Re: Why is the actual disk usage of btrfs considered unknowable?

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On Sun, Dec 07, 2014 at 10:20:27AM -0800, ashford@xxxxxxxxxxxxx wrote:
[snip]
> > 3) From what I gathered it is planned to allow different raid /
> > redundancy levels for different subvolumes. BTRFS can´t know
> > beforehand where applications request to save future data, i.e.
> > in which subvolume.
> 
> Same as above.
> 
> If a user will be requesting to use a specific subvolume, it is up to them
> to verify that adequate free space exists there, or handle the exception.

   OK, so let's say I've got a filesystem with 100 GiB of unallocated
space. I have two subvolumes, one configured for RAID-1 and one
configured for single storage.

   What number should be shown in the free output of df?

   100 GiB? But I can only write 50 GiB to the RAID-1 subvolume before
it runs out of space.

   50 GiB? I can get twice that much on the single subvolume.

   *Any* value shown here is going to be inaccurate, and whatever way
round we show it, someone will complain.

   Hugo.

-- 
Hugo Mills             | My doctor tells me that I have a malformed
hugo@... carfax.org.uk | public-duty gland, and a natural deficiency in moral
http://carfax.org.uk/  | fibre.
PGP: 65E74AC0          |                                     Zaphod Beeblebrox

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