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Flash memory is a write-once medium in which reprogramming cells requiresfirst erasing the block that contains them. The lifetime of the flash is afunction of the number of block erasures and can be as small as severalthousands. To reduce the number of block erasures, pages, which are thesmallest write unit, are rewritten out-of-place in the memory. A Write-oncememory (WOM) code is a coding scheme which enables to write multiple times tothe block before an erasure. However, these codes come with significant rateloss. For example, the rate for writing twice (with the same rate) is at most0.77.  In this paper, we study WOM codes and their tradeoff between rate loss andreduction in the number of block erasures, when pages are written uniformly atrandom. First, we introduce a new measure, called erasure factor, that reflectsboth the number of block erasures and the amount of data that can be written oneach block. A key point in our analysis is that this tradeoff depends upon thespecific implementation of WOM codes in the memory. We consider two systemsthat use WOM codes; a conventional scheme that was commonly used, and a newrecent design that preserves the overall storage capacity. While the firstsystem can improve the erasure factor only when the storage rate is at most0.6442, we show that the second scheme always improves this figure of merit.

When Do WOM Codes Improve the Erasure Factor in Flash Memories?