Comment # 1 on bug 908066 from Filipe Manana

(In reply to Andreas Schneider from comment #0)
> I get errors with ENOSPCE writing on my btrfs partition but the btrfs tools
> tells me there are 25G free.
> 
> magrathea:~ # btrfs fi df /
> Data, single: total=77.12GiB, used=54.62GiB
> System, single: total=4.00MiB, used=16.00KiB
> Metadata, single: total=3.01GiB, used=2.49GiB
> GlobalReserve, single: total=512.00MiB, used=0.00B

Hi Andreas,

What you can read from that output is 77Gb are allocated for data, of which
54.62 are in use. It likely means more metadata space wasn't possible to
allocate because all the remaining space is allocated for data.

Have you tried the usual recommendations for running balance to reclaim data
space so that it can be reused for metadata?

See: https://btrfs.wiki.kernel.org/index.php/Balance_Filters
In particular the section titled "Balancing to fix filesystem full errors"