Mercurial_ The Definitive Guide - Bryan O'Sullivan [9]
Before you read on, please understand that this section necessarily reflects my own experiences, interests, and (dare I say it) biases. I have used every one of the revision control tools listed below, in most cases for several years at a time.
Subversion
Subversion is a popular revision control tool, developed to replace CVS. It has a centralized client/server architecture.
Subversion and Mercurial have similarly named commands for performing 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.
Prior to version 1.5, Subversion had no useful support for merges. At the time of writing, its merge tracking capability is new, and known to be complicated and buggy.
Mercurial has a substantial performance advantage over Subversion on every revision control operation I have benchmarked. I have measured its advantage as ranging from a factor of two to a factor of six when compared with Subversion 1.4.3’s ra_local file store, which is the fastest access method available. In more realistic deployments 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 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 (status) and displaying modifications against the current revision (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.
Git
Git is a distributed revision control tool that was developed for managing the Linux kernel source tree. Like Mercurial, its early design was somewhat influenced by Monotone (described at the end of this chapter).
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