Retired-page utilization in write-once memory — A coding perspective

In write-once memory (e.g., Flash), a cell´s level can only be raised, and erasure is only in bulk. The total number of erasures (endurance) is limited, and drops sharply with technology shrinkage and with cell-capacity increase. The normalized write capacity (ratio of total amount of data that can be written to storage capacity) drops similarly. Various coding schemes enable overwrites at the expense of storage capacity. With all of them, whenever desired data cannot be written to its current page, the page is “retired” for subsequent erasure. Interestingly, however, a retired page can still be used for writing other data, and it has been proposed to try and use retired pages. Simulation results are promising. In this paper, after briefly presenting Retired Page Utilization, we cast it as a cross-page coding technique. We employ Markovian analysis to derive the expected number of random-data writes with RPU, and show how the required state space can sometimes be substantially reduced.