Improving performance and lifetime of the SSD RAID-based host cache through a log-structured approach

This paper proposes a cost-effective and reliable SSD host cache solution that we call SRC (SSD RAID Cache). Cost-effectiveness is brought about by using multiple low-cost SSDs and reliability is enhanced through RAID-based data redundancy. RAID, however, is managed in a log-structured manner on multiple SSDs effectively eliminating the detrimental read-modify-write operations found in conventional RAID-5. Within the proposed framework, we also propose to eliminate parity blocks for stripes that are composed of clean blocks as the original data resides in primary storage. We also propose the use of destaging, instead of garbage collection, to make space in the cache when the SSD cache is full. We show that the proposed techniques have significant implications on the performance of the cache and lifetime of the SSDs that comprise the cache. Finally, we study various ways in which stripes can be formed based on data and parity block allocation policies. Our experimental results using different realistic I/O workloads show using the SRC scheme is on average 59% better than the conventional SSD cache scheme supporting RAID-5. In case of lifetime, our results show that SRC reduces the erase count of the SSD drives by an average of 47% compared to the RAID-5 scheme.