Strategies for Data Protection - Brocade

Highly Available Storagecharacteristic of RAID 1. The inverse of RAID 0+1 is RAID 10, in whichcase the mirroring occurs first as a virtual disk before striping isexecuted.Other RAID techniques attempt to integrate the performance advantageof data striping with alternative means to reconstruct data in theevent of disk failure. The most commonly deployed is RAID 5, whichstripes data across a disk set and uses block parity instead of mirroringto rebuild data. As data blocks are striped across multiple disks, aparity block is calculated using an eXclusive OR (XOR) algorithm andwritten to disk. If a disk fails, the data can be reconstructed on a newdisk from the parity blocks. In RAID 4, the parity blocks are written to asingle dedicated disk. This creates some vulnerability if the parity diskitself fails and incurs a write penalty, since every write must be parityprocessed on a single drive. RAID 5 reduces the write penalty by placingthe parity information across multiple disks in the RAID set. As theparity data is generated, the array controller does not have to wait forthe availability of a dedicated disk. As shown in Figure 10, RAID 5arrays typically house spare disks that can automatically be broughtonline and reconstructed in the event of disk failure. In this example, ifthe third disk in the set fails, the parity block on the fifth disk (P abcd)can be used to recreate both block C and the parity block (P efgh) forblocks E, F, G, and H.Figure 10. RAID 5 with distributed parity blocksThe primary benefit of RAID 5 is its ability to protect block data whileminimizing the number of disks required to guard against failure. Onthe other hand, the write penalty generated by parity calculation needshardware acceleration to improve performance and avoid an adverseimpact to upper-layer applications.Strategies for Data Protection 19