Trade-offs between Instantaneous and Total Capacity in Multi-Cell Flash Memories

The limited endurance of flash memories is a major design concern for enterprise storage systems. We propose a method to increase it by using relative (as opposed to fixed) cell levels and by representing the information with Write Asymmetric Memory (WAM) codes. Overall, our new method enables faster writes, improved reliability as well as improved endurance by allowing multiple writes between block erasures. We study the capacity of the new WAM codes with relative levels, where the information is represented by multiset permutations induced by the charge levels, and show that it achieves the capacity of any other WAM codes with the same number of writes. Specifically, we prove that it has the potential to double the total capacity of the memory. Since capacity can be achieved only with cells that have a large number of levels, we propose a new architecture that consists of multi-cells - each an aggregation of a number of floating gate transistors.