hgbook
view es/concepts.tex @ 409:04ba1c7785ae
Fixing build prevention, goofy translator. Taking a new chapter
| author | Igor TAmara <igor@tamarapatino.org> |
|---|---|
| date | Sun Nov 09 23:50:07 2008 -0500 (2008-11-09) |
| parents | 0f2170fcb79a |
| children | 7c84967093e1 |
line source
1 \chapter{Tras bambalinas}
2 \label{chap:concepts}
4 A diferencia de varios sistemas de control de revisiones, los
5 conceptos en los que se fundamenta Mercurial son lo suficientemente
6 simples como para entender fácilmente cómo funciona el software.
7 Saber esto no es necesario, pero considero útil tener un ``modelo
8 mental'' de qué es lo que sucede.
10 Comprender esto me da la confianza de que Mercurial ha sido
11 cuidadosamente diseñado para ser tanto \emph{seguro} como
12 \emph{eficiente}. Y tal vez con la misma importancia, si es fácil
13 para mí hacerme a una idea adecuada de qué está haciendo el software
14 cuando llevo a cabo una tarea relacionada con control de revisiones,
15 es menos probable que me sosprenda su comportamiento.
17 En este capítulo, cubriremos inicialmente los conceptos centrales
18 del diseño de Mercurial, y luego discutiremos algunos detalles
19 interesantes de su implementación.
21 \section{Registro del historial de Mercurial}
23 \subsection{Seguir el historial de un único fichero}
25 Cuando Mercurial sigue las modificaciones a un fichero, guarda el
26 historial de dicho fichero en un objeto de metadatos llamado
27 \emph{filelog}\ndt{Fichero de registro}. Cada entrada en el fichero
28 de registro contiene suficiente información para reconstruir una
29 revisión del fichero que se está siguiendo. Los ficheros de registro
30 son almacenados como ficheros el el directorio
31 \sdirname{.hg/store/data}. Un fichero de registro contiene dos tipos
32 de información: datos de revisiones, y un índice para ayudar a
33 Mercurial a buscar revisiones eficientemente.
35 El fichero de registro de un fichero grande, o con un historial muy
36 largo, es guardado como ficheros separados para datos (sufijo
37 ``\texttt{.d}'') y para el índice (sufijo ``\texttt{.i}''). Para
38 ficheros pequeños con un historial pequeño, los datos de revisiones y
39 el índice son combinados en un único fichero ``\texttt{.i}''. La
40 correspondencia entre un fichero en el directorio de trabajo y el
41 fichero de registro que hace seguimiento a su historial en el
42 repositorio se ilustra en la figura~\ref{fig:concepts:filelog}.
44 \begin{figure}[ht]
45 \centering
46 \grafix{filelog}
47 \caption{Relación entre ficheros en el directorio de trabajo y
48 ficheros de registro en el repositorio}
49 \label{fig:concepts:filelog}
50 \end{figure}
52 \subsection{Administración de ficheros monitoreados}
54 Mercurial usa una estructura llamada \emph{manifiesto} para
55 % TODO collect together => centralizar
56 centralizar la información que maneja acerca de los ficheros que
57 monitorea. Cada entrada en el manifiesto contiene información acerca
58 de los ficheros involucrados en un único conjunto de cambios. Una
59 entrada registra qué ficheros están presentes en el conjunto de
60 cambios, la revisión de cada fichero, y otros cuantos metadatos del
61 mismo.
63 \subsection{Registro de información del conjunto de cambios}
65 La \emph{bitácora de cambios} contiene información acerca de cada
66 conjunto de cambios. Cada revisión indica quién consignó un cambio, el
67 comentario para el conjunto de cambios, otros datos relacionados con
68 el conjunto de cambios, y la revisión del manifiesto a usar.
70 \subsection{Relaciones entre revisiones}
72 Dentro de una bitácora de cambios, un manifiesto, o un fichero de
73 registro, cada revisión conserva un apuntador a su padre inmediato
74 (o sus dos padres, si es la revisión de una fusión). Como menciońe
75 anteriormente, también hay relaciones entre revisiones \emph{a través}
76 de estas estructuras, y tienen naturaleza jerárquica.
78 Por cada conjunto de cambios en un repositorio, hay exactamente una
79 revisión almacenada en la bitácora de cambios. Cada revisión de la
80 bitácora de cambios contiene un apuntador a una única revisión del
81 manifiesto. Una revisión del manifiesto almacena un apuntador a una
82 única revisión de cada fichero de registro al que se le hacía
83 seguimiento cuando fue creado el conjunto de cambios. Estas relaciones
84 se ilustran en la figura~\ref{fig:concepts:metadata}.
86 \begin{figure}[ht]
87 \centering
88 \grafix{metadata}
89 \caption{Relaciones entre metadatos}
90 \label{fig:concepts:metadata}
91 \end{figure}
93 Como lo muestra la figura, \emph{no} hay una relación ``uno a uno''
94 entre las revisiones en el conjunto de cambios, el manifiesto, o el
95 fichero de registro. Si el manifiesto no ha sido modificado de un
96 conjunto de cambios a otro, las entradas en la bitácora de cambios
97 para esos conjuntos de cambios apuntarán a la misma revisión del
98 manifiesto. Si un fichero monitoreado por Mercurial no sufre ningún
99 cambio de un conjunto de cambios a otro, la entrada para dicho fichero
100 en las dos revisiones del manifiesto apuntará a la misma revisión de
101 su fichero de registro.
103 \section{Almacenamiento seguro y eficiente}
105 La base común de las bitácoras de cambios, los manifiestos, y los
106 ficheros de registros es provista por una única estructura llamada el
107 \emph{revlog}\ndt{Contracción de \emph{revision log}, registro de
108 revisión.}.
110 \subsection{Almacenamiento eficiente}
112 El revlog provee almacenamiento eficiente de revisiones por medio del
113 mecanismo de \emph{deltas}\ndt{Diferencias.}. En vez de almacenar una
114 copia completa del fichero por cada revisión, almacena los cambios
115 necesarios para transformar una revisión anterior en la nueva
116 revisión. Para muchos tipos de fichero, estos deltas son típicamente
117 de una fracción porcentual del tamaño de una copia completa del
118 fichero.
120 Algunos sistemas de control de revisiones obsoletos sólo pueden
121 manipular deltas de ficheros de texto plano. Ellos o bien almacenan
122 los ficheros binarios como instantáneas completas, o codificados en
123 alguna representación de texto plano adecuada, y ambas alternativas
124 son enfoques que desperdician bastantes recursos. Mercurial puede
125 manejar deltas de ficheros con contenido binario arbitrario; no
126 necesita tratar el texto plano como un caso especial.
128 \subsection{Safe operation}
129 \label{sec:concepts:txn}
131 Mercurial only ever \emph{appends} data to the end of a revlog file.
132 It never modifies a section of a file after it has written it. This
133 is both more robust and efficient than schemes that need to modify or
134 rewrite data.
136 In addition, Mercurial treats every write as part of a
137 \emph{transaction} that can span a number of files. A transaction is
138 \emph{atomic}: either the entire transaction succeeds and its effects
139 are all visible to readers in one go, or the whole thing is undone.
140 This guarantee of atomicity means that if you're running two copies of
141 Mercurial, where one is reading data and one is writing it, the reader
142 will never see a partially written result that might confuse it.
144 The fact that Mercurial only appends to files makes it easier to
145 provide this transactional guarantee. The easier it is to do stuff
146 like this, the more confident you should be that it's done correctly.
148 \subsection{Fast retrieval}
150 Mercurial cleverly avoids a pitfall common to all earlier
151 revision control systems: the problem of \emph{inefficient retrieval}.
152 Most revision control systems store the contents of a revision as an
153 incremental series of modifications against a ``snapshot''. To
154 reconstruct a specific revision, you must first read the snapshot, and
155 then every one of the revisions between the snapshot and your target
156 revision. The more history that a file accumulates, the more
157 revisions you must read, hence the longer it takes to reconstruct a
158 particular revision.
160 \begin{figure}[ht]
161 \centering
162 \grafix{snapshot}
163 \caption{Snapshot of a revlog, with incremental deltas}
164 \label{fig:concepts:snapshot}
165 \end{figure}
167 The innovation that Mercurial applies to this problem is simple but
168 effective. Once the cumulative amount of delta information stored
169 since the last snapshot exceeds a fixed threshold, it stores a new
170 snapshot (compressed, of course), instead of another delta. This
171 makes it possible to reconstruct \emph{any} revision of a file
172 quickly. This approach works so well that it has since been copied by
173 several other revision control systems.
175 Figure~\ref{fig:concepts:snapshot} illustrates the idea. In an entry
176 in a revlog's index file, Mercurial stores the range of entries from
177 the data file that it must read to reconstruct a particular revision.
179 \subsubsection{Aside: the influence of video compression}
181 If you're familiar with video compression or have ever watched a TV
182 feed through a digital cable or satellite service, you may know that
183 most video compression schemes store each frame of video as a delta
184 against its predecessor frame. In addition, these schemes use
185 ``lossy'' compression techniques to increase the compression ratio, so
186 visual errors accumulate over the course of a number of inter-frame
187 deltas.
189 Because it's possible for a video stream to ``drop out'' occasionally
190 due to signal glitches, and to limit the accumulation of artefacts
191 introduced by the lossy compression process, video encoders
192 periodically insert a complete frame (called a ``key frame'') into the
193 video stream; the next delta is generated against that frame. This
194 means that if the video signal gets interrupted, it will resume once
195 the next key frame is received. Also, the accumulation of encoding
196 errors restarts anew with each key frame.
198 \subsection{Identification and strong integrity}
200 Along with delta or snapshot information, a revlog entry contains a
201 cryptographic hash of the data that it represents. This makes it
202 difficult to forge the contents of a revision, and easy to detect
203 accidental corruption.
205 Hashes provide more than a mere check against corruption; they are
206 used as the identifiers for revisions. The changeset identification
207 hashes that you see as an end user are from revisions of the
208 changelog. Although filelogs and the manifest also use hashes,
209 Mercurial only uses these behind the scenes.
211 Mercurial verifies that hashes are correct when it retrieves file
212 revisions and when it pulls changes from another repository. If it
213 encounters an integrity problem, it will complain and stop whatever
214 it's doing.
216 In addition to the effect it has on retrieval efficiency, Mercurial's
217 use of periodic snapshots makes it more robust against partial data
218 corruption. If a revlog becomes partly corrupted due to a hardware
219 error or system bug, it's often possible to reconstruct some or most
220 revisions from the uncorrupted sections of the revlog, both before and
221 after the corrupted section. This would not be possible with a
222 delta-only storage model.
224 \section{Revision history, branching,
225 and merging}
227 Every entry in a Mercurial revlog knows the identity of its immediate
228 ancestor revision, usually referred to as its \emph{parent}. In fact,
229 a revision contains room for not one parent, but two. Mercurial uses
230 a special hash, called the ``null ID'', to represent the idea ``there
231 is no parent here''. This hash is simply a string of zeroes.
233 In figure~\ref{fig:concepts:revlog}, you can see an example of the
234 conceptual structure of a revlog. Filelogs, manifests, and changelogs
235 all have this same structure; they differ only in the kind of data
236 stored in each delta or snapshot.
238 The first revision in a revlog (at the bottom of the image) has the
239 null ID in both of its parent slots. For a ``normal'' revision, its
240 first parent slot contains the ID of its parent revision, and its
241 second contains the null ID, indicating that the revision has only one
242 real parent. Any two revisions that have the same parent ID are
243 branches. A revision that represents a merge between branches has two
244 normal revision IDs in its parent slots.
246 \begin{figure}[ht]
247 \centering
248 \grafix{revlog}
249 \caption{}
250 \label{fig:concepts:revlog}
251 \end{figure}
253 \section{The working directory}
255 In the working directory, Mercurial stores a snapshot of the files
256 from the repository as of a particular changeset.
258 The working directory ``knows'' which changeset it contains. When you
259 update the working directory to contain a particular changeset,
260 Mercurial looks up the appropriate revision of the manifest to find
261 out which files it was tracking at the time that changeset was
262 committed, and which revision of each file was then current. It then
263 recreates a copy of each of those files, with the same contents it had
264 when the changeset was committed.
266 The \emph{dirstate} contains Mercurial's knowledge of the working
267 directory. This details which changeset the working directory is
268 updated to, and all of the files that Mercurial is tracking in the
269 working directory.
271 Just as a revision of a revlog has room for two parents, so that it
272 can represent either a normal revision (with one parent) or a merge of
273 two earlier revisions, the dirstate has slots for two parents. When
274 you use the \hgcmd{update} command, the changeset that you update to
275 is stored in the ``first parent'' slot, and the null ID in the second.
276 When you \hgcmd{merge} with another changeset, the first parent
277 remains unchanged, and the second parent is filled in with the
278 changeset you're merging with. The \hgcmd{parents} command tells you
279 what the parents of the dirstate are.
281 \subsection{What happens when you commit}
283 The dirstate stores parent information for more than just book-keeping
284 purposes. Mercurial uses the parents of the dirstate as \emph{the
285 parents of a new changeset} when you perform a commit.
287 \begin{figure}[ht]
288 \centering
289 \grafix{wdir}
290 \caption{The working directory can have two parents}
291 \label{fig:concepts:wdir}
292 \end{figure}
294 Figure~\ref{fig:concepts:wdir} shows the normal state of the working
295 directory, where it has a single changeset as parent. That changeset
296 is the \emph{tip}, the newest changeset in the repository that has no
297 children.
299 \begin{figure}[ht]
300 \centering
301 \grafix{wdir-after-commit}
302 \caption{The working directory gains new parents after a commit}
303 \label{fig:concepts:wdir-after-commit}
304 \end{figure}
306 It's useful to think of the working directory as ``the changeset I'm
307 about to commit''. Any files that you tell Mercurial that you've
308 added, removed, renamed, or copied will be reflected in that
309 changeset, as will modifications to any files that Mercurial is
310 already tracking; the new changeset will have the parents of the
311 working directory as its parents.
313 After a commit, Mercurial will update the parents of the working
314 directory, so that the first parent is the ID of the new changeset,
315 and the second is the null ID. This is shown in
316 figure~\ref{fig:concepts:wdir-after-commit}. Mercurial doesn't touch
317 any of the files in the working directory when you commit; it just
318 modifies the dirstate to note its new parents.
320 \subsection{Creating a new head}
322 It's perfectly normal to update the working directory to a changeset
323 other than the current tip. For example, you might want to know what
324 your project looked like last Tuesday, or you could be looking through
325 changesets to see which one introduced a bug. In cases like this, the
326 natural thing to do is update the working directory to the changeset
327 you're interested in, and then examine the files in the working
328 directory directly to see their contents as they werea when you
329 committed that changeset. The effect of this is shown in
330 figure~\ref{fig:concepts:wdir-pre-branch}.
332 \begin{figure}[ht]
333 \centering
334 \grafix{wdir-pre-branch}
335 \caption{The working directory, updated to an older changeset}
336 \label{fig:concepts:wdir-pre-branch}
337 \end{figure}
339 Having updated the working directory to an older changeset, what
340 happens if you make some changes, and then commit? Mercurial behaves
341 in the same way as I outlined above. The parents of the working
342 directory become the parents of the new changeset. This new changeset
343 has no children, so it becomes the new tip. And the repository now
344 contains two changesets that have no children; we call these
345 \emph{heads}. You can see the structure that this creates in
346 figure~\ref{fig:concepts:wdir-branch}.
348 \begin{figure}[ht]
349 \centering
350 \grafix{wdir-branch}
351 \caption{After a commit made while synced to an older changeset}
352 \label{fig:concepts:wdir-branch}
353 \end{figure}
355 \begin{note}
356 If you're new to Mercurial, you should keep in mind a common
357 ``error'', which is to use the \hgcmd{pull} command without any
358 options. By default, the \hgcmd{pull} command \emph{does not}
359 update the working directory, so you'll bring new changesets into
360 your repository, but the working directory will stay synced at the
361 same changeset as before the pull. If you make some changes and
362 commit afterwards, you'll thus create a new head, because your
363 working directory isn't synced to whatever the current tip is.
365 I put the word ``error'' in quotes because all that you need to do
366 to rectify this situation is \hgcmd{merge}, then \hgcmd{commit}. In
367 other words, this almost never has negative consequences; it just
368 surprises people. I'll discuss other ways to avoid this behaviour,
369 and why Mercurial behaves in this initially surprising way, later
370 on.
371 \end{note}
373 \subsection{Merging heads}
375 When you run the \hgcmd{merge} command, Mercurial leaves the first
376 parent of the working directory unchanged, and sets the second parent
377 to the changeset you're merging with, as shown in
378 figure~\ref{fig:concepts:wdir-merge}.
380 \begin{figure}[ht]
381 \centering
382 \grafix{wdir-merge}
383 \caption{Merging two heads}
384 \label{fig:concepts:wdir-merge}
385 \end{figure}
387 Mercurial also has to modify the working directory, to merge the files
388 managed in the two changesets. Simplified a little, the merging
389 process goes like this, for every file in the manifests of both
390 changesets.
391 \begin{itemize}
392 \item If neither changeset has modified a file, do nothing with that
393 file.
394 \item If one changeset has modified a file, and the other hasn't,
395 create the modified copy of the file in the working directory.
396 \item If one changeset has removed a file, and the other hasn't (or
397 has also deleted it), delete the file from the working directory.
398 \item If one changeset has removed a file, but the other has modified
399 the file, ask the user what to do: keep the modified file, or remove
400 it?
401 \item If both changesets have modified a file, invoke an external
402 merge program to choose the new contents for the merged file. This
403 may require input from the user.
404 \item If one changeset has modified a file, and the other has renamed
405 or copied the file, make sure that the changes follow the new name
406 of the file.
407 \end{itemize}
408 There are more details---merging has plenty of corner cases---but
409 these are the most common choices that are involved in a merge. As
410 you can see, most cases are completely automatic, and indeed most
411 merges finish automatically, without requiring your input to resolve
412 any conflicts.
414 When you're thinking about what happens when you commit after a merge,
415 once again the working directory is ``the changeset I'm about to
416 commit''. After the \hgcmd{merge} command completes, the working
417 directory has two parents; these will become the parents of the new
418 changeset.
420 Mercurial lets you perform multiple merges, but you must commit the
421 results of each individual merge as you go. This is necessary because
422 Mercurial only tracks two parents for both revisions and the working
423 directory. While it would be technically possible to merge multiple
424 changesets at once, the prospect of user confusion and making a
425 terrible mess of a merge immediately becomes overwhelming.
427 \section{Other interesting design features}
429 In the sections above, I've tried to highlight some of the most
430 important aspects of Mercurial's design, to illustrate that it pays
431 careful attention to reliability and performance. However, the
432 attention to detail doesn't stop there. There are a number of other
433 aspects of Mercurial's construction that I personally find
434 interesting. I'll detail a few of them here, separate from the ``big
435 ticket'' items above, so that if you're interested, you can gain a
436 better idea of the amount of thinking that goes into a well-designed
437 system.
439 \subsection{Clever compression}
441 When appropriate, Mercurial will store both snapshots and deltas in
442 compressed form. It does this by always \emph{trying to} compress a
443 snapshot or delta, but only storing the compressed version if it's
444 smaller than the uncompressed version.
446 This means that Mercurial does ``the right thing'' when storing a file
447 whose native form is compressed, such as a \texttt{zip} archive or a
448 JPEG image. When these types of files are compressed a second time,
449 the resulting file is usually bigger than the once-compressed form,
450 and so Mercurial will store the plain \texttt{zip} or JPEG.
452 Deltas between revisions of a compressed file are usually larger than
453 snapshots of the file, and Mercurial again does ``the right thing'' in
454 these cases. It finds that such a delta exceeds the threshold at
455 which it should store a complete snapshot of the file, so it stores
456 the snapshot, again saving space compared to a naive delta-only
457 approach.
459 \subsubsection{Network recompression}
461 When storing revisions on disk, Mercurial uses the ``deflate''
462 compression algorithm (the same one used by the popular \texttt{zip}
463 archive format), which balances good speed with a respectable
464 compression ratio. However, when transmitting revision data over a
465 network connection, Mercurial uncompresses the compressed revision
466 data.
468 If the connection is over HTTP, Mercurial recompresses the entire
469 stream of data using a compression algorithm that gives a better
470 compression ratio (the Burrows-Wheeler algorithm from the widely used
471 \texttt{bzip2} compression package). This combination of algorithm
472 and compression of the entire stream (instead of a revision at a time)
473 substantially reduces the number of bytes to be transferred, yielding
474 better network performance over almost all kinds of network.
476 (If the connection is over \command{ssh}, Mercurial \emph{doesn't}
477 recompress the stream, because \command{ssh} can already do this
478 itself.)
480 \subsection{Read/write ordering and atomicity}
482 Appending to files isn't the whole story when it comes to guaranteeing
483 that a reader won't see a partial write. If you recall
484 figure~\ref{fig:concepts:metadata}, revisions in the changelog point to
485 revisions in the manifest, and revisions in the manifest point to
486 revisions in filelogs. This hierarchy is deliberate.
488 A writer starts a transaction by writing filelog and manifest data,
489 and doesn't write any changelog data until those are finished. A
490 reader starts by reading changelog data, then manifest data, followed
491 by filelog data.
493 Since the writer has always finished writing filelog and manifest data
494 before it writes to the changelog, a reader will never read a pointer
495 to a partially written manifest revision from the changelog, and it will
496 never read a pointer to a partially written filelog revision from the
497 manifest.
499 \subsection{Concurrent access}
501 The read/write ordering and atomicity guarantees mean that Mercurial
502 never needs to \emph{lock} a repository when it's reading data, even
503 if the repository is being written to while the read is occurring.
504 This has a big effect on scalability; you can have an arbitrary number
505 of Mercurial processes safely reading data from a repository safely
506 all at once, no matter whether it's being written to or not.
508 The lockless nature of reading means that if you're sharing a
509 repository on a multi-user system, you don't need to grant other local
510 users permission to \emph{write} to your repository in order for them
511 to be able to clone it or pull changes from it; they only need
512 \emph{read} permission. (This is \emph{not} a common feature among
513 revision control systems, so don't take it for granted! Most require
514 readers to be able to lock a repository to access it safely, and this
515 requires write permission on at least one directory, which of course
516 makes for all kinds of nasty and annoying security and administrative
517 problems.)
519 Mercurial uses locks to ensure that only one process can write to a
520 repository at a time (the locking mechanism is safe even over
521 filesystems that are notoriously hostile to locking, such as NFS). If
522 a repository is locked, a writer will wait for a while to retry if the
523 repository becomes unlocked, but if the repository remains locked for
524 too long, the process attempting to write will time out after a while.
525 This means that your daily automated scripts won't get stuck forever
526 and pile up if a system crashes unnoticed, for example. (Yes, the
527 timeout is configurable, from zero to infinity.)
529 \subsubsection{Safe dirstate access}
531 As with revision data, Mercurial doesn't take a lock to read the
532 dirstate file; it does acquire a lock to write it. To avoid the
533 possibility of reading a partially written copy of the dirstate file,
534 Mercurial writes to a file with a unique name in the same directory as
535 the dirstate file, then renames the temporary file atomically to
536 \filename{dirstate}. The file named \filename{dirstate} is thus
537 guaranteed to be complete, not partially written.
539 \subsection{Avoiding seeks}
541 Critical to Mercurial's performance is the avoidance of seeks of the
542 disk head, since any seek is far more expensive than even a
543 comparatively large read operation.
545 This is why, for example, the dirstate is stored in a single file. If
546 there were a dirstate file per directory that Mercurial tracked, the
547 disk would seek once per directory. Instead, Mercurial reads the
548 entire single dirstate file in one step.
550 Mercurial also uses a ``copy on write'' scheme when cloning a
551 repository on local storage. Instead of copying every revlog file
552 from the old repository into the new repository, it makes a ``hard
553 link'', which is a shorthand way to say ``these two names point to the
554 same file''. When Mercurial is about to write to one of a revlog's
555 files, it checks to see if the number of names pointing at the file is
556 greater than one. If it is, more than one repository is using the
557 file, so Mercurial makes a new copy of the file that is private to
558 this repository.
560 A few revision control developers have pointed out that this idea of
561 making a complete private copy of a file is not very efficient in its
562 use of storage. While this is true, storage is cheap, and this method
563 gives the highest performance while deferring most book-keeping to the
564 operating system. An alternative scheme would most likely reduce
565 performance and increase the complexity of the software, each of which
566 is much more important to the ``feel'' of day-to-day use.
568 \subsection{Other contents of the dirstate}
570 Because Mercurial doesn't force you to tell it when you're modifying a
571 file, it uses the dirstate to store some extra information so it can
572 determine efficiently whether you have modified a file. For each file
573 in the working directory, it stores the time that it last modified the
574 file itself, and the size of the file at that time.
576 When you explicitly \hgcmd{add}, \hgcmd{remove}, \hgcmd{rename} or
577 \hgcmd{copy} files, Mercurial updates the dirstate so that it knows
578 what to do with those files when you commit.
580 When Mercurial is checking the states of files in the working
581 directory, it first checks a file's modification time. If that has
582 not changed, the file must not have been modified. If the file's size
583 has changed, the file must have been modified. If the modification
584 time has changed, but the size has not, only then does Mercurial need
585 to read the actual contents of the file to see if they've changed.
586 Storing these few extra pieces of information dramatically reduces the
587 amount of data that Mercurial needs to read, which yields large
588 performance improvements compared to other revision control systems.
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