RB-Explorer: An Accurate and Practical Approach to Write Amplification Measurement for SSDs

A large write amplification ratio degrades the program/erase cycles (P/Es) of NAND Flashes and reduces the endurance and performance of solid state disks (SSDs). The lack of a practical way to measure write amplification for SSDs motivates us to propose a novel measuring method called RB-Explorer at the SSD level rather than the NAND Flash level. The goal of RB-Explorer is two-fold: (1) to accurately measure the write amplification of SSDs to quantify SSD endurance and (2) to study the impacts of I/O techniques on write amplification of SSDs. RB-Explorer incorporates a Ready/Busy (R/B) signal of one of the NAND Flashes in an SSD in a proposed write amplification model for SSDs with four full-parallelism levels (i.e., the channel, chip, die, and plane levels). RBExplorer takes two steps toward measuring write amplification. First, RB-Explorer quantifies the number of page programs using the low R/B signal level, the duration of which varies with the different operation (i.e., read, program, and erase) in NAND Flash. Second, RB-Explorer measures data volume written to NAND Flashes by considering parallelisms at four levels. Data volume written to a die in a NAND Flash is obtained as a product of the number N_p of programs and page size P_a. Given the number N_channel of channels, the number N_chip of chips per channel, and the number N_die of dies per chip, one can obtain the data volume written to NAND Flashes as a product of N_p, P_a, N_die, N_chip, and N_channel. RB-Explorer is applied to analyzing write amplification ratios of SSDs to track SSD endurance. Furthermore, we implement a real-world SSD (i.e., SSD-v) and employ a fine-tuned SSD simulator (i.e., SSDsim) to validate the accuracy of RB-Explorer. Our experimental results show that RB-Explorer improves on the accuracy of SSDsim-the state-of-the-art SSD simulator-in most tested cases. We conduct a series of me- surements using micro-benchmarks and I/O traces to demonstrate how RB-Explorer may be applied to investigate SSDs.