Message-ID: <41B376B4.2060605@arbash-meinel.com>
Date: Sun, 05 Dec 2004 14:59:32 -0600
From: John A Meinel <john@arbash-meinel.com>
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To: Tom Browder <tbrowder@cox.net>
Subject: Re: [Gnu-arch-users] Arch Versus CVS Versus Subversoin
References: <20041205000613.PNEI7152.lakermmtao09.cox.net@nonerjsnum1tkq>
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Tom Browder wrote:

>One major thing that subversion offers that I haven't seen on the
>comparisons between arch and other scm programs is the capability to version
>control binary files with binary diffing.  This is an important feature for
>me and I would like to use subversion.
>
>However, and unfortunately, the subversion folks refuse to listen to my
>CVS-user suggestions for the same type tags as rtag and use of an equivalent
>to CVSROOT, otherwise I would jump on subversion.
>
>Does arch offer or plan to offer those three features (CVS rtag equivalent,
>binary diff, CVSROOT equivalent)?
>
>Thanks.
>
>Tom Browder
>  
>
Arch does not support binary diff in the subversion sense. Because 
binary diff only makes sense if you have complete history. For each 
change you must have the bit-wise identical previous version. (binary 
diff only supports exact patching). One of the ideas of arch is because 
patch supports fuzzy patching, each revision actually has meaning by 
itself. Say you fixed a bug, and committed it as patch-23 into your 
archive. My archive is similar to yours, but I've made changes in the 
mean-time (even to the same files that you've changed). I can take your 
patch-23, and in the normal (non-conflicting) case, I can apply it to my 
tree (usually successfully) without knowing much else about your tree.

This is possible because the diffs generated contain context, and this 
context can be used for patching purposes. In the case of a binary diff, 
there isn't really the same idea of context. For instance, say the 
binary file is a bitmap, and I add a circle to the bottom left corner. 
You add a circle to the bottom right corner. Now the proper "diff" would 
be just that change, and if you applied your patch and my patch, you 
should end up with a circle in the bottom left, and the bottom right. 
But in reality the diffs would at the very least conflict. (In the case 
of a bitmap, data is stored row-wise, so you changed the same rows I 
changed). Things get even worse if you were dealing with something like 
png images, where the data is compressed. Because again, the proper 
'diff' would have to know about the uncompressed image and what changed, 
so that patch could only change the altered pixels.

In general, to do appropriate binary diffs, you must know the file 
format, and have an intelligent diff program. The hardest part of this, 
is that everyone who would use your archive must also have the ability 
to understand the diffs, so that they can run their "patch" and get an 
exact copy of what you had.

I think the reason people want binary diff (it's why I wanted it) was to 
decrease the amount of space used in the archive. At first glance, it 
seems silly to keep 10 copies of a 100k file, when only part of it 
changes. However, afterwards it means that every one of your 10 
revisions is independently useful. You don't need the previous revision 
to understand the next one. Also, unless your binary file is really big, 
and the changes are very localized, you don't really save that much 
space, and usually space is relatively cheap. It only costs $1/GB of 
space. $200 (less than a weeks salary for most people) gets you enough 
space to store lots and lots of copies of your files. Bandwidth isn't 
quite as cheap (and downloading a full copy of the file 20 times is 
probably expensive), but cacherevs can help prevent that. Arch also 
supports other enhancements (local archives, revision libraries), which 
trade off bandwidth versus space. And finally, all changes are stored 
compressed in the archive, so in many cases the compression will do 
better than a binary diff would.

Sorry this got so long, but there are specific reasons that arch doesn't 
support binary diffs, and it isn't just that it hasn't implemented them yet.

In response to rtag, arch's idea of tags vs branches, etc are 
considerably different. I believe you are trying to create a tag on a 
file without having a local copy. 'tla tag' does exactly this, but you 
are more likely to want to use a configuration, or something like that. 
It is very easy to create a meta-project that keeps track of any sort of 
tags, etc that you want for some other project (these are the 
configurations I speak of.) Then you get version-controlled 
meta-information. (In CVS when you move a tag, it is moved for good.)

John
=:->

PS> If anyone else wants to comment, please do, but this is my feeling 
about how arch operates.

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