快閃記憶體下的治癒不平均之虛擬耗損平均技術

Abstract

隨著快閃記憶體的技術不斷往高容量、低成本的方向發展，快閃記憶體可承受的抹寫次數急遽地下降，因此快閃記憶體的壽命成為一個很重要的議題。抹除和寫入的動作會在快閃記憶體的氧化層中產生缺陷，當缺陷大到一個程度時，快閃記憶體單元就會無法使用。之前的研究指出快閃記憶體可以經由一段時間的休息而被修復，且在高溫下可以加速修復的速度，因此有研究提出自我修復技術，他會對壞掉的單元進行加熱來延長快閃記憶體的壽命。然而加熱的動作會需要比較長的延遲時間以及消耗比較高的電量，現有的磨損平均技術是以平均所有區塊的抹除次數為原則，這會使所有的區塊在短時間內一起壞掉，而導致自我修復快閃記憶體必須以集中的方式對大量的區塊進行加熱，而使得反應時間急遽上升，還可能產生電能耗損的問題。 為了解決自我修復快閃記憶體的集中式加熱的問題，我們提出了一個新的磨損平均策略，叫虛擬耗損平均技術。我們會利用平均虛擬磨損次數來分散加熱的時間，而且最終我們也會平均所有區塊的磨損次數。此外我們的方法可以用在任何一個快閃記憶體轉換層以及磨損平均方法上，且不會產生額外的負擔。我們實作了一系列的實驗來評估我們提出的方法，結果顯示集中加熱的問題成功地被解決，而且使用了虛擬耗損平均技術的自我修復快閃記憶體的壽命顯著地被改善。

As the technology continues to scale down, the program/erase cycles (P/E cycles) which flash memory can survive is degraded. Short lifetimes are becoming a critical problem in flash memory. The P/E cycles will induce trap generation on oxide layer, which cause the degradation of flash cell. Prior studies have shown that a cell can be recovered as the time elapses and speed up the process under high temperature, so the self-healing technology have been proposed. Self-healing flash can heal the worn-out cell by high temperature to prolong the flash lifetime. However, the healing process has long latency and consumes a substantial amount of power. The existing wear leveling scheme, based on evenly distribute P/E cycles on blocks, will tend to wear out all blocks in a short time period. It will cause self-healing flash do healing in a concentrated manner, which will seriously worsen response time and may cause power outage. To solve concentrated healing problem in self-healing flash, we proposed a novel wear leveling scheme called Virtual wear leveling. It uniformly disperse the healing time by doing wear leveling with virtual erase count, and it also evenly distribute the block erase counts eventually. Besides, it can be applied on any FTL and wear leveling algorithm without additional overhead. We conducted a serious of experiments to evaluate the proposed design. The results showed that the concentrated healing problem was successfully solved and the device lifetime was significantly improved with Virtual wear leveling on self-healing flash.