Don't Let RAID Raid the Lifetime of Your SSD Array

Parity protection at system level is typically employed to compose reliable storage systems. However, careful consideration is required when SSD based systems employ parity protection. First, additional writes are required for parity updates. Second, parity consumes space on the device, which results in write amplification from less efficient garbage collection at higher space utilization. This paper analyzes the effectiveness of SSD based RAID and discusses the potential benefits and drawbacks in terms of reliability. A Markov model is presented to estimate the lifetime of SSD based RAID systems in different environments. In a single array, our preliminary results show that parity protection provides benefit only with considerably low space utilizations and low data access rates. However, in a large system, RAID improves data lifetime even when we take write amplification into account.