A User’s Guide to Darcs 2

I’ve been slowly working to push all my systems to using darcs 2. Darcs 2 has some major new features and more importantly some great new ways to get more performance out of darcs. However, to truly make use of these improvements there are a few changes from a darcs 1 setup that are quick to make and well worth the effort.

Enable a Global Cache

Before anything else you should enable a global cache, as this one of the biggest performance enhancing tools of darcs 2 and you’ll benefit the most from the following suggestions if you enable the global cache first. The global cache acts as a giant patch pool where darcs first looks for a patch when grabbing new patches, thus you want it to be on the same file system as your repositories. On file systems that support the cached patches are going to be hardlinked (the patch is only stored once, but represented in multiple places) across all of your repositories.

To enable a Global Cache:

$ mkdir -p $HOME/.darcs/cache
$ echo cache:$HOME/.darcs/cache > $HOME/.darcs/sources

In XP or Vista you can run the same commands from cmd.exe (Command Prompt) ignore the $ prompts and drop the -p from the mkdir, replacing $HOME with C:\Documents and Settings\*Username* or C:\Users\*Username*, respectively.

There are some other advanced things you can do in a sources file, such as create per-repository caches, read-only caches and even set a primary source repository above any used in a darcs get or darcs pull command.

Grab Hashed Repositories

Once you’ve got a global cache set up the fastest way to start making good use of it is to start working with hashed repositories. In addition to making use of the global cache and automatic “lazy” loading of patches, hashed repositories have better patch and pristine management, making darcs repositories safer from corruption and bad tools.

To get a hashed version of a darcs 1 repository simply:

$ darcs get --hashed old-repo new-repo

To get the most from darcs 2 you may want to convert, at the very least, all of your local working versions of darcs 1 repositories to hashed repositories. Darcs 2 can push and send from a hashed repository to a non-hashed “classic” darcs 1 repository, without any problems. You just have to be aware that if you share a hashed repository other people accessing the hashed repository will need darcs 2. You can use get or put to create a non-hashed copy for publishing to darcs 1 users.

You can initialize a new repository in the hashed format by darcs init —hashed, but if you are starting a new project and expect it to be used entirely by darcs 2 users your best bet is to use the new darcs 2 format.

Use Darcs 2 Format

For new projects it makes a good amount of sense to require darcs 2 for all developers and to make new repositories in darcs 2’s new format. The darcs 2 format fixes some long-standing darcs 1 format issues, at the expense of direct interoperation with darcs 1 installations. If you must support darcs 1 users, use the hashed format above as much as possible. If you are starting a new project, push developers to darcs 2 and use darcs 2 format. Starting a new darcs-2 format repository is easy and quick:

$ darcs init --darcs-2

Convert Old Repositories to Darcs 2

Once all developers on your project have darcs 2 installed, it’s worth considering converting active repositories to darcs 2 format as well. Conversion is not trivial, but it’s mostly painless. The biggest issue is that conversion can only be done once for each project (as conversion results in new versions of some patches that won’t convert the same more than once). Your best bet is to take a branch that is the largest superset of your project and convert it, recreating the mainline and other subset branches from it. It may be a good time to re-evaluate some of your extent branches, before converting, and deprecate or merge them all into your biggest unstable branch.

Conversion is a very simple command in darcs 2, and it will warn of the above problem that projects should only be converted once (and all at once):

$ darcs convert old-repo new-repo


Darcs 2 provides some new useful tools and it might take a short bit to become accustomed to them.

Format Darcs 1 push/pull Darcs 2 push/pull
darcs-1 Yes Yes
hashed No Yes
darcs-2 No Yes
Can Be Converted To
Format darcs-1 hashed darcs-2
darcs-1 Yes Yes (Once!)
hashed Yes Yes (Once!)
darcs-2 No No
5 years, 8 months ago

Posted on August 4, 2008 @11:41. Last Updated on August 7, 2008 @04:55.

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