private function parseDu($duOutput, array $mountPaths)
    {
        $mountSizes = [];
        $duOutputSplit = explode("\n", $duOutput, count($mountPaths));
        foreach ($mountPaths as $i => $mountPath) {
            if (!isset($duOutputSplit[$i])) {
                throw new \RuntimeException("Failed to find row $i of 'du' command output: \n" . $duOutput);
            }
            list($mountSizes[$mountPath],) = explode("\t", $duOutputSplit[$i], 2);
        }

        return $mountSizes;
    }