[CentOS-devel] deltarpm and presto for centos

Sun Nov 15 19:45:25 UTC 2009
Jonathan Dieter <jdieter at gmail.com>

On Sun, 2009-11-15 at 13:44 -0500, Jeff Johnson wrote:
> FWIW, there are deep fundamental design issues wrto Courgette
> that have nothing to do with with whether Google is choosing Courgette
> for Chromium peculier updates.
> For starters:
> 1) RFC 3229 at http://www.rfc-editor.org/rfc/rfc3229.txt
> 	This is what subversion uses (afaik) instead of xdelta
> 	While I privately like xdelta _A LOT_ and I think
> 	that Josh McDonald's master's thesis, and xdelta[123] are
> 	the cat's pajama's, xdelta code is quite obscure and
> 	hard to justify deploying generally. YMMV, everyone's does, but
> 	(objectively) subevrsion chose vdelta rather than xdelta
> 	because xdelta code is insanely difficult and uncommented,
> 	and uses vdelta (ala RFC 3229) instead.

FWIW, I think the delta algorithm is one of the smaller problems
deltarpm needs to deal with right now.

> 2) disassembling code to remove pointer entropy (as in Courgette)
> maye be a win for executables, but is not generally useful for presto
> (or packaging).

To clarify for others following this thread, most of the files in an rpm
are data, *not* executables.

Deltarpm currently has two big problems that keep it from having hugely
efficient deltas even when two rpms have barely changed.

1) Any colored binaries that aren't in a multilib directory
(i.e. /usr/bin/*) are never delta'd at all.  This was because we didn't
want to lose the complete delta because some 32-bit package on a 64-bit
machine was missing some file in /usr/bin.  We may want to rethink this
now as 64-bit installs tend to have fewer 32-bit packages then when this
decision was made.
2) A small change in an uncompressed file will result in a huge change
after it's been compressed.  Many of the larger packages have at least
some compressed files and those files are essentially not delta'd at

In my mind, at least, solving these two problems will have a far bigger
effect in reducing deltarpm size than adopting Courgette.

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