Chapter 10. Data protection with RAID Double Parity 13110.2 Why use RAID-DPAs mentioned earlier, traditional single-parity RAID offers adequate protection against asingle event. This event can be either a complete disk failure or a bit error during a read. Ineither event, data is re-created by using both parity data and data that remains on unaffecteddisks in the array or volume.If the event is a read error, re-creating data happens almost instantaneously and the array orvolume remains in an online mode. However, if a disk fails, the lost data must be re-created.The array or volume remains in a vulnerable degraded mode until data is reconstructed onto areplacement disk or global hot spare disk. This degraded mode is where traditionalsingle-parity RAID fails to meet the demands of modern disk architectures. In single-parityRAID, the chance of secondary disk failure is increased during rebuild times, increasing therisk of data loss.Modern disk architectures have continued to evolve, as have other computer-relatedtechnologies. Disk drives are orders of magnitude larger than they were when RAID was firstintroduced. As disk drives have gotten larger, their reliability has not improved, and the biterror likelihood per drive has increased proportionally with larger media. These three factors(larger disks, unimproved reliability, and increased bit errors with larger media) have seriousconsequences for the ability of single-parity RAID to protect data.Given that disks are as likely to fail now as when RAID technology was first introduced, RAIDis still vital. Integrating RAID-DP when one disk fails, RAID re-creates data from both paritiesand the remaining disks in the array or volume onto a hot spare disk. But because RAID wasintroduced, the significant increases in disk size have resulted in much longer reconstructiontimes for data lost on the failed disk.It takes much longer to re-create lost data when a 274 GB disk fails than when a 36 GB diskfails (Figure 10-2). In addition, reconstruction times are longer because the larger disk drivesin use today tend to be ATA-based. ATA-based drives run more slowly and are less reliablethan smaller, SCSI-based drives.Figure 10-2 Disk size versus reconstruction time10.2.1 Single-parity RAID using larger disksThe various options to extend the ability of single-parity RAID to protect data as diskscontinue to get larger are not attractive. The first option is to continue to buy and implementstorage using the smallest disk sizes possible so that reconstruction completes quicker.However, this approach is impractical. Capacity density is critical in space-constrained data
