I've come up with a method of splitting repodata into chunks that can
be downloaded and combined with chunks that are already on the local
system to create a byte-for-byte copy of the compressed repodata. 
Tools and scripts are at:

With DNF, we're currently downloading ~20MB of repository data every
time the updates repository changes.

When casync was released, I wondered if we could use it to only
download the deltas for the repodata.  At Flock last summer, I ran some
tests against the uncompressed repodata and saw a reduction of 30-40%
from one day to the next, which seemed low, but was a good starting

Unfortunately, due to the way casync separates each file into thousands
of compressed chunks, building each file required thousands of (serial)
downloads which, even on a decent internet connection, took *forever*.

When I talked through the idea with Kevin and Patrick, they also
pointed out that our mirrors might not be too keen on the idea of
adding thousands of tiny files that change every day.

The Solution(?):
One potential solution to the "multitude of files" problem is to merge
the chunks back into a single file, and use HTTP ranges to only
download the parts of the file we want.  An added bonus is that most
web servers are configured to support hundreds of ranges in one
request, which greatly reduces the number of requests we have to make.

The other problem with casync is that it's chunk separation is naïve,
which is why we were only achieving 30-40% savings.  But we know what
the XML file is supposed to look like, so we can separate the chunks on
the tag boundaries in the XML.

So I've ditched casync altogether and put together a proof-of-concept
(tentatively named zchunk) that takes an XML file, compresses each tag
separately, and then concatenates all of them into one file.  The tool
also creates an index file that tells you the sha256sum for each
compressed chunk and the location of the chunk in the file.

I've also written a small script that will download a zchunk off the
internet.  If you don't specify an old file, it will just download
everything, but if you specify an old file, it will download the index
of the new file and compare the sha256sums of each chunk.  Any
checksums that match will be taken from the old file, and the rest will
be downloaded.

In testing, I've seen savings ranging from 10% (December 17 to today)
to 95% (yesterday to today).

Remaining problems:
 * Zchunk files are bigger than their gzip equivalents.  This ranges
   from 5% larger for filelists.xml to 300% larger for primary.xml. 
   This can be greatly reduced by chunking primary.xml based on srpm
   rather than rpm, which brings the size increase for primary.xml down
   to roughly 30%.

 * Many changes to the metadata can mean a large number of ranges
   requested.  I ran a check on our mirrors, and three (out of around
   150 that had the file I was testing) don't honor range requests at
   all, and three others only honor a small number in a single request.
    A further seven didn't respond at all (not sure if that had
   anything to do with the range requests), and the rest supported
   between 256 and 512 ranges in a single request.  We can reduce the
   number of ranges requested by always ordering our packages by date. 
   This would ensure that new packages are grouped at the end of the
   xml where they will be grabbed in one contiguous range.

 * Zchunk files use zlib (it gives better compression than xz with such
   small chunks), but, because they use a custom zdict, they are not gz
   files.  This means that we'll need new tools to read and write them.
   (And I am volunteering to do the work here)

The tools:
The proof-of-concept tools are all sitting in

They are full of ugly hacks, especially when it comes to parsing the
XML, there's little to no error reporting, and I didn't comment them
well at all, but they should work.

If all you want to do is download zchunks, you need to run dl_zchunk.py
with the url you want to download (ending in .zck) as the first
parameter.  Repodata for various days over the last few weeks is at:
https://www.jdieter.net/downloads/zchunk-test/  You may need to hover
over the links to see which is which.  The downloads directory is also
available over rsync at rsync://jdieter.net/downloads/zchunk-test.

dl_zchunk.py doesn't show anything if you download the full file, but
if you run the command with an old file as the second parameter, it
will show four numbers: bytes taken from the old file, bytes downloaded
from the new, total downloaded bytes and total uploaded bytes.

zchunk.py creates a .zck file.  To group chunks by source rpm in
primary.xml, run
./zchunk.py <file> rpm:sourcerpm

unzchunk.py decompresses a .zck file to stdout

I realize that there's a lot to digest here, and it's late, so I know I
missed something.  Please let me know if you have any suggestions,
criticisms or flames, though it might be a few hours before I respond.

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