Performance, reliability, and performability of a hybrid RAID array and a comparison with traditional RAID1 arrays

摘要

We describe a hybrid mirrored disk organization patented by LSI Logic Corp. and compare its performance, reliability, and performability with traditional mirrored RAID1 disk organizations and RAID(4+ℓ), ℓ≥1. LSI RAID has the same level of redundancy as mirrored disks, but also utilizes parity coding. Unlike RAID1, which cannot tolerate all two disk failures, LSI RAID similarly to RAID6 is 2 Disk Failure Tolerant (2DFT), but in addition it can tolerate almost all three disk failures, while RAID1 organizations are generally 1DFT. We list analytic expressions for the reliability of various RAID1 organizations and use enumeration when the reliability expression cannot be obtained analytically. An asymptotic expansion method based on disk unreliabilities is used for an easy comparison of RAID reliabilities. LSI RAID performance is evaluated with the Read-Modify-Write (RMW) and ReConstruct Write (RCW) methods to update parities. The combination of the two methods is used to balance data and parity disk loads, which results in maximizing the I/O throughput. The analysis shows that LSI RAID has an inferior performance with respect to basic mirroring in processing an OLTP workload, but it outperforms RAID6. LSI RAID in spite of its higher Mean Time to Data Loss (MTTDL) is outperformed by other RAID1 organizations as far as its performability is concerned, i.e., the number of I/Os carried out by the disk array operating at maximum I/Os Per Second (IOPS) until data loss occurs. A survey of RAID1 organizations and distributed replicated systems is also included.