func getSegmentSize(dir string, index int) (int64, error) {
	i := int64(-1)
	fi, err := os.Stat(wal.SegmentName(dir, index))
	if err == nil {
		i = fi.Size()
	}
	return i, err
}
func getSegmentSize(dir string, index int) (int64, error) {
	i := int64(-1)
	fi, err := os.Stat(wal.SegmentName(dir, index))
	if err == nil {
		i = fi.Size()
	}
	return i, err
}
// Get size of segment.