Redundant Arrays of Independent Disks RAID is a popular storage architecture with high performance and reliability. RAID-6 with a higher level of reliability based on MDS (Maximum Distance Separable) code is well studied, for its optimal storage efficiency. RAID-6 could offer continuous services in degraded mode, during the period of online failure recovery. However, the online recovery would bring a considerable I/O workflow to the storage system, that almost all the surviving data in the system need to be accessed. Due to the limitation of disk bandwidth, user response time would be significantly affected by the recovery workflow. In this paper, we examine the online recovery performance of two typical MDS RAID-6 codes RDP code and P-code. To our observation, P-code significiantly outperforms RDP in user response time and recovery duration during a single disk failure recovery. To our analysis, the difference comes from not only the parity layout but also the parity organization. Therefore, we propose a new categorization for existing MDS RAID-6 codes, based on the methodology of parity organization. By our approach, all the MDS RAID-6 codes could be categorized to Sym-codes with only one type of parity, and Asym-codes with at least two different types of parity.