# HG changeset patch # User Bryan O'Sullivan # Date 1197659324 28800 # Node ID 4700dd38384cdf9654789e8883b1d11655d915d9 # Parent 3c14c304d8234b30e908b2972113762a9d76924a Bring the comparison section up to date and more into balance. diff -r 3c14c304d823 -r 4700dd38384c en/intro.tex --- a/en/intro.tex Tue Dec 11 09:31:58 2007 -0800 +++ b/en/intro.tex Fri Dec 14 11:08:44 2007 -0800 @@ -306,7 +306,7 @@ fall over under the combined load of just a few dozen concurrent users. Once again, the typical response tends to be an expensive and clunky replication facility. Since the load on a central server---if -you have one at all---is orders of magnitude lower with a distributed +you have one at all---is many times lower with a distributed tool (because all of the data is replicated everywhere), a single cheap server can handle the needs of a much larger team, and replication to balance load becomes a simple matter of scripting. @@ -362,20 +362,24 @@ have used every one of the revision control tools listed below, in most cases for several years at a time. + \subsection{Subversion} Subversion is a popular revision control tool, developed to replace CVS. It has a centralised client/server architecture. Subversion and Mercurial have similarly named commands for performing -the same operations, so it is easy for a person who is familiar with -one to learn to use the other. Both tools are portable to all popular +the same operations, so if you're familiar with one, it is easy to +learn to use the other. Both tools are portable to all popular operating systems. Subversion lacks a history-aware merge capability, forcing its users to manually track exactly which revisions have been merged between branches. If users fail to do this, or make mistakes, they face the prospect of manually resolving merges with unnecessary conflicts. +Subversion also fails to merge changes when files or directories are +renamed. Subversion's poor merge support is its single biggest +weakness. Mercurial has a substantial performance advantage over Subversion on every revision control operation I have benchmarked. I have measured @@ -385,24 +389,44 @@ involving a network-based store, Subversion will be at a substantially larger disadvantage. Because many Subversion commands must talk to the server and Subversion does not have useful replication facilities, -server capacity becomes a bottleneck for modestly large projects. - -Additionally, Subversion incurs a substantial storage overhead to -avoid network transactions for a few common operations, such as -finding modified files (\texttt{status}) and displaying modifications -against the current revision (\texttt{diff}). A Subversion working -copy is, as a result, often approximately the same size as, or larger -than, a Mercurial repository and working directory, even though the -Mercurial repository contains a complete history of the project. - -Subversion is currently more widely supported by -revision-control-aware third party tools than is Mercurial, although -this gap is closing. Like Mercurial, Subversion has an excellent user -manual. - -Several tools exist to accurately and completely import revision -history from a Subversion repository into a Mercurial repository, -making the transition from the older tool relatively painless. +server capacity and network bandwidth become bottlenecks for modestly +large projects. + +Additionally, Subversion incurs substantial storage overhead to avoid +network transactions for a few common operations, such as finding +modified files (\texttt{status}) and displaying modifications against +the current revision (\texttt{diff}). As a result, a Subversion +working copy is often the same size as, or larger than, a Mercurial +repository and working directory, even though the Mercurial repository +contains a complete history of the project. + +Subversion is widely supported by third party tools. Mercurial +currently lags considerably in this area. This gap is closing, +however, and indeed some of Mercurial's GUI tools now outshine their +Subversion equivalents. Like Mercurial, Subversion has an excellent +user manual. + +Because Subversion doesn't store revision history on the client, it is +well suited to managing projects that deal with lots of large, opaque +binary files. If you check in fifty revisions to an incompressible +10MB file, Subversion's client-side space usage stays constant The +space used by any distributed SCM will grow rapidly in proportion to +the number of revisions, because the differences between each revision +are large. + +In addition, it's often difficult or, more usually, impossible to +merge different versions of a binary file. Subversion's ability to +let a user lock a file, so that they temporarily have the exclusive +right to commit changes to it, can be a significant advantage to a +project where binary files are widely used. + +Mercurial can import revision history from a Subversion repository. +It can also export revision history to a Subversion repository. This +makes it easy to ``test the waters'' and use Mercurial and Subversion +in parallel before deciding to switch. History conversion is +incremental, so you can perform an initial conversion, then small +additional conversions afterwards to bring in new changes. + \subsection{Git} @@ -410,15 +434,16 @@ managing the Linux kernel source tree. Like Mercurial, its early design was somewhat influenced by Monotone. -Git has an overwhelming command set, with version~1.5.0 providing~139 -individual commands. It has a reputation for being difficult to -learn. It does not have a user manual, only documentation for -individual commands. - -In terms of performance, git is extremely fast. There are several -common cases in which it is faster than Mercurial, at least on Linux. -However, its performance on (and general support for) Windows is, at -the time of writing, far behind that of Mercurial. +Git has a very large command set, with version~1.5.0 providing~139 +individual commands. It has something of a reputation for being +difficult to learn. Compared to Git, Mercurial has a strong focus on +simplicity. + +In terms of performance, Git is extremely fast. In several cases, it +is faster than Mercurial, at least on Linux, while Mercurial performs +better on other operations. However, on Windows, the performance and +general level of support that Git provides is, at the time of writing, +far behind that of Mercurial. While a Mercurial repository needs no maintenance, a Git repository requires frequent manual ``repacks'' of its metadata. Without these, @@ -432,14 +457,17 @@ The core of Git is written in C. Many Git commands are implemented as shell or Perl scripts, and the quality of these scripts varies widely. -I have encountered a number of instances where scripts charged along +I have encountered several instances where scripts charged along blindly in the presence of errors that should have been fatal. +Mercurial can import revision history from a Git repository. + + \subsection{CVS} CVS is probably the most widely used revision control tool in the -world. Due to its age and internal untidiness, it has been -essentially unmaintained for many years. +world. Due to its age and internal untidiness, it has been only +lightly maintained for many years. It has a centralised client/server architecture. It does not group related file changes into atomic commits, making it easy for people to @@ -470,6 +498,9 @@ remained locked for over a decade are just two of the less interesting problems I can recall from personal experience). +Mercurial can import revision history from a CVS repository. + + \subsection{Commercial tools} Perforce has a centralised client/server architecture, with no @@ -482,6 +513,54 @@ Modestly large Perforce installations require the deployment of proxies to cope with the load their users generate. + +\subsection{Choosing a revision control tool} + +With the exception of CVS, all of the tools listed above have unique +strengths that suit them to particular styles of work. There is no +single revision control tool that is best in all situations. + +As an example, Subversion is a good choice for working with frequently +edited binary files, due to its centralised nature and support for +file locking. If you're averse to the command line, it currently has +better GUI support than other free revision control tools. However, +its poor merging is a substantial liability for busy projects with +overlapping development. + +I personally find Mercurial's properties of simplicity, performance, +and good merge support to be a compelling combination that has served +me well for several years. + + +\section{Switching from another tool to Mercurial} + +Mercurial is bundled with an extension named \hgext{convert}, which +can incrementally import revision history from several other revision +control tools. By ``incremental'', I mean that you can convert all of +a project's history to date in one go, then rerun the conversion later +to obtain new changes that happened after the initial conversion. + +The revision control tools supported by \hgext{convert} are as +follows: +\begin{itemize} +\item Subversion +\item CVS +\item Git +\item Darcs +\end{itemize} + +In addition, \hgext{convert} can export changes from Mercurial to +Subversion. This makes it possible to try Subversion and Mercurial in +parallel before committing to a switchover, without risking the loss +of any work. + +The \hgxcmd{conver}{convert} command is easy to use. Simply point it +at the path or URL of the source repository, optionally give it the +name of the destination repository, and it will start working. After +the initial conversion, just run the same command again to import new +changes. + + %%% Local Variables: %%% mode: latex %%% TeX-master: "00book"