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git unpack: efficient tree filter

published on Sunday, June 11, 2017

I recently had the pleasure of migrating an SVN repository with about 6300 revisions and 370 tags/branches to git. One of the peculiarities with the original repository was that they decided to gzip many of their large (>100KiB) text files. The idea behind this was, I guess, to reduce checkout and transfer sizes.

In git, of course, gzipping your files will in fact increase the overall size of the repository! The reason is that you generally fetch the entire history in git (not only the current work-tree as in SVN) and that gzipping prevents git from packing similar objects efficiently (packfiles).

Furthermore, gzipping text files isn't too nice because it doesn't play well with diffs and some text-processing tools (yes, git can be configured to diff with zcat, but this has to be done by every user…). Clearly, I should get rid of these files before publishing your new git repository – but how?

git filter-branch

No problem, I thought, let's just git filter-branch these files to hell:

CWD="$(pwd)" git filter-branch \
    --index-filter '$CWD/extract_gz_files.sh' \
    -- --branches --tags

With the following script in the current directory:

extract_gz_files.sh

#! /bin/zsh
mkdir -p .known-objects

git ls-tree -lr $GIT_COMMIT | while read -r MODE TYPE OBJ SIZE NAME; do
    if [[ $NAME == *.gz ]]; then
        if [[ ! -e .known-objects/$OBJ ]]; then
            git cat-file blob $OBJ |
                gunzip |
                git hash-object -w -t blob --stdin > .known-objects/$OBJ
        fi
        OBJ=$(< .known-objects/$OBJ)
        git update-index --remove $NAME
        git update-index --add --cacheinfo $MODE,$OBJ,${NAME%.gz}
    fi
done

This works fine. BUT:

Have you ever tried to use git filter-branch --index-filter on a large repository? The above command needs about 2 hours (with 99% of the time spent in only those 2000 commits where files actually had to be extracted).

Efficient rewrites

Part of the reason that git filter-branch can be so slow, is that it is fully sequential and takes no advantage of parallelization. While sequential processing is required for the commits themselves, the rewriting of the trees can be efficiently parallelized. The other main reason is that it is hard make use of caching for already computed subtrees.

Large scale Git history rewrites is a great article which pushes this to the extreme and shows a lot of tricks to speed up your performance. However, his blink_history_rewrite module is perhaps too heavily optimized: it makes a lot of assumptions about how the internal structure of the git object database and does not make use of higher-level tools. For this reason, the code is very fragile. And indeed: I could not get it to run even though I performed all the listed preparation steps – maybe because I'm using a newer version of git, and maybe because of something else entirely.

Nonetheless inspired by this article, I created a similar module that uses git commands and makes no assumptions about the underlying storage model (whether objects are stored in packfiles, etc).

Of course, my module is likely a factor 10 or more slower than his, but on the other hand: you should be able to use this module with every git repository; and without having to perform any preparation steps as described in the above article (they may, however, still result in performance improvements).

Very first, clone your repository, you don't want to accidentally lose data:

git clone ORIG DEST --mirror
cd DEST

Okay, we're ready to rewrite. Instead of the single filter-branch command, we proceed now in two phases.

First, rewrite the trees using the python module (parallelized). This creates a folder objmap where it stores for each top level tree, the hash of the tree with which it should be replaced:

git clone https://github.com/coldfix/git-filter-tree

git log --format='%T' --branches --tags | \
    python git-filter-tree/git_filter_tree unpack

And second, rewrite the commits using git filter-branch --commit-filter, making use of the objmap/ folder created in phase 1 (still sequential, but fast enough):

git filter-branch --commit-filter '
    obj=$1; shift; git commit-tree $(cat $GIT_DIR/objmap/$obj) "$@"' \
    -- --branches --tags

Voilà, the 2 hour job is now done in 4 minutes, factor 30 speedup, not bad.

Remove unneeded objects

After you're finished with either filter-branch command, you may find that the repository still takes up more space than than the original repository. So all of that for nothing? No, it's just that we haven't performed a final step:

We have to to tell git to clean up, delete all the unreferenced objects and compress all the others. Be sure to do this only on your cloned repository – otherwise you will lose data:

rm -rf refs/original/
git reflog expire --expire=now --all
git gc --prune=now
git gc --aggressive --prune=now

Give me a one-liner!

Sorry, two lines:

git clone https://github.com/coldfix/git-tree-filter

./git-tree-filter/git-unpack ORIG DEST

While my particular use-case may be rather rare, the pattern is genuinely generic. So, if you're interested to do a similar but different tree-rewrite, and you don't mind writing a few lines of python code, you may be able to adapt the unpack module for your own purposes.

Also, please don't hesitate to open issues and/or submit pull-requests with more examples.

This entry was tagged extract, filter-branch, git, history, plumbing, rewrite and unpack

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