From: Morten Welinder <mwelinder@gmail.com>
Date: Sun, 29 Jan 2006 14:19:23 GMT
Subject: Re: [Census] So who uses git?
Message-ID: <118833cc0601290619k1e9c6bb8gc63937f1a2d2b31e@mail.gmail.com>
In-Reply-To: <7v7j8jpu48.fsf@assigned-by-dhcp.cox.net>

> I think that 40% sounds about right.  My understanding of the
> underlying format CVS uses, RCS, is that it stores an full copy
> of the tip of trunk uncompressed, and other versions of the file
> are represented as incremental delta from that.  The packed git
> format does not favor particular version based on the distance
> from the tip, and stores either a compressed full copy, or a
> delta from some other revision (which may not necessarily be
> represented as a full copy).  When we store something as a delta
> from something else, we limit the length of the delta chain to a
> full copy to 10 (by default), so that you can get to a specific
> object with at most 10 applications of delta on top of a full
> copy.

If I understand this right, that means that for a log file (in this
case a ChangeLog file) that is appended to linearly as a
function of revision number, we have...

cvs: O(n) archive size
git: O(n*n) archive size

At least that is what we get if revision N is always deltad over
revision N-1.  A good deal could be saved if instead of dumping
a full copy every 10 revisions, that revision would instead be
deltad off an earlier revision, but I think it'll still be O(n*n).

(/me prepares for Linus chiming in and telling me I should not
keep ChangeLog files, :-)

M.