How Many MLCs Should Impersonate SLCs to Optimize SSD Performance?

Since an MLC (multi-level cell) can be used in an SLC (single-level cell) mode, an MLC-based flash SSD typically uses a fixed small portion (called log partition) in the SLC mode to accommodate hot data so that its overall performance can be improved. In this paper, we show that a fixed capacity of a log partition without considering workload characteristics can lead to an unexpected overall performance degradation. Contrary to intuition, we notice that blindly enlarging the capacity of a log partition would also result in worse performance due to the increased garbage collection cost in a data partition, which serves cold data. How many MLCs should impersonate SLCs under a particular workload to achieve an optimized performance is still an open question. To answer this question, we first measure write costs on each partition and their impact on the overall performance of an SSD. Next, a hardware-validated write cost model is built. Based on the model, we demonstrate that for each workload there always exists an optimal partitioning scheme. Further, to verify the effectiveness of our workload-aware dynamic partitioning strategy, we implement an FTL (flash translation layer) called BROMS (Best Ratio Of MLC to SLC), which adaptively adjusts the capacities of two partitions according to the workload characteristics. Experimental results from a hardware platform show that BROMS outperforms a fixed partitioning scheme by up to 86%.