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Re: Proposal: A new approach to differential debs




You are assuming the savings are substantial.  That's not clear.  When
files are compressed, if you then start doing binary diffs, well it
isn't clear that they will consistently be much smaller than plain new
files.  it also isn't clear what the impact on repo disk usage would
be.

The most straigforward option:
The least intrusive way to do this is to add differential files in
addition to the existing binaries, and any time the differential file,
compared to a new version exceeds some threhold size (for example:
50%) of the original file, then you end up adding the sum total of the
diff files in addition to the regular files to the repos.  I haven't
done the math, but it's clear to me that it ends up being about double
the disk space with this approach. it's also costly in that all those
files have to be built and managed, which is likely a substantial
ongoing load (cpu/io/people)  I think this is what people are
objecting to.

A more intrusive and less obvious way to do this is to use zsync (
http://zsync.moria.org.uk/. ) With zsync, you build tables of content
for each file, using the same 4k blocking that rsync does. To handle
compression efficiently, there needs to be an understanding of
blocking, so a customized gzip needs to be used.  With such a format,
you produce the same .deb's as today, with the .zsyncs (already in
use?) and the author already provides some debian Packages files as
examples.  The space penalty here is probably only a few percent.

the resource penalty is one read through each file to build the
indices, and you can save that by combining the index building with
the compression phase.  To get differential patches, one just fetches
byte-ranges in the existing main files, so no separate diff files
needed.  And since the same mechanisms can (should?) be used for repo
replication, the added cost is likely a net savings in bandwidth
usage, and relatively little complexity.

The steps would be:
   -- add gzip-ware flavour to package creation logic
   -- add zsync index creation on repos. (potentially combined with first step.)
   -- add zsync client to apt-* friends.

To me, this makes a lot of sense to do just for repo replication,
completely ignoring the benefits for people on low bandwidth lines,
but it does work for both.

On Sun, Aug 13, 2017 at 9:20 AM, Paul Wise <pabs@xxxxxxxxxx> wrote:
> On Sun, Aug 13, 2017 at 5:38 AM, Adrian Bunk wrote:
>
>> It sounds like something that would have been a cool feature 20 years
>> ago when I was downloading Debian updates over an analog modem.
>>
>> Today the required effort, infrastructure and added complexity would
>> IMHO not be worth it for a potential few percent of bandwidth decrease.
>
> Low-speed connections and low bandwidth quotas (especially on mobile)
> are still common enough around the world that delta upgrades make a
> difference right now, IIRC even Google uses them for Chrome.
>
> --
> bye,
> pabs
>
> https://wiki.debian.org/PaulWise
>