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Git, Compression, and Deltas - An explanation

 1 month ago
source link: https://gist.github.com/matthewmccullough/2695758
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Git Compression of Blobs and Packfiles.

Many users of Git are curious about the lack of delta compression at the object (blob) level when commits are first written. This efficiency is saved until the pack file is written. Loose objects are written in compressed, but non-delta format at the time of each commit.

A simple run though of a commit sequence with only the smallest change to the image (in uncompressed TIFF format to amplify the observable behavior) aids the understanding of this deferred and different approach efficiency.

The command sequence:

Create the repo:

$ git init test6
Initialized empty Git repository in /Users/mccm06/Documents/Temp/Scratch/test6/.git/
[master (root-commit) 05e9c3e] First-commit
 0 files changed
 create mode 100644 README

$ du -c
72	./.git/hooks
8	./.git/info
8	./.git/logs/refs/heads
8	./.git/logs/refs
16	./.git/logs
8	./.git/objects/05
8	./.git/objects/54
8	./.git/objects/e6
0	./.git/objects/info
0	./.git/objects/pack
24	./.git/objects
8	./.git/refs/heads
0	./.git/refs/tags
8	./.git/refs
0	./.git/rr-cache
168	./.git
168	.
168	total

There's only a total of 168kb for the entire repo and working directory.

Now copy in the white image:

$ cp ../completely-white.tiff .

And show how large that image is (5294kb) in an uncompressed TIFF format:

$ ls -al
total 10344
drwxr-xr-x   5     170 .
drwxrwxr-x   6     204 ..
drwxr-xr-x  15     510 .git
-rw-r--r--   1       0 README
-rw-r--r--   1 5294996 completely-white.tiff

And show the size of the entire repo and working copy:

$ du -c
72	./.git/hooks
8	./.git/info
8	./.git/logs/refs/heads
8	./.git/logs/refs
16	./.git/logs
8	./.git/objects/05
8	./.git/objects/54
8	./.git/objects/e6
0	./.git/objects/info
0	./.git/objects/pack
24	./.git/objects
8	./.git/refs/heads
0	./.git/refs/tags
8	./.git/refs
0	./.git/rr-cache
168	./.git
10512	.
10512	total

Now add that file to the staging area and commit it.

$ git add .
$ git commit -m"White image added"
[master f93724b] White image added
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 completely-white.tiff

$ du -c
72	./.git/hooks
8	./.git/info
8	./.git/logs/refs/heads
8	./.git/logs/refs
16	./.git/logs
8	./.git/objects/05
8	./.git/objects/54
8	./.git/objects/87
56	./.git/objects/90
8	./.git/objects/e6
8	./.git/objects/f9
0	./.git/objects/info
0	./.git/objects/pack
96	./.git/objects
8	./.git/refs/heads
0	./.git/refs/tags
8	./.git/refs
0	./.git/rr-cache
240	./.git
10584	.
10584	total

The file is compressed when saved to its blob in the objects/90 directory and thus is only 56kb in size.

Make minor edits to the image:

$ open completely-white.tiff -a Pixelmator 

$ git status
# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#	modified:   completely-white.tiff
#
no changes added to commit (use "git add" and/or "git commit -a")

$ git add .
$ git commit -m"Dot added to white image"
[master 3015c4a] Dot added to white image
 1 file changed, 0 insertions(+), 0 deletions(-)

Now, after the second minor alteration, we can see there is an equally sized 56kb directory called 31 that contains the blob for the modified image.

$ du -c
72	./.git/hooks
8	./.git/info
8	./.git/logs/refs/heads
8	./.git/logs/refs
16	./.git/logs
8	./.git/objects/05
8	./.git/objects/30
56	./.git/objects/31
8	./.git/objects/47
8	./.git/objects/54
8	./.git/objects/87
56	./.git/objects/90
8	./.git/objects/e6
8	./.git/objects/f9
0	./.git/objects/info
0	./.git/objects/pack
168	./.git/objects
8	./.git/refs/heads
0	./.git/refs/tags
8	./.git/refs
0	./.git/rr-cache
312	./.git
10656	.
10656	total

Lastly, we will compress the history into a single packfile instead of loose objects:

$ git gc --aggressive
Counting objects: 9, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (7/7), done.
Writing objects: 100% (9/9), done.
Total 9 (delta 1), reused 0 (delta 0)
$ du -c
72	./.git/hooks
16	./.git/info
8	./.git/logs/refs/heads
8	./.git/logs/refs
16	./.git/logs
8	./.git/objects/info
32	./.git/objects/pack
40	./.git/objects
0	./.git/refs/heads
0	./.git/refs/tags
0	./.git/refs
0	./.git/rr-cache
192	./.git
10536	.
10536	total

And you can see that the combined packfile is only 40kb instead of 168kb when stored separately in two unique blobs in separate object directories.

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