HP StorageWorks Modular Smart Array 1000 HP StorageWorks 1000/1500 Modular Sma - Page 48

In each mirrored pair, the physical drive that is not busy answering other requests answers any read requests that are sent to the array. (This behavior is called load balancing.) If a physical drive fails, the remaining drive in the mirrored pair can still provide all the necessary data. Several drives in the array can fail without incurring data loss, as long as no two failed drives belong to the same mirrored pair. RAID 1+0 is useful when high performance and data protection are more important than the cost of physical drives. Table 4 RAID 1, RAID 1+0 features Advantages Highest read and write performance of any fault-tolerant configuration. No loss of data as long as no failed drive is mirrored to another failed drive. Disadvantages Expensive (half of the drives are used for fault tolerance). Only half of total drive capacity usable for data storage. RAID 5-Distributed data guarding In this method, a block of parity data is calculated for each stripe from the data that is in all other blocks within that stripe. The blocks of parity data are distributed across every physical drive within the logical drive (Figure 7). When a physical drive fails, data that was on the failed drive can be calculated from the data on the remaining drives and the parity data. This recovered data is written to the assigned spare or to a replacement drive in a process called a rebuild. S1 B1 S2 B3 S3 P5,6 S4 B7 D1 B2 P3,4 B5 B8 D2 P1,2 B4 B6 P7,8 D3 15316 Figure 7 RAID 5 array, with three physical hard drives (D1, D2, and D3) showing distributed parity information (Px,y) This configuration is useful when cost, performance, and data availability are equally important. Table 5 RAID 5 features Advantages Highest read performance. No loss of data if one physical drive fails. More usable drive capacity than RAID 1+0, because parity information requires the storage space equivalent to one physical drive. Disadvantages Relatively low write performance. Loss of data if a second drive fails before data from the first failed drive is rebuilt. RAID 6-Advanced data guarding RAID 6 (also called RAID ADG) is similar to RAID 5, because both methods generate and store parity information to protect against data loss caused by drive failure. With RAID 6, however, two different sets of parity data are distributed across the physical drives, allowing data to be preserved even if two drives fail. Each set of parity data uses up a capacity equivalent to that of one of the constituent drives, as shown in Figure 8. 48 Storage overview