Re: Curious about details of optimization of object database...

From: Boyd Stephen Smith Jr. <bss@iguanasuicide.net>
Date: Jan 9, 2009, 19:07 UTC
Message-ID: <200901091307.33483.bss@iguanasuicide.net>
In-Reply-To: <20090109174623.GC12552@seberino.org>

On Friday 2009 January 09 11:46:23 chris@seberino.org wrote:

>I'm told a commit is *not* a patch (diff), but, rather a copy of the entire
>tree.

It's even more than that. A commit object contains its message, the SHA of the tree, and zero or more SHAs for its parents.

>Can anyone say, in a few sentences, how git avoids needing to keep multiple
>slightly different copies of entire files without just storing lots of
>patches/diffs?

Loose objects can have large swaths of duplicated data. However, git also supports storing objects in a packed format, which uses delta compression to reduce the duplication to close to nothing.

Some examples: Sizes are from "du -sh .git ."; The .git directory stores all the objects as well as the repository configuration, refs, reflogs, etc. The . directory has .git and a clean checkout of master.

The LinuxPMI (http://linuxpmi.org/) tree: 41M .git 83M . (So, the storage is actually a bit smaller than the checkout; 984 objects; 140 commits)

A small project between me an my flatmates: 309K .git 3.6M . (Here, the storage is significantly smaller than the checkout; 786 objects; 155 commits)

My repository that tracks my dotfiles: 124K .git 176K . (113 objects; 28 commits)

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