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Re: Are my repos properly using ZLIB compression level 9?

From: Branko Čibej <brane_at_apache.org>
Date: Wed, 12 Dec 2018 14:14:45 +0100

On 12.12.2018 10:40, Thorsten Schöning wrote:
> Hi all,
> I upgraded all my repos using dump/load and changed the compression
> settings to use ZLIB 9, because I manage some pretty good compressable
> files at a low commit rate and want to preserve as much storage as
> possible for those. I used the following version of svnadmin and
> changed the following setting in fsfs.conf only:
>> svnadmin, Version 1.9.3 (r1718519)
>> [deltification]
>> compression-level = 9
> Overall memory of all my repos dropped significantly,

^^^ this is a pretty good indicator that "something" is working as you
expect. :)

> so I guess
> thinks work as expected. But I didn't have a look at each individual
> revision file, instead I simply committed some textual changes in some
> text file in some test repo and found clear, readable text as DELTA in
> the created revision file on the server.
> Is this expected behaviour? I would have thought that all datas are
> unreadable because of compression. Or is there some threshold or such?
> What do I need to look for to make sure that ZLIB 9 is used?

In general, you can't just look at the compressed data and guess what
the compression level setting was.

Regarding plain text in "compressed" data, it really depends on the size
of the changes and type of data. Many compression algorithms — including
the text-delta algorithm used by Subversion, and I presume ZLIB, too —
will at some point decide that it makes no sense to try compressing some
parts of the data, because the compressed representation would be larger
than the plain source (due to overhead for dictionaries etc.). However
if you're seeing pages of uncompressed plain text, then we'll have to
investigate further.

-- Brane
Received on 2018-12-12 14:14:54 CET

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