  SSDs are emerging storage devices which unlike HDDs, do not have mechanical
parts and therefore, have superior performance compared to HDDs. Due to the
high cost of SSDs, entirely replacing HDDs with SSDs is not economically
justified. Additionally, SSDs can endure a limited number of writes before
failing. To mitigate the shortcomings of SSDs while taking advantage of their
high performance, SSD caching is practiced in both academia and industry.
Previously proposed caching architectures have only focused on either
performance or endurance and neglected to address both parameters in suggested
architectures. Moreover, the cost, reliability, and power consumption of such
architectures is not evaluated. This paper proposes a hybrid I/O caching
architecture that while offers higher performance than previous studies, it
also improves power consumption with a similar budget. The proposed
architecture uses DRAM, Read-Optimized SSD, and Write-Optimized SSD in a
three-level cache hierarchy and tries to efficiently redirect read requests to
either DRAM or RO-SSD while sending writes to WO-SSD. To provide high
reliability, dirty pages are written to at least two devices which removes any
single point of failure. The power consumption is also managed by reducing the
number of accesses issued to SSDs. The proposed architecture reconfigures
itself between performance- and endurance-optimized policies based on the
workload characteristics to maintain an effective tradeoff between performance
and endurance. We have implemented the proposed architecture on a server
equipped with industrial SSDs and HDDs. The experimental results show that as
compared to state-of-the-art studies, the proposed architecture improves
performance and power consumption by an average of 8% and 28%, respectively,
and reduces the cost by 5% while increasing the endurance cost by 4.7% and
negligible reliability penalty.
