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From Mark Phippard <markp...@gmail.com>
Subject Re: not storing diffs of binary files
Date Tue, 09 Aug 2011 17:50:22 GMT
On Tue, Aug 9, 2011 at 1:19 PM, Jon Stafford
<Jon.Stafford@complyserve.com>wrote:

> Thanks everyone for the responses.  To check my understanding, and to give
> half a conclusion -
>
> Every revision apart from the very initial revision of a file is stored as
> a delta against some previous version.  Subversion would typically probably
> use the least disk space *if* each revision was stored as a delta against
> the immediately preceding revision.  But that would be really slow for
> reconstructing the 1000th revision.  So instead, each revision is stored as
> a delta against a base of flip-rightmost-1.
>
> This generally gives a balance between space used up and time to recreate
> any given revision of the file.
>
> OK, how does all that sound so far?
>
> Knowing this I was hoping I'd look again and understand what was going on
> with my repository with successive zips of my database data checked in.  Not
> quite...
>
> I can see that the deltas aren't necessarily against the immediately
> preceding version - in fact with 15 revisions it's satisfying/reassuring to
> see them doing exactly as billed in the skip deltas document.
>
> The bit I still can't reconcile is the difference in the delta size between
> xdelta standalone (small) and the delta stored by subversion (large - almost
> the size of the file itself sometimes).
>
> I've checked in various versions of my database data zipped.  Some with a
> month of changes between each revision, some with the most trivial change
> possible between revisions.
>
> For a trivial change:
> xdelta delta size = 300KB, subversion db\revs file size = 300KB
>
> For a month of database edits:
> xdelta delta size = 3 or 4MB, subversion db\revs file size = 50MB
>
> Obviously for fair comparison I'm only picking on revisions where
> subversion did delta against the immediately preceding revision.
>
> So does subversion (version 1.6.11) use an old, not quite so good, xdelta?
>  Or is it just that it applies xdelta after its already done some format
> manipulation on the file, which then makes it less delta-able?  Or something
> else...
>

I do not understand it enough to give a lot of details so let me point you
to an old thread on the list:

http://svn.haxx.se/dev/archive-2007-03/1277.shtml

The xdelta algorithm has a configurable window that determines the amount of
memory used.  The more memory you give it, the smaller the delta it can
often produce.  It is likely the xdelta binary you are using uses a larger
window than Subversion.

-- 
Thanks

Mark Phippard
http://markphip.blogspot.com/

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