Performance and Reliability Analysis of Cross-Layer Optimizations of NAND Flash Controllers

NAND flash memories are becoming the predominant technology in the implementation of mass storage systems for both embedded and high-performance applications. However, when considering data and code storage in Non-Volatile Memories (NVMs), such as NAND flash memories, reliability and performance become a serious concern for systems designers. Designing NAND flash-based systems based on worst-case scenarios leads to waste of resources in terms of performance, power consumption, and storage capacity. This is clearly in contrast with the request for runtime reconfigurability, adaptivity, and resource optimization in modern computing systems. There is a clear trend toward supporting differentiated access modes in flash memory controllers, each one setting a differentiated tradeoff point in the performance-reliability optimization space. This is supported by the possibility of tuning the NAND flash memory performance, reliability, and power consumption through several tuning knobs such as the flash programming algorithm and the flash error correcting code. However, to successfully exploit these degrees of freedom, it is mandatory to clearly understand the effect that the combined tuning of these parameters has on the full NVM subsystem. This article performs a comprehensive quantitative analysis of the benefits provided by the runtime reconfigurability of an MLC NAND flash controller through the combined effect of an adaptable memory programming circuitry coupled with runtime adaptation of the ECC correction capability. The full NVM subsystem is taken into account, starting from a characterization of the low-level circuitry to the effect of the adaptation on a wide set of realistic benchmarks in order to provide readers a clear view of the benefit this combined adaptation may provide at the system level.