FlashDefibrillator: A Data Recovery Technique for Retention Failures in NAND Flash Memory

Although NAND flash memory is known as a nonvolatile memory device, the non-volatility of the data stored in the NAND flash memory is guaranteed only for a specified retention time. Since the NAND retention time assumes specific operation conditions, when the NAND flash memory is exposed to an abnormal environment beyond the specified operation conditions, stored data cannot be reliably retrieved due to retention failures. In this paper, we propose a novel data recovery technique, called FlashDefibrillator (FD), for recovering retention failures in recent NAND flash memory. By reversely exploiting charge-transient behavior observed in recent 20-nm node (or below) NAND flash memory, FD can identify retention-failed cells in a progressive fashion using a novel selective error-correction procedure. FD repeatedly applies the selective error-correction procedure until retention failures are fully recovered. Our measurement results with recent 20-nm node NAND chips show that FD outperforms the existing recovery technique in both the data recovery speed and the data recovery capability. FD can recover retention failures up to 23 times faster over the existing data recovery technique. Furthermore, FD can successfully recover severely retention-failed data (such as ones experienced eight times longer retention times than the retention-time specification) which were not recoverable with the existing technique.