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Re: RFC: Support for zstd in .deb packages?

On Fri, Apr 27, 2018 at 07:02:12AM +0200, Guillem Jover wrote:
> * Eternity contract: This would add yet another format that would need
>   to be supported pretty much forever, to be able to at least unpack
>   .deb's that might be available in the wild. This also increases the
>   (Build-)Essential-set.
> * Format stability: Although it's supposedly frozen now, it has
>   changed quite often in recent times. AFAIR it was also mentioned at
>   least in the past that the target was mainly real-time data streaming,
>   so long-term data storage might not be a priority? Would need
>   clarification from upstream I guess.

This does not sound like something that should be done soon.

> As a replacement for gzip, it would
> definitely make sense, but otherwise I'm not sure I see it.

The number of packages that use gzip as compressor if rebuilt should be 
pretty close to 0. We need gzip for compatibility with older packages,
but no replacement for it.

> An area where there's still room for improvement with xz f.ex. when it
> comes to decompression speed, is lack of multi-threaded support, as
> liblzma currently only supports it for compression.

This sounds like a much less invasive change to me.
And it could already deliver benefits for buster if
someone would be willing to implement that.

And there's another potential low hanging fruit for buster:

xz decompression time is linear with the compressed size,
higher compression would bring both smaller packages and
faster decompression.

There are two downsides to switching xz compression from -6 to -9:[1]

1. higher compression time
This will increase build times.
If this turns out to be a problem, most of our biggest packages are the 
-dbgsym packages that could continue to use a lower compression if needed.

2. more memory usage for uncompression
64 MB memory usage can be a problem for some lower end machines.

One way to mitigate both downsides might be different default compression 
levels on different ports, the ports where something like faster setup
of cloud servers would matter are not the ports with slow buildds or
where 64 MB memory usage would matter much on any supported hardware.

> Thanks,
> Guillem


[1] we are talking about perhaps 5-10% smaller packages and faster 
    decompression speed


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       "Only a promise," Lao Er said.
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