Research On Flash-based Hybrid Storage System

Recent advances in flash memory show great potential to replace traditional hard drives (HDDs) with flash-based solid state drives (SSDs) from personal computing to distributed systems. However, it is still a long way to go before completely using SSDs for enterprise data storage. Considering the cost, performance, and reliability of SSDs, a practical solution is to combine both SSDs and HDDs. This paper proposes a hybrid storage system named PR-HSS (Performance-Reliability-based Hybrid Storage System) to tradeoff between I/O performance and data reliability. PR-HSS includes a high-performance SSD and a traditional HDD to store mirrored data for reliability.All of the I/O requests are redirected to the primary SSD first and then the updated data blocks are copied to the backup HDD asynchronously. In order to hide the latency of copying operations, we use an I/O window to coalesce write requests and maintain an ordered I/O queue to shorten the HDD seeks and rotation time. To adapt to different I/O workloads, we develop an adaptive policy to balance the foreground I/O processing and background mirroring. We implement a prototype system of PR-HSS by developing a Linux device driver and conduct experiments on the IoMeter, PostMark, and TPC-C benchmarks.Our results show that PR-HSS can achieve up to 10 times the performance of a RAID1 storage system for the IoMeter and PostMark workloads while reducing data reliability less than 2%. More interestingly, while PR-HSS does not produce any performance benefit for the TPC-C benchmark, it does allow the system to scale to larger sizes than when using an HDD-based RAID system alone. The result of temporal burstiness analysis indicates that TPC-C workload has more idle time to do the background backup than IoMeter and PostMark.