DuraCache: A durable SSD cache using MLC NAND flash

Abstract

Adopting SSDs as caches for HDD arrays has gained popularity in datacenters because SSDs are superior in handling random reads that HDDs cannot efficiently deal with. Two types of flash memory cells are available for building SSD caches, single-level cells (SLC) and multi-level cells (MLC). MLC is more appealing than SLC because it can achieve higher cache capacity at the same cost. However, we see a critical issue for SSD caches to adopt MLC NAND flash: the endurance of modern MLC NAND flash is too low to sustain datacenter workloads. In this paper, we propose DuraCache that addresses the durability issue of SSD caches. DuraCache exploits the fact that SSD caches are write-through caches in datacenters. Therefore, uncorrectable errors in SSD caches can be handled like cache misses which bring in correct data from HDD arrays. In addition, DuraCache gradually allocates more ECC parities associated with data when NAND flash reaches wearout thresholds. This allows SSD caches to continue operating by sacrificing available capacity. We conduct empirical experiments and demonstrate that DuraCache enables MLC SSD caches to achieve 4.1 years of service life assuming a TPC-C workload.