I've accidentally overwritten an important file on BTRFS.
Knowing that this filesystem is COW, it should be in theory possible to recover an old version of the file.
Unfortunately googling didn't reveal relevant information. Can someone please advice me, what steps can I do (if any)?
You should try the btrfs-undelete tool posted on this mailing list thread by Jörg Walter.
http://comments.gmane.org/gmane.comp.file-systems.btrfs/22560