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Re: Offer of help with move to git
- From: Joseph Myers <joseph at codesourcery dot com>
- To: "Eric S. Raymond" <esr at thyrsus dot com>
- Cc: <gcc at gcc dot gnu dot org>
- Date: Mon, 24 Aug 2015 16:54:51 +0000
- Subject: Re: Offer of help with move to git
- Authentication-results: sourceware.org; auth=none
- References: <20150823144340 dot GA7448 at thyrsus dot com> <alpine dot DEB dot 2 dot 10 dot 1508241350060 dot 19249 at digraph dot polyomino dot org dot uk> <20150824163157 dot GB23430 at thyrsus dot com>
On Mon, 24 Aug 2015, Eric S. Raymond wrote:
> Joseph Myers <email@example.com>:
> > Hence my suggestion in <https://gcc.gnu.org/ml/gcc/2015-08/msg00150.html>
> > of reconverting and then combining with the existing git-svn history via
> > renaming all the refs in the existing git repository, so as to preserve
> > the validity of commit references and git-only branches there while having
> > the main copy of the history properly converted.
> Sorry, but I can't even imagine how to recombine in that way with the tools
> I have. If you still think it's worth trying after seeing the reposurgeon
> conversion I deliver, we can investigate that I suppose.
I'm pretty sure it should be doable with pure git. Do something with git
for-each-ref on (a copy of) the old repository to script renaming of all
refs so the two repositories don't have any conflicting refs. Then use
git fast-import to import all the content of one repository into the other
(or add one repository as a remote to the other, fetch and then rename all
the refs from remotes/). Then git gc --aggressive to repack it all.
Optionally, add a "git merge -s ours" commit to new master to show it as
merged from the git-svn master, if it's considered beneficial to converge
the history like that.
People with existing git-only branches might need to take extra care the
first time they merge after this is done (maybe merge up to the last
revision of the git-svn master then do their own -s ours merge from the
new master, if git doesn't get it right automatically given such a merge
commit on master), but I don't see any reason this approach shouldn't work
to keep existing references to git-svn commit hashes meaningful (without
needing to have a renamed git-svn repository sit on the side for that
purpose) and to keep existing git-only branches (of which there are lots)
usable. And with blobs and hopefully most tree objects shared between the
two histories, I hope this won't make the repository too much larger.
> The GCC repo is pretty huge, but I've been hunting mastodons like it
> for years now - there's a row of trophy heads in the reposurgeon
> documentation. I ended up building a machine with a processor and
> cache specifically designed to handle non-parallelizable graph-theory
> computations multiple gigabytes wide - SMP is no help here and you
> want extra-large primary memory caches. On this hardware, conversion
> runs will merely be painfully slow rather than die-of-old-age
FWIW, Jason's own trial conversion with reposurgeon got up to at least
45GB memory consumption on a 32GB repository.
Joseph S. Myers