good test plan provided in https://bugs.kde.org/show_bug.cgi?id=400610 . Works like a charm, tested in KDE Neon dev-ustable
The reason for bug was that return value of posix_fallocate() was assumed to be negative on error, but in fact it is a positive integer. The check was `< 0`, whi should be `!= 0`. ( http://man7.org/linux/man-pages/man3/posix_fallocate.3.html )