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List:       fedora-devel-list
Subject:    Re: F34 Change: Reduce installation media size by improving the compression ratio of SquashFS filesy
From:       Zbigniew =?utf-8?Q?J=C4=99drzejewski-Szmek?= <zbyszek () in ! waw ! pl>
Date:       2020-09-05 10:43:59
Message-ID: 20200905104359.GZ10369 () in ! waw ! pl
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On Thu, Aug 27, 2020 at 11:13:26AM -0400, Ben Cotton wrote:
> https://fedoraproject.org/wiki/Changes/OptimizeSquashFS
...
> Based on the results above, I'd suggest selecting the following
> ''optimal configuration'': XZ algorithm, with block size of 1MiB and
> without BCJ filter (plain xz -b 1M, without -Xbcj x86).
> On the right, you can see the impact of the compression algorithms on
> installation time.
> 
> As can be seen from the picture on the right hand side, selecting
> 'plain xz -b 1M configuration' has minimal impact on the installation
> time and CPU usage during the installation. The compression will
> result in +6.51% or, in real terms, +24.94s additional installation
> time, compared to the savings of 142 MiB on the installation media,

> == Benefit to Fedora ==
> * Reduction of the installation media size and the cost of storing and
> distributing Fedora.
> * Reduction of the CPU usage at build time. Depending on which
> compression parameters chosen.

Hi Bohdan,

I think there's a misalignment of priorities.

My evaluation is the following: users won't care. The image size difference
is not big enough for people to notice. OTOH, slower installation will
impact QA and VM installations. We're doing more and more automated
installations and tests, and this change impacts those tests negatively.

> This increase in installation time will be compensated by the change
> in the installer: https://github.com/rhinstaller/anaconda/pull/2292

This PR is very interesting. But it seems that the more we optimize
things, the more slow decompression will be noticable. I.e. in some
way, right now, the slow decompression is obscured by the slow IO
speed, multiple levels of block device, or slow processing of the
uncompressed data. Any time the input or output speed is improved,
slow compression will be more of a bottleneck. So the approach of
increasing XZ compression levels has bad perspectives.

Zbyszek
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