自適應單位原密度與就地覆寫進行快閃記憶體讀取干擾管理

Abstract

讀取干擾是由快閃記憶體讀取操作所引起的電路層級噪音，讀取刷新(Read refreshing)利用資料搬移來避免讀取干擾破壞現存的資料。然而在讀取密集地的工作負載中，讀取刷新造成頻繁的資料塊抹除。受到軟體控制單位原密度的啟發，我們提出保留閥值電壓(Threshold Voltage)為守衛層級(Guard levels)來提升對讀取干擾的容忍。保留守衛層級的資料塊雖然降低了單位原密度但可以儲存頻繁讀取的資料且避免頻繁讀取刷新。我們更進一步提出利用就地覆寫轉換高位元密度成低位元密度來降低資料搬移的需求，我們的結果顯示我們的方法最高可以降低85\%讀取刷新造成的資料塊抹除且降低平均讀取回應時間22\%

Read disturbance is a circuit-level noise induced by flash read operations. Read refreshing employs data migration to prevent read disturbance from corrupting existing data. However, it costs frequent block erasure under read-intensive workloads. Inspired by software-controlled cell bit-density, we propose to reserve selected threshold voltage levels as guard levels to extend the tolerance of read disturbance. Blocks with guard levels have low cell bit-density, but they can store frequently read data without frequent read refreshing. We further propose to convert a high-density block into a low-density one using in-place reprogramming to reduce the need for data migration. Our approach reduced the number of blocks erased due to read refreshing by up to 85\% and the average read response time by up to 22\%.