From mboxrd@z Thu Jan  1 00:00:00 1970
From: Li Zefan <lizf@cn.fujitsu.com>
Subject: Re: Inefficient storing of ISO images with compress=lzo
Date: Mon, 19 Sep 2011 10:53:45 +0800
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Cc: linux-btrfs@vger.kernel.org
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Maciej Marcin Piechotka wrote:
> I've noticed that:
> 
>  - with x86-64 Fedora 15 DVD install images:
>    - du -sh <ROOT VOLUME> was 36 GB
>    - btrfs df | grep -i data have shown over 40 GB used
>  - without
>    - du -sh <ROOT VOLUME> is 34 GB
>    - btrfs df | grep -i data have shown less then 34 GB used
> 
> It seems that iso files are considered compressable while they may not be (and penalty is severe - 3x).
> 

With compress option specified, btrfs will try to compress the file, at most
128K at one time, and if the compressed result is not smaller, the file will
be marked as uncompressable.

I just tried with Fedora-14-i386-DVD.iso, and the first 896K is compressed,
with a compress ratio about 71.7%, and the remaining data is not compressed.

--
Li Zefan