Exploiting Write Heterogeneity of Morphable MLC/SLC SSDs in Datacenters with Service-Level Objectives

Given the needs of data-intensive web services and cloud computing applications, storage centers play an important role in serving the demanded data access while jointly considering low cost, qualified performance, and good scalability. To manage peak workloads with performance requirements for read/write latencies, overprovisioning more storage nodes is common but also increases total cost as well as power consumption. Recently, due to the growing capacity and dropping price, NAND-flash-based Solid-State Drives (SSDs) have become an attractive storage solution in datacenters. In this work, we exploit the write heterogeneity in Multi-Level-Cell (MLC) NAND flash memory to meet Service-Level Objectives (SLOs) of applications and to avoid storage overprovision. In MLC NAND flash memory, a memory cell can be programmed as a Single-Level Cell (SLC) or a multi-level cell at runtime, and SLC writes take shorter latency with the cost of larger consumed capacity. The proposed SLO-aware morphable SSD design seeks to meet the SLO requirement by deciding the write mode of each write request while minimizing the number of SLC writes. Experimental results show that the proposed design meets the SLO requirement for all of the tested I/O traces with less than 2.8 percent extra erase counts in average, while conventional MLC SSDs require up to 2.375 times storage overprovision to meet the SLO requirement.