HP StorageWorks 4000s NAS 4000s and 9000s Administration Guide - Page 59
RAID ADG-Advanced Data Guarding and RAID 5DP-Double Parity
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Storage Management Overview Disadvantages Some disadvantages of distributed data guarding are: ■ Lower write performance than RAID 0 or RAID 1+0. ■ Possibility of data loss if a second drive fails before data from the first failed drive has been rebuilt. RAID ADG-Advanced Data Guarding and RAID 5DP-Double Parity RAID ADG and RAID 5DP are similar to RAID 5 in that parity information is generated (and stored) to protect against data loss caused by drive failure. With RAID ADG and RAID 5DP, however, two different sets of parity data are used. This allows data to still be preserved if two drives fail. As can be seen from Figure 30, each set of parity data uses up a capacity equivalent to that of one of the constituent drives, for a total parity usage of two drives of space. This method is most useful when data loss is unacceptable, but cost must also be minimized. The probability that data loss will occur when configured with RAID ADG or RAID 5DP is less than when configured with RAID 5. Note: The ADG feature is available only with the MSA1000. RAID 5DP is available only with HP Virtual Arrays and is equivalent to ADG. B1 B3 P5,6 Q7,8 B2 P3,4 Q5,6 B7 P1,2 Q3,4 B5 B8 Q1,2 B4 B6 P7,8 Figure 30: RAID ADG (advanced data guarding) with two sets of parity data Advanced Data Guarding technology offers the best combination of fault tolerance and usable disk space among RAID levels. This technology allows the safe deployment of large capacity disk drives and the creation of very large storage volumes without expensive overhead to protect business critical data. This technology provides more flexibility in responding to drive failures without the fear of costly server downtime. Advance Data Guarding protects against multiple disk failures, while requiring the capacity of two drives in an array of up to 56 disk drives to be set aside for dual sets of distributed parity data. It provides data protection greater than RAID 5, and also has the capacity utilization efficiency similar to RAID 5. NAS 4000s and 9000s Administration Guide 59