Does git reuse blobs?

Question

Let's say I committed a binary file, then changed it a couple of commits later and now I changed it back in a new commit.

Out of curiosity, I wondered if git creates a new blob for it? Or does it detect it's in the history and reuse it? If so, how does it detect that? Checksum?

Answer 1

Git will reuse the same blob.

I have done a test. I made 3 commits. First I commit a binary file, then I modified the binary file and commit it again. Then finally I overwrote the file by original binary used in the first commit and commit again.

The binary files content in 1st & 3rd commits are the same. Each commit is the HEAD of the follow branches:

1st commit: "FIRST". 2nd commit: "SECOND". 3rd commit: "master"

Then if you run "git cat-file -p FIRST^{tree}" , it shows hash code of the binary file.

$ git cat-file -p FIRST^{tree}
100644 blob ec049240a47b472bd7c31d1fa27118c4fe2f1229    test.db3
$ git cat-file -p SECOND^{tree}
100644 blob a47bb3727e5aefe3ec386bec5520f3e4ffb3a4c5    test.db3
$ git cat-file -p master^{tree}
100644 blob ec049240a47b472bd7c31d1fa27118c4fe2f1229    test.db3

You will find that the hash code of the blob of 1st and 3rd commit are the same.

git is so smart enough to check whether a blob exists for a hash code and reuse that blob if found.