A High Performance Reliable Storage System Using HDDs as the Backup of SSDs

SSD (solid state drive) has been widely deployed due to its high performance. But, its cost and reliability have not met the demand of large-scale storage systems. RAID (redundant arrays of inexpensive disks) is a conventional scheme to enhance reliability. However, existing RAID schemes are not effective for SSDs anymore. Instead, we propose a hybrid array consisting of SSDs and HDDs (hard disk drive): SSDs are used for responding to I/O requests, and HDDs supply backup for SSDs. The hybrid array guarantees both performance and reliability at a reasonable cost. However, HDDs lose to SSDs in term of both latency and bandwidth. Therefore, we employ a nonvolatile memory to bridge the latency gap, and take other two measures to improve HDD’s bandwidth. Firstly, workloads within an HDD are reconfigured to be more sequential. Secondly, multiple HDDs collaborate with each other to supply much higher aggregate bandwidth. By these ways, HDDs can replicate SSDs’ data in time. We implement a prototype to evaluate the proposed array. First of all, we demonstrate that the hybrid array is feasible. Then, we compare the hybrid array with other solutions. Experimental results show that the hybrid array covers both performance and reliability, and is also cost-effective.