bug#60335: du should warn about btrfs

[Paul Eggert]

On 12/26/22 00:55, Krzysztof Żelechowski wrote:
> The total reported on my
> system is almost twice the volume capacity.  This is unexpected and should be
> indicated with a warning, both in the output and in the documentation.

Although we can document the issue, I don't think a warning is needed. 
Nor is it implementable, since there's no reliable way to know whether 
copy-on-write, compression, etc. are affecting the numbers the operating 
system is reporting.

Perhaps in some distant future the kernel, Btrfs, ZFS etc. can find a 
way to report underlying space usage in a better way. For example, if 
CoW is being used for a page they might divide the page's size by the 
number of times it's used. Whatever. In the meantime the best we can do 
is document the issue, which I've attempted to do by installing the 
attached patch.

Thanks for reporting the problem.

0001-doc-improve-doc-of-du-with-CoW-etc.patch
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