RAID4-structured Heterogeneous-chip-based SSD

SSDs consisting of NAND-flash memory chips have low access delay and are a promising alternative to traditional HDDs in the I/O intensive application environment, but the capacity of single disk is one order of magnitude lower than HDDs. To increase the capacity of those SSDs, MLC techniques are adopted while the feature size of flash memory are continuously scaling down, which induce high and continuously increasing raw bit error rate thus make the conventional in-Page ECCs(e.g., BCH codes) to be inadequate to ensure the reliability of data.To address this problem, a novel RAID4-structured Heterogeneous-chip-based SSD is proposed, named as RH-SSD. RH-SSD replaces a small portion of MLC chips with SLC chips which have high life endurance and P/E performance to store parity data, thus improve the reliability of data by employing two-level data redundancy and significantly reduce influence of performance degradation caused by updates of parity data. In RH-SSD imbalance-stripe technique is proposed smooth the capacity gap of SLC and MLC chips while splitted-page technique is proposed to smooth page size gap. In the meanwhile, RH-SSD is implemented in Micro-Add solid state disk simulator.We evaluate RH-SSD against RAID0-SSD, RAID4-SSD and RAID5-SSD which are respectively based on RAID0, RAID4 and RAID5. The result shows that RH-SSD is 149.5% higher than RAID4-SSD, 33.3% higher than RAID5-SSD and only 8.6% lower than RAID0-SSD on average IOPS; 11.7% lower than RAID4-SSD, 52.2% lower than RAID5-SSD and only 11.2% high than RAID0-SSD on average chip wear rate. Considering RAID0-SSD has no parity data and poor of reliability, RH-SSD has a better balance of high data reliability and availability, write throughput and life endurance.