Log-ROC: Log Structured RAID on Open-Channel SSD

With the development of high-density flash memory, the price decreases, and the program/erase capability for each storage cell also decreases. Error correcting should be applied at all levels in SSD (Solid State Drive) based storage systems. This paper focuses on how to build a redundant array of Open-Channel SSDs, and proposes Log-ROC to enforce reliability at the drive level. Log-ROC adopts log structure, which means each write will be firstly buffered in the internal cache, and then the data in the cache are encoded in corresponding RAID level and flushed out in new locations as logs. Such mechanism is integrated with the host-side FTL (flash translation layer), effectively eliminating the parity update issue existing in RAID5 systems. We have built Log-ROC on 4 Open-Channel SSDs and evaluated Log-ROC with FIO and Filebench workloads. Compared with RAID5 on conventional SSDs, Log-ROC can achieve 1.91∼5.29× speedup in throughput on different workloads.