[Rpm-ecosystem] Proposal: Zchunked rpms to reduce compose time and eliminate need for deltarpms

[Michael Schroeder]

On Sat, Nov 17, 2018 at 06:10:56PM +0000, Jonathan Dieter wrote:
> In Fedora, there was a call for ideas on, among other things, reducing
> the compose time.  Currently, a good chunk of Fedora's compose time is
> spent generating deltarpms,

Why's that? Delta generation is actually pretty fast *if* you
limit the memory usage with the '-m' option.

> and I've been thinking about a way to use
> zchunk as rpm's compression payload, which would make deltarpms
> redundant.  Neal suggested I bring it up here, so here's my proposal:
> 
> <tl;dr>
> Have rpm use zchunk as its compression format, removing the need for
> deltarpms, and thus reducing compose time.  This will require changes
> to both the rpm format and new features in the zchunk format.
> </tl;dr>

You'll need to provide some real life numbers to make a convincing
argument for this. A delta algorithm is way different to chunking:

- the minimal chunk size is *really* small (16 bytes iirc)
- there is also an additional "offset" stream that improves the delta
  sizes significantly

So zchunk will be much worse than a deltarpm. Deltarpms also work
with files from the filesystem, so you don't have to keep the
installed rpms around for the update.

Cheers,
  Michael.

-- 
Michael Schroeder                                   mls at suse.de
SUSE LINUX GmbH,           GF Jeff Hawn, HRB 16746 AG Nuernberg
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