Enabling cost-effective flash based caching with an array of commodity SSDs

SSD based cache solutions are being widely utilized to im- prove performance in network storage systems. With a goal of providing a cost-effective, high performing SSD cache so- lution, we propose a new caching solution called SRC (SSD RAID as a Cache) for an array of commodity SSDs. In designing SRC, we borrow both the well-known RAID tech- nique and the log-structured approach and adopt them into the cache layer. In so doing, we explore a wide variety of de- sign choices such as flush issue frequency, write units, form- ing stripes without parity, and garbage collection through copying rather than destaging that become possible as we make use of RAID and a log-structured approach at the cache level. Using an implementation in Linux under the Device Mapper framework, we quantitatively present and analyze results of the design space options that we consid- ered in our design. Our experiments using realistic work- load traces show that SRC performs at least 2 times better in terms of throughput than existing open source solutions. We also consider cost-effectiveness of SRC with a variety of SSD products. In particular, we compare SRC config- ured with MLC and TLC SATA SSDs and a single high-end NVMe SSD. We find that SRC configured as RAID-5 with low-cost MLC and TLC SATA SSDs generally outperforms that configured with a single high-end SSD in terms of both performance and lifetime per dollars spent.