Reliability and security of RAID storage systems and D2D archives using SATA disk drives

Abstract

Information storage reliability and security is addressed by using personal computer disk drives in enterprise-class nearline and archival storage systems. The low cost of these serial ATA (SATA) PC drives is a tradeoff against drive reliability design and demonstration test levels, which are higher in the more expensive SCSI and Fibre Channel drives. This article discusses the tradeoff between SATA which has the advantage that fewer higher capacity drives are needed for a given system storage capacity, which further reduces cost and allows higher drive failure rates, and the use of additional storage system redundancy and drive failure prediction to maintain system data integrity using less reliable drives. RAID stripe failure probability is calculated using typical ATA and SCSI drive failure rates, for single and double parity data reconstruction failure, and failure due to drive unrecoverable block errors. Reliability improvement from drive failure prediction is also calculated, and can be significant. Today's SATA drive specifications for unrecoverable block errors appear to allow stripe reconstruction failure, and additional in-drive parity blocks are suggested as a solution. The possibility of using low cost disks data for backup and archiving is discussed, replacing higher cost magnetic tape. This requires significantly better RAID stripe failure probability, and suitable drive technology alternatives are discussed. The failure rate of nonoperating drives is estimated using failure analysis results from ≈4000 drives. Nonoperating RAID stripe failure rates are thereby estimated. User data security needs to be assured in addition to reliability, and to extend past the point where physical control of drives is lost, such as when drives are removed from systems for data vaulting, repair, sale, or discard. Today, over a third of resold drives contain unerased user data. Security is proposed via the existing SATA drive secure-erase command, or via the existing SATA drive password commands, or by data encryption. Finally, backup and archival disc storage is compared to magnetic tape, a technology with a proven reliability record over the full half-century of digital data storage. In contrast, tape archives are not vulnerable to tape transport failure modes. Only failure modes in the archived tapes and reels will make data unrecoverable.