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compress-force not really forcing compression?

DISCUSSION Tomasz Chmielewski 1 participant 1 message Dec 24, 2018 Btrfs file system

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If a file has portions which are not easily compressible does that imply all future writes are also incompressible. IMO no, so I think what will be prudent is remove FORCE_COMPRESS altogether and make the code act as if it's always on.

Any opinions?

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That is a good idea. If I turn on compression I would expect everything to be compressed, except in cases where there is no size benefit.

I expect that the vast majority of files consist of blocks of similar compressibility. Thus, finding a block that fails to compress strongly suggests other blocks are either incompressible as well or compress only minimally. Refusing to waste time, electricity and fragmentation in such case is a good default, I think.

Please see this thread for an example where btrfs compression fails to detect that text files compress very well:

https://marc.info/?t=144905015000003&r=1&w=4

To put it short - with rsync server receiving text logs from remote servers, sometimes at slow speeds (due to how logfiles are appended):

- compress + zlib - some 20% compression ratio
- compress-force + zlib - some 80% compression ratio

Tomasz Chmielewski
https://lxadm.com