CODEWITHSUNDEEP
gitbackup
Evgeniy Berezovsky
Evgeniy Berezovsky

Reputation: 19248

Differential backups of git repositories using git bundle

I would like to know if it is possible to do differential backups of a git repository, using git bundle like this:

First time:

git clone --mirror https://github.com/me/myrepo
git bundle create base.bundle --all

Every time I want to create a differential bundle:

cd myrepo
git fetch --all # is this necessary? (because "git bundle" doesn't check remote)
git bundle create diff.bundle $(git rev-parse HEAD)..HEAD --all

My main question is if above method ensures that base.bundle and diff.bundle, when used together, contain the complete repository right from repository creation up until the point when diff.bundle was taken, including branches, tags, and whatever else there may be in a git repo that I'm not aware of.

Upvotes: 1

Views: 291

Answers (1)

torek
torek

Reputation: 489203

Your first two commands work fine to create a base bundle. However, your second group of commands won't do the right thing. You do want the git fetch (--all is needed only if you have multiple remotes), but you want the newly created differential bundle to be done with "negative refspecs" for every ref that was a positive refspec in the previous bundle. That is:

git bundle create diff.bundle $(git rev-parse HEAD)..HEAD --all

is clearly wrong for two reasons:

  1. The inner git rev-parse HEAD uses the current HEAD, which may not be correct;
  2. If the previous bundle (initial or previous differential) used refs/heads/br1, refs/heads/br2, refs/tags/t1, refs/remotes/origin/r1, and refs/remotes/origin/r2 as its positive refspecs via --all, you need negative refspecs that will produce each hash ID from all of the positive refspecs.

The easiest way to fix both of these is to:

  1. have the initial sequence end with git rev-parse --all with output saved somewhere;
  2. for creating a new differential bundle, use ^$hash for each hash ID listed in the saved output from the last save;
  3. after creating the new differential bundle, use git rev-parse --all again to get the positive refspec hash IDs.

So you'll end up with something along these lines:

git clone --mirror https://github.com/me/myrepo
git bundle create $HOME/b/base.bundle --all
git rev-list --all > $HOME/b/hashes

followed by:

cd myrepo
git fetch  # --all if needed, but per the above there's just one remote
git bundle create $HOME/b/diff.bundle $(sed s/^/^/ < $HOME/b/hashes) --all
git rev-list --all > $HOME/b/hashes

Warning: this is entirely untested. I'm also assuming that each diff.bundle is an increment to the previous diff.bundle here, i.e., these each need to be saved separately.

(You're probably best off using real backup software anyway, but this is likely to work.)

Upvotes: 2

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