On Fri, 21 Aug 2015 11:15:23 +0200
Swâmi Petaramesh [off-list ref] wrote:

Le vendredi 21 août 2015 10:47:45 Karsten Heymann a écrit :

quoted

did you check with lsof that no process has an open file descriptor in
that directory?

Yes, I even renamed it and rebooted the machine to be double-sure, but to no 
avail...

And did you try the obvious, i.e. running btrfsck?

-- 
With respect,
Roman