On Wed, 20 Mar 2024 at 22:01, Kevin Kofler via devel <devel@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
Aoife Moloney wrote:
> The zstd compression type was chosen to match createrepo_c settings.
> As an alternative, we might want to choose xz,
Since xz consistently compresses better than zstd, I would strongly suggest
using xz everywhere to minimize download sizes. However:
> especially after zlib-ng has been made the default in Fedora and brought
> performance improvements.
zlib-ng is for gz, not xz, and gz is fast, but compresses extremely poorly
(which is mostly due to the format, so, while some implementations manage to
do better than others at the expense of more compression time, there is a
limit to how well they can do and it is nowhere near xz or even zstd) and
should hence never be used at all.
There are two parts to this which users will see as 'slowness'. Part one is downloading the data from a mirror. Part two is uncompressing the data. In work I have been a part of, we have found that while xz gave us much smaller files, the time to uncompress was so much larger that our download gains were lost. Using zstd gave larger downloads (maybe 10 to 20% bigger) but uncompressed much faster than xz. This is data dependent though so it would be good to see if someone could test to see if xz uncompression of the datafiles will be too slow.
Kevin Kofler
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