Message-ID: <41B3BF5A.903@arbash-meinel.com>
Date: Sun, 05 Dec 2004 20:09:30 -0600
From: John A Meinel <john@arbash-meinel.com>
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To: Andrew Suffield <asuffield@debian.org>
Subject: Re: [Gnu-arch-users] Arch Versus CVS Versus Subversoin
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	<20041206015722.GJ2799@suffields.me.uk>
In-Reply-To: <20041206015722.GJ2799@suffields.me.uk>
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Andrew Suffield wrote:
> On Sun, Dec 05, 2004 at 07:27:37PM -0600, John A Meinel wrote:
> 
>>Andrew Suffield wrote:
>>
>>>On Sun, Dec 05, 2004 at 02:59:32PM -0600, John A Meinel wrote:
>>>

> 
> Yes, arch supports binary diffs in exactly the same sense as subversion.
> 
> It doesn't do delta compression on them. That's irrelevant.

It is not irrelevant, people care about it.

> 
> xdelta is not a diff. It's delta compression. That's even in the *name*.
> 
> 
>>This is most definitely NOT what the original poster was meaning by 
>>"binary diff".
> 
> 
> You mean they were out of their tree and using the wrong names for
> things? You'll never get anywhere like that. "$foo diff" is an
> operation which generates the input to "$foo patch", such that:
> 
> patch(A, diff(A, B)) == B, where A and B are of type $foo
> 
> That's the definition.
> 

This is the first time that I've heard it (properly) called delta 
compression.

> 
>>>Compression is not related in any way.
>>>
>>
>>compression is not binary diff. True. But the reason people want binary 
>>diff is so that when you have your 10MB Word document, and you change 2 
>>lines in it, when you do the commit it doesn't have to upload 10MB of 
>>data.
> 
> 
> No, the reason people want binary diffs is so that they can have
> binary files stored in revision control. That's what it means and
> that's what it does.
> 

I do believe that what the original poster was asking for was "binary 
deltas". Yes we support "binary diff" as you name it. Your terminology 
is probably better than mine. But CVS has long supported binary files, 
what people didn't like is that every time they made a change, the size 
of their repository went up by the complete file size, instead of just 
the differences. The reason every one else calls it a binary diff is 
because a normal diff only contains the changes, your version of a 
binary diff contains the entire file pre and post.

Yes, I understand why it does that. And I'm not really advocating that 
it changes. I might argue that we would be better off storing the md5sum 
of the previous version rather than a complete copy of it, but generally 
that is irrelevant.

The only reason I can think of to use SVN is if I ever try to revision 
control a lot of binary files. Think most office document formats. MS 
and OOo. (ooo is zipped xml, but you are still committing the zip file 
which is binary)

Admittedly, I'm talking about a small population (about 10+ people), but 
when I was taught CVS I was warned that binary files are not "diffed" 
but store in complete form each time. (Obviously this should have been 
"delta compressed"). I have also heard this argument from independent 
sources (as mentioned, about 10 times). I realize all of us are using 
the wrong terminology here. But I know that is what they meant, because 
the complaint was about the size of the archive, and how much better SVN 
is because it only stores the changes.

See here:
http://osdir.com/Article203.phtml
> Subversion stores all files in a binary representation and uses an efficient 
> binary diff algorithm to compute differences between them. This means multiple 
> revisions of binary files take up a lot less space on the server

"efficient binary diff" => delta compression.

John
=:->


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