WOM-FTL: An Efficient FTL for High-Density Flash Memory Through WOM-v Codes

High-density NAND flash memory, such as quadruple-level cell (QLC) flash, has has been widely adopted in emerging storage systems. However, its limited endurance and performance challenges necessitate novel solutions. Voltage-based write-once memory (WOM-v) codes have demonstrated their effectiveness in extending flash memory lifespan by reducing the erase count of flash blocks. Concurrently, secure deletion is essential to ensure data privacy in flash-based storage systems. Existing secure deletion approaches—encryption-based, erasure-based, and scrubbing-based—often face limitations such as susceptibility to deciphering or significant performance overheads. Additionally, the inherent “big block problem” in high-density flash memory complicates garbage collection (GC), further degrading system performance. To address these challenges, this paper proposes WOM-FTL, a flash translation layer (FTL) that integrates secure deletion and garbage collection (GC) with WOM-v codes to enhance both security and performance. WOM-FTL classifies request data into four categories based on access frequency and privacy requirements: hot-secure (HS), cold-secure (CS), hot-unsecure (HU) and cold-unsecure (CU). Additionally, WOM-FTL divides each block into several equal-sized sub-blocks and further classifies them into top sub-blocks and bottom sub-blocks according to their data storage characteristics. When a secure data deletion command is issued to the storage device, WOM-FTL leverages unsecure data (HU and CU) to overwrite the secure data (HS and CS). Furthermore, WOM-FTL allocates different types of request data to the corresponding sub-blocks, creating a data allocation pattern that is both scrubbing-friendly and GC-friendly. Experimental results demonstrate that WOM-FTL improves the I/O performance of storage systems by 60.91% compared to state-of-the-art solutions, providing a significant advancement in secure and efficient management of high-density flash memory.