Re: Git: CVS to Git import

[Matthew Ogilvie <mmogilvi_git@miniinfo.net>]

On Fri, Nov 11, 2011 at 03:17:33PM -0800, Jvsrvcs wrote:
> Git:  CVS to Git import
> 
> We are moving from CVS to Git and want to know if anyone has had any
> experience there doing this and could share do's  / dont's, best practices
> when doing the initial import.

Some ideas:

I wouldn't trust "git cvsimport".  In my testing, it was actaully fairly
common for the resulting git tags and branches to be inconsistent with the
original CVS tags and branches: checking out a tag from CVS and the same
tag from GIT, the trees were often different.  See the manpage
for a list of some of the known issues. 

Use cvs2git instead.

Write up your own script to do the conversion.  Iteratively inspect
the results, find ways to fix up anything you don't like,
and re-run the script.  Any "fixups" you want should be
scripted, so that you can try different things, examine
the result.  Then when the actual "real" conversion
happens, you have a minimal amount of downtime as you your
already-tested script runs.

The exact fixups your script should do depend on your
circumstances, but in my case, some of things my script did included:

  - First, copy the CVS repository, and work with the copy:
  - Delete some ",v" files we didn't interested in importing into git for
    various reasons.
  - Tweak some CVS commit timestamps in some files (such as a version
    file), to reduce import odditities.  (The most common oddities
    resulted from an old CVS workflow that would often sequence:
    (a) checkout, (b) modify version number file, (c) build, (d) commit
    the new version number file, and (e) tag the sandbox.  It was
    was moderately common for other changes (in other files) to
    be committed between (a) and (d), which will either cause
    strange import artifacts or actually break import tools, due to
    the out-of-order timestamps.  Tweaking back the timestamp in the
    CVS file typically allows the import tool to avoid the
    oddity.  Completely cleaning this up would have been a
    lot of work, so I focused just on just improving recent
    history.)  (sed -i ...)
  - Do the bulk of the import work using cvs2git.
  - Graft on appropriate merge history (multiple parents) for
    CVS merges.  To save time, I only worried about recent merges.
     - If you have a nice consistent tag naming
       convention, there are ways to do this as part of cvs2git.
       Unfortunately, we didn't.
     - Do not refer to a previous run's commit SHA-1's; they'll
       likely change as things change.  Use CVS tags instead.
     - git rev-parse is useful for looking up current references
       to construct graft lines.
  - Use git filter-branch to both make the above grafts permanent,
    and to fix commiter/author username/email.
  - Move imported tags and branches to refs/oldcvstags/*
    and refs/oldcvsbranches/*, to bury a lot of the noise
    (automatic build tags, tags applied as part of doing a
    merge, etc) to where a normal "git clone" will not grab
    them, but they can still be fetched manually if necessary.
  - Copy/rename a few recent release tags and branches to
    normal refs/tags/* and refs/heads/*, when they are actually
    useful. (git pack-refs and sed)
  - Something like: sleep 5 ; git gc --aggressive --prune='1 second ago'

--
Matthew Ogilvie   [mmogilvi_git@miniinfo.net]