complain and stop whatever it's doing.</para>

      <para id="x_303">In addition to the effect it has on retrieval efficiency,
	Mercurial's use of periodic snapshots makes it more robust
	against partial data corruption.  If a revlog becomes partly
	corrupted due to a hardware error or system bug, it's often
	possible to reconstruct some or most revisions from the
	uncorrupted sections of the revlog, both before and after the
	corrupted section.  This would not be possible with a
	delta-only storage model.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Revision history, branching, and merging</title>

    <para id="x_304">Every entry in a Mercurial revlog knows the identity of its
      immediate ancestor revision, usually referred to as its
      <emphasis>parent</emphasis>.  In fact, a revision contains room
      for not one parent, but two.  Mercurial uses a special hash,
      called the <quote>null ID</quote>, to represent the idea
      <quote>there is no parent here</quote>.  This hash is simply a
      string of zeroes.</para>

    <para id="x_305">In <xref linkend="fig:concepts:revlog"/>, you can see
      an example of the conceptual structure of a revlog.  Filelogs,
      manifests, and changelogs all have this same structure; they
      differ only in the kind of data stored in each delta or
      snapshot.</para>

    <para id="x_306">The first revision in a revlog (at the bottom of the image)
      has the null ID in both of its parent slots.  For a
      <quote>normal</quote> revision, its first parent slot contains
      the ID of its parent revision, and its second contains the null
      ID, indicating that the revision has only one real parent.  Any
      two revisions that have the same parent ID are branches.  A
      revision that represents a merge between branches has two normal
      revision IDs in its parent slots.</para>

    <figure id="fig:concepts:revlog">
      <title>The conceptual structure of a revlog</title>
      <mediaobject>
	<imageobject><imagedata fileref="figs/revlog.png"/></imageobject>
	<textobject><phrase>XXX add text</phrase></textobject>
      </mediaobject>
    </figure>

  </sect1>
  <sect1>
    <title>The working directory</title>

    <para id="x_307">In the working directory, Mercurial stores a snapshot of the
      files from the repository as of a particular changeset.</para>

    <para id="x_308">The working directory <quote>knows</quote> which changeset
      it contains.  When you update the working directory to contain a
      particular changeset, Mercurial looks up the appropriate
      revision of the manifest to find out which files it was tracking
      at the time that changeset was committed, and which revision of
      each file was then current.  It then recreates a copy of each of
      those files, with the same contents it had when the changeset
      was committed.</para>

    <para id="x_309">The <emphasis>dirstate</emphasis> is a special
      structure that contains Mercurial's knowledge of the working
      directory.  It is maintained as a file named
      <filename>.hg/dirstate</filename> inside a repository.  The
      dirstate details which changeset the working directory is
      updated to, and all of the files that Mercurial is tracking in
      the working directory. It also lets Mercurial quickly notice
      changed files, by recording their checkout times and
      sizes.</para>

    <para id="x_30a">Just as a revision of a revlog has room for two parents, so
      that it can represent either a normal revision (with one parent)
      or a merge of two earlier revisions, the dirstate has slots for
      two parents.  When you use the <command role="hg-cmd">hg
	update</command> command, the changeset that you update to is
      stored in the <quote>first parent</quote> slot, and the null ID
      in the second. When you <command role="hg-cmd">hg
	merge</command> with another changeset, the first parent
      remains unchanged, and the second parent is filled in with the
      changeset you're merging with.  The <command role="hg-cmd">hg
	parents</command> command tells you what the parents of the
      dirstate are.</para>

    <sect2>
      <title>What happens when you commit</title>

      <para id="x_30b">The dirstate stores parent information for more than just
	book-keeping purposes.  Mercurial uses the parents of the
	dirstate as <emphasis>the parents of a new
	  changeset</emphasis> when you perform a commit.</para>

      <figure id="fig:concepts:wdir">
	<title>The working directory can have two parents</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/wdir.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_30d"><xref linkend="fig:concepts:wdir"/> shows the
	normal state of the working directory, where it has a single
	changeset as parent.  That changeset is the
	<emphasis>tip</emphasis>, the newest changeset in the
	repository that has no children.</para>

      <figure id="fig:concepts:wdir-after-commit">
	<title>The working directory gains new parents after a
	  commit</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/wdir-after-commit.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_30f">It's useful to think of the working directory as
	<quote>the changeset I'm about to commit</quote>.  Any files
	that you tell Mercurial that you've added, removed, renamed,
	or copied will be reflected in that changeset, as will
	modifications to any files that Mercurial is already tracking;
	the new changeset will have the parents of the working
	directory as its parents.</para>

      <para id="x_310">After a commit, Mercurial will update the
	parents of the working directory, so that the first parent is
	the ID of the new changeset, and the second is the null ID.
	This is shown in <xref
	  linkend="fig:concepts:wdir-after-commit"/>. Mercurial
	doesn't touch any of the files in the working directory when
	you commit; it just modifies the dirstate to note its new
	parents.</para>

    </sect2>
    <sect2>
      <title>Creating a new head</title>

      <para id="x_311">It's perfectly normal to update the working directory to a
	changeset other than the current tip.  For example, you might
	want to know what your project looked like last Tuesday, or
	you could be looking through changesets to see which one
	introduced a bug.  In cases like this, the natural thing to do
	is update the working directory to the changeset you're
	interested in, and then examine the files in the working
	directory directly to see their contents as they were when you
	committed that changeset.  The effect of this is shown in
	<xref linkend="fig:concepts:wdir-pre-branch"/>.</para>

      <figure id="fig:concepts:wdir-pre-branch">
	<title>The working directory, updated to an older
	  changeset</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/wdir-pre-branch.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_313">Having updated the working directory to an
	older changeset, what happens if you make some changes, and
	then commit?  Mercurial behaves in the same way as I outlined
	above.  The parents of the working directory become the
	parents of the new changeset.  This new changeset has no
	children, so it becomes the new tip.  And the repository now
	contains two changesets that have no children; we call these
	<emphasis>heads</emphasis>.  You can see the structure that
	this creates in <xref
	  linkend="fig:concepts:wdir-branch"/>.</para>

      <figure id="fig:concepts:wdir-branch">
	<title>After a commit made while synced to an older
	  changeset</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/wdir-branch.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <note>
	<para id="x_315">If you're new to Mercurial, you should keep
	  in mind a common <quote>error</quote>, which is to use the
	  <command role="hg-cmd">hg pull</command> command without any
	  options.  By default, the <command role="hg-cmd">hg
	    pull</command> command <emphasis>does not</emphasis>
	  update the working directory, so you'll bring new changesets
	  into your repository, but the working directory will stay
	  synced at the same changeset as before the pull.  If you
	  make some changes and commit afterwards, you'll thus create
	  a new head, because your working directory isn't synced to
	  whatever the current tip is.  To combine the operation of a
	  pull, followed by an update, run <command>hg pull
	    -u</command>.</para>

	<para id="x_316">I put the word <quote>error</quote> in quotes
	  because all that you need to do to rectify the situation
	  where you created a new head by accident is
	  <command role="hg-cmd">hg merge</command>, then <command
	    role="hg-cmd">hg commit</command>.  In other words, this
	  almost never has negative consequences; it's just something
	  of a surprise for newcomers.  I'll discuss other ways to
	  avoid this behavior, and why Mercurial behaves in this
	  initially surprising way, later on.</para>
      </note>

    </sect2>
    <sect2>
      <title>Merging changes</title>

      <para id="x_317">When you run the <command role="hg-cmd">hg
	  merge</command> command, Mercurial leaves the first parent
	of the working directory unchanged, and sets the second parent
	to the changeset you're merging with, as shown in <xref
	  linkend="fig:concepts:wdir-merge"/>.</para>

      <figure id="fig:concepts:wdir-merge">
	<title>Merging two heads</title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="figs/wdir-merge.png"/>
	  </imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_319">Mercurial also has to modify the working directory, to
	merge the files managed in the two changesets.  Simplified a
	little, the merging process goes like this, for every file in
	the manifests of both changesets.</para>
      <itemizedlist>
	<listitem><para id="x_31a">If neither changeset has modified a file, do
	    nothing with that file.</para>
	</listitem>
	<listitem><para id="x_31b">If one changeset has modified a file, and the
	    other hasn't, create the modified copy of the file in the
	    working directory.</para>
	</listitem>
	<listitem><para id="x_31c">If one changeset has removed a file, and the
	    other hasn't (or has also deleted it), delete the file
	    from the working directory.</para>
	</listitem>
	<listitem><para id="x_31d">If one changeset has removed a file, but the
	    other has modified the file, ask the user what to do: keep
	    the modified file, or remove it?</para>
	</listitem>
	<listitem><para id="x_31e">If both changesets have modified a file,
	    invoke an external merge program to choose the new
	    contents for the merged file.  This may require input from
	    the user.</para>
	</listitem>
	<listitem><para id="x_31f">If one changeset has modified a file, and the
	    other has renamed or copied the file, make sure that the
	    changes follow the new name of the file.</para>
	</listitem></itemizedlist>
      <para id="x_320">There are more details&emdash;merging has plenty of corner
	cases&emdash;but these are the most common choices that are
	involved in a merge.  As you can see, most cases are
	completely automatic, and indeed most merges finish
	automatically, without requiring your input to resolve any
	conflicts.</para>

      <para id="x_321">When you're thinking about what happens when you commit
	after a merge, once again the working directory is <quote>the
	  changeset I'm about to commit</quote>.  After the <command
	  role="hg-cmd">hg merge</command> command completes, the
	working directory has two parents; these will become the
	parents of the new changeset.</para>

      <para id="x_322">Mercurial lets you perform multiple merges, but
	you must commit the results of each individual merge as you
	go.  This is necessary because Mercurial only tracks two
	parents for both revisions and the working directory.  While
	it would be technically feasible to merge multiple changesets
	at once, Mercurial avoids this for simplicity.  With multi-way
	merges, the risks of user confusion, nasty conflict
	resolution, and making a terrible mess of a merge would grow
	intolerable.</para>

    </sect2>

    <sect2>
      <title>Merging and renames</title>

      <para id="x_69a">A surprising number of revision control systems pay little
	or no attention to a file's <emphasis>name</emphasis> over
	time.  For instance, it used to be common that if a file got
	renamed on one side of a merge, the changes from the other
	side would be silently dropped.</para>

      <para id="x_69b">Mercurial records metadata when you tell it to perform a
	rename or copy. It uses this metadata during a merge to do the
	right thing in the case of a merge.  For instance, if I rename
	a file, and you edit it without renaming it, when we merge our
	work the file will be renamed and have your edits
	applied.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Other interesting design features</title>

    <para id="x_323">In the sections above, I've tried to highlight some of the
      most important aspects of Mercurial's design, to illustrate that
      it pays careful attention to reliability and performance.
      However, the attention to detail doesn't stop there.  There are
      a number of other aspects of Mercurial's construction that I
      personally find interesting.  I'll detail a few of them here,
      separate from the <quote>big ticket</quote> items above, so that
      if you're interested, you can gain a better idea of the amount
      of thinking that goes into a well-designed system.</para>

    <sect2>
      <title>Clever compression</title>

      <para id="x_324">When appropriate, Mercurial will store both snapshots and
	deltas in compressed form.  It does this by always
	<emphasis>trying to</emphasis> compress a snapshot or delta,
	but only storing the compressed version if it's smaller than
	the uncompressed version.</para>

      <para id="x_325">This means that Mercurial does <quote>the right
	  thing</quote> when storing a file whose native form is
	compressed, such as a <literal>zip</literal> archive or a JPEG
	image.  When these types of files are compressed a second
	time, the resulting file is usually bigger than the
	once-compressed form, and so Mercurial will store the plain
	<literal>zip</literal> or JPEG.</para>

      <para id="x_326">Deltas between revisions of a compressed file are usually
	larger than snapshots of the file, and Mercurial again does
	<quote>the right thing</quote> in these cases.  It finds that
	such a delta exceeds the threshold at which it should store a
	complete snapshot of the file, so it stores the snapshot,
	again saving space compared to a naive delta-only
	approach.</para>

      <sect3>
	<title>Network recompression</title>

	<para id="x_327">When storing revisions on disk, Mercurial uses the
	  <quote>deflate</quote> compression algorithm (the same one
	  used by the popular <literal>zip</literal> archive format),
	  which balances good speed with a respectable compression
	  ratio.  However, when transmitting revision data over a
	  network connection, Mercurial uncompresses the compressed
	  revision data.</para>

	<para id="x_328">If the connection is over HTTP, Mercurial recompresses
	  the entire stream of data using a compression algorithm that
	  gives a better compression ratio (the Burrows-Wheeler
	  algorithm from the widely used <literal>bzip2</literal>
	  compression package).  This combination of algorithm and
	  compression of the entire stream (instead of a revision at a
	  time) substantially reduces the number of bytes to be
	  transferred, yielding better network performance over most
	  kinds of network.</para>

	<para id="x_329">If the connection is over
	  <command>ssh</command>, Mercurial
	  <emphasis>doesn't</emphasis> recompress the stream, because
	  <command>ssh</command> can already do this itself.  You can
	  tell Mercurial to always use <command>ssh</command>'s
	  compression feature by editing the
	  <filename>.hgrc</filename> file in your home directory as
	  follows.</para>

	<programlisting>[ui]
ssh = ssh -C</programlisting>

      </sect3>
    </sect2>
    <sect2>
      <title>Read/write ordering and atomicity</title>

      <para id="x_32a">Appending to files isn't the whole story when
	it comes to guaranteeing that a reader won't see a partial
	write.  If you recall <xref linkend="fig:concepts:metadata"/>,
	revisions in the changelog point to revisions in the manifest,
	and revisions in the manifest point to revisions in filelogs.
	This hierarchy is deliberate.</para>

      <para id="x_32b">A writer starts a transaction by writing filelog and
	manifest data, and doesn't write any changelog data until
	those are finished.  A reader starts by reading changelog
	data, then manifest data, followed by filelog data.</para>

      <para id="x_32c">Since the writer has always finished writing filelog and
	manifest data before it writes to the changelog, a reader will
	never read a pointer to a partially written manifest revision
	from the changelog, and it will never read a pointer to a
	partially written filelog revision from the manifest.</para>

    </sect2>
    <sect2>
      <title>Concurrent access</title>

      <para id="x_32d">The read/write ordering and atomicity guarantees mean that
	Mercurial never needs to <emphasis>lock</emphasis> a
	repository when it's reading data, even if the repository is
	being written to while the read is occurring. This has a big
	effect on scalability; you can have an arbitrary number of
	Mercurial processes safely reading data from a repository
	all at once, no matter whether it's being written to or
	not.</para>

      <para id="x_32e">The lockless nature of reading means that if you're
	sharing a repository on a multi-user system, you don't need to
	grant other local users permission to
	<emphasis>write</emphasis> to your repository in order for
	them to be able to clone it or pull changes from it; they only
	need <emphasis>read</emphasis> permission.  (This is
	<emphasis>not</emphasis> a common feature among revision
	control systems, so don't take it for granted!  Most require
	readers to be able to lock a repository to access it safely,
	and this requires write permission on at least one directory,
	which of course makes for all kinds of nasty and annoying
	security and administrative problems.)</para>

      <para id="x_32f">Mercurial uses locks to ensure that only one process can
	write to a repository at a time (the locking mechanism is safe
	even over filesystems that are notoriously hostile to locking,
	such as NFS).  If a repository is locked, a writer will wait
	for a while to retry if the repository becomes unlocked, but
	if the repository remains locked for too long, the process
	attempting to write will time out after a while. This means
	that your daily automated scripts won't get stuck forever and
	pile up if a system crashes unnoticed, for example.  (Yes, the
	timeout is configurable, from zero to infinity.)</para>

      <sect3>
	<title>Safe dirstate access</title>

	<para id="x_330">As with revision data, Mercurial doesn't take a lock to
	  read the dirstate file; it does acquire a lock to write it.
	  To avoid the possibility of reading a partially written copy
	  of the dirstate file, Mercurial writes to a file with a
	  unique name in the same directory as the dirstate file, then
	  renames the temporary file atomically to
	  <filename>dirstate</filename>.  The file named
	  <filename>dirstate</filename> is thus guaranteed to be
	  complete, not partially written.</para>

      </sect3>
    </sect2>
    <sect2>
      <title>Avoiding seeks</title>

      <para id="x_331">Critical to Mercurial's performance is the avoidance of
	seeks of the disk head, since any seek is far more expensive
	than even a comparatively large read operation.</para>

      <para id="x_332">This is why, for example, the dirstate is stored in a
	single file.  If there were a dirstate file per directory that
	Mercurial tracked, the disk would seek once per directory.
	Instead, Mercurial reads the entire single dirstate file in
	one step.</para>

      <para id="x_333">Mercurial also uses a <quote>copy on write</quote> scheme
	when cloning a repository on local storage.  Instead of
	copying every revlog file from the old repository into the new
	repository, it makes a <quote>hard link</quote>, which is a
	shorthand way to say <quote>these two names point to the same
	  file</quote>.  When Mercurial is about to write to one of a
	revlog's files, it checks to see if the number of names
	pointing at the file is greater than one.  If it is, more than
	one repository is using the file, so Mercurial makes a new
	copy of the file that is private to this repository.</para>

      <para id="x_334">A few revision control developers have pointed out that
	this idea of making a complete private copy of a file is not
	very efficient in its use of storage.  While this is true,
	storage is cheap, and this method gives the highest
	performance while deferring most book-keeping to the operating
	system.  An alternative scheme would most likely reduce
	performance and increase the complexity of the software, but
	speed and simplicity are key to the <quote>feel</quote> of
	day-to-day use.</para>

    </sect2>
    <sect2>
      <title>Other contents of the dirstate</title>

      <para id="x_335">Because Mercurial doesn't force you to tell it when you're
	modifying a file, it uses the dirstate to store some extra
	information so it can determine efficiently whether you have
	modified a file.  For each file in the working directory, it
	stores the time that it last modified the file itself, and the
	size of the file at that time.</para>

      <para id="x_336">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 <command
	  role="hg-cmd">hg copy</command> files, Mercurial updates the
	dirstate so that it knows what to do with those files when you
	commit.</para>

      <para id="x_337">The dirstate helps Mercurial to efficiently
	  check the status of files in a repository.</para>

      <itemizedlist>
	<listitem>
	  <para id="x_726">When Mercurial checks the state of a file in the
	    working directory, it first checks a file's modification
	    time against the time in the dirstate that records when
	    Mercurial last wrote the file. If the last modified time
	    is the same as the time when Mercurial wrote the file, the
	    file must not have been modified, so Mercurial does not
	    need to check any further.</para>
	</listitem>
	<listitem>
	  <para id="x_727">If the file's size has changed, the file must have
	    been modified.  If the modification time has changed, but
	    the size has not, only then does Mercurial need to
	    actually read the contents of the file to see if it has
	    changed.</para>
	</listitem>
      </itemizedlist>

      <para id="x_728">Storing the modification time and size dramatically
	reduces the number of read operations that Mercurial needs to
	perform when we run commands like <command>hg status</command>.
	This results in large performance improvements.</para>
    </sect2>
  </sect1>
</chapter>

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