Re: BTRFS constantly reports "No space left on device" even with a huge unallocated space

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-09-01 12:34, Ronan Arraes Jardim Chagas wrote:
> Em Qui, 2016-09-01 às 09:21 -0400, Austin S. Hemmelgarn escreveu:
>> Yes, you can just run `btrfs quota disable /` and it should
>> work.  This
>> ironically reiterates that one of the bigger problems with BTRFS is
>> that
>> distros are enabling unstable and known broken features by default
>> on
>> install.  I was pretty much dumbfounded when I first learned that
>> OpenSUSE is enabling BTRFS qgroups by default since they are known
>> to
>> not work reliably and cause all kinds of issues.
>
> Thanks Austin! I executed the command and now I get:
>
> btrfs qgroup show /
> ERROR: can't perform the search - No such file or directory
> ERROR: can't list qgroups: No such file or directory
>
> as expected. Now I will wait for +- 1 week to see if the problem will
> occur and, if not, I will send an e-mail to openSUSE factory mailing
> list to start a discussion if it is better to not enable qgroups by
> default.
I have a feeling that you'll probably have no issues.

As far as having qgroups enabled by default, I think the reasoning is to 
emulate having separate filesystems with their own space limits.  I can 
entirely understand this use case, and TBH it's about the only use case 
I'd consider quota groups for (per-user subvolumes for home directories 
are great, but there are numerous perfectly legitimate reasons to have 
very large amounts of data in your home directory for very short periods 
of time, so I wouldn't personally use qgroups there).  The problem 
arises from the fact that it doesn't _look_ like separate filesystems 
(single entry in df, all the mounts point at the same device, etc), and 
the standard of overloading ENOSPC to mean you've hit your quota leads 
to lots of confusion in this particular case (especially considering the 
free space issues that BTRFS is known to have from time to time).