Re: CVS -> SVN -> Git

Michael Haggerty <mhagger@alum.mit.edu>:

Excuse me, I missed Michael's Haggerty's original post. But as it happens I've been doing quite a lot of work with VCSes and migration tools recently and I have some opinions and experience that I think are relevant.

In slightly more detail: I just finished forward-porting sccs2rcs to Python, I've been moving some Subversion-hosted stuff to Mercurial, and I've been thinking about writing a simple rcs2svn because the last time I tried using cvs2svn on a large RCS history (the Jargon File, as it happens, a couple years back) it did a very poor job of coalescing related commits without the CVS metadata. I'm going to hope 2.0.0 has fixed that; I'll experiment and see.

Also, I'm in the process of rewriting Emacs VC mode and testing it with three different VCSes. One consequence is that the Subversion support in Emacs is going to cease sucking badly in the very near future -- the VC-mode rewrite is giving VC mode the ability to make atomic fileset commits if the underlying VCS will support them. I'm putting the finishing touches on that code today, as it happens.

Another consequence is that Mercurial and git support will get really good, oh, about ten minutes after the new Subversion backend lands. The blocker on all three was the same weakness in the engine of VC mode, for whicch I was (alas) responsible as its original author and *which I have now fixed*.

So, I hear about plans to make cvs2svn generate something other than Subversion, and here's my instant reaction:

	    	       	   DON'T DO IT!

This is not because I think Subversion is some kind of final answer to the VCS problem. Fame from it -- I'm moving towards Mercurial. No, the real reason I think this would be a waste of time is subtler than that.

Subversion, by design, is very good at capturing the metadata from SCCS and RCS and the various CVS variants floating around. In fact, lifting from those into Subversion is basically lossless - the real problems are that (a) as Michael notes, the data you're losslessly lifting is scratchy, and (b) as I've noted, you have to use heuristics to coalesce file histories into changesets and those don't always make the links they should.

That being the case, two-step conversion with tools that import CVS to SVN and export from SVN to whatever actually works extremely well. I'm speaking from direct recent experience here, not just theory. In fact, it works so well well that I'm convinced a tool for direct conversion from CVS to the third-generation systems would be misplaced effort.

I'd much rather see the effort go into improving import to Subversion from CVS and older, cruftier systems. Subversion is like the Heinlein quote about low Earth orbit being halfway to anywhere -- once your code and metadata are there, export to advanced alien VCSes is easy. So, Michael; I know it's nice to think about building space probes that can go direct to the aliens -- but please concentrate on building a better heavy-lift vehicle, because low earth orbit is the hard part.