ZombieNAND: Resurrecting Dead NAND Flash for Improved SSD Longevity

ABSTRACT

As consumer pressure for more bits per dollar and higher density-per-solid-state disk (SSD) forces manufacturers to squeeze more than one bit per flash cell and feature sizes downwards, wear-out is again becoming an increasing concern. Specifically, while single-level cell flash at larger feature sizes used to boast over 100,000 program/erase (P/E) cycles, modern triple-level cell flash can only sustain a measly 3,000 P/E cycles before it can no longer be reliably used. However, one lesser known facet of NAND flash design is that there is no material difference between cells that store one, two, or three bits per cell -- it is merely a logical interpretation of the cells contents.

Therefore, in this work we leverage this interesting property to explore how resurrecting dead flash cells to create "Zombie-NAND" flash can improve an SSD's lifetime, and what, if any, impact on latency results in doing such. Specifically, we analyze the impact of switching a TLC or MLC cell down one bit upon death; this allows the voltage thresholds to rise and life, though at a lower capacity, to continue for that cell. Finding that traditional wear-leveling techniques actually inhibit the benefits of this scheme, we propose and explore how controlled "wear-unleveling" can work in tandem with Zombie-NAND cells to provide vastly increased life and decreased latencies for the drive. In this exploration, we perform rigorous performance measurement over a number of parameters representative of a variety of commodity and commercial SSDs.