WOM codes reduce write amplification in NAND flash memory

This paper proposes a NAND flash system that uses Write-Once Memory (WOM) codes to encode the data stored. It is shown through both analysis and simulation that, with proper parameters, flash memories which use WOM codes to encode data can achieve a lower write amplification than in a non-WOM-coded system. For example, in a 16-level per cell flash memory, when a two-write MLC WOM code is used with a total overprovisioning of 0.8, the write amplification is 15% lower than a non-WOM-coded system. A closed-form expression for the write amplification in a WOM-coded system is given for a system with a greedy garbage collection policy and a uniform random workload. The proposed expression is a function of the total overprovisioning factor, number of WOM code writes, and number of values per cell. The expression is applicable for both SLC and MLC flash.