Hybrid RAID: A Solution for Enhancing the Reliability of SSD-Based RAIDs

The failure probability in SSDs increases when the number of Program/Erase (P/E) cycles increases. Traditionally, a group of SSDs are protected with parity disks, called SSD-based RAID. It has been shown that the reliability of RAIDs depends on the distribution of parities among SSDs. There are two main policies to distribute parities among SSDs in RAIDs, i.e., evenly and unevenly. By distributing parities evenly, all SSDs would wear out with the same rate, causing simultaneous failures of SSDs. By distributing parities unevenly, one of the SSDs in RAID may fail much earlier than the others. Both these two drawbacks, i.e., the simultaneous failures of SSDs and the rapid first failure of one SSD, reduce the reliability of SSD-based RAIDs. This paper proposes a Hybrid RAID, called Hy-RAID, as a solution to mitigate the above-mentioned drawbacks of the pure evenlyand unevenly-based RAIDs. Hy-RAID changes the policy of parity distribution when the number of P/E cycles of SSDs reaches to a predefined limit. To evaluate the reliability of Hy-RAID, two new quantitative metrics are proposed. The results show that Hy-RAID enhances reliability as compared with traditional RAIDs by omitting the simultaneous disk failures and postponing the first disk failure.