The devil is in the details: on the real costs and benefits of WOM codes for Flash

NAND flash, used in modern SSDs, is a write-once medium, where each memory cell must be erased prior to writing. The lifetime of an SSD is limited by the number of erasures allowed on each cell. Thus, minimizing erasures is a key objective in SSD design.

A promising approach to eliminate erasures and extend SSD lifetime is to use write-once memory (WOM) codes, designed to accommodate additional writes on write-once media. However, these codes inflate the physically stored data by at least 29%, require an extra read operation before each additional write, and their applicability to MLC and TLC flash is restricted.

I will describe the first evaluation of the possible benefit from reusing flash pages with WOM codes on real flash chips and an end-to-end FTL design and implementation. Our results expose a considerable gap between the theoretical benefits of page reuse and those that can be achieved on current state-of-the-art hardware. However, they also lay the ground for promising future improvements.

Joint work with Fabio Margaglia, Eitan Yaakobi, Yue Li, Assaf Schuster and Andre Brinkmann.