Prolongation of Lifetime and the Evaluation Method of Dependable SSD

Since high-density flash memory has high error rate, strong error control is necessary for the solid-state drive (SSD). The number of erasure cycles of each memory cell is limited, where the cell should be erased before writing. Wear-leveling is used for leveling the erasure cycles in a flash memory. Since the existing wear-leveling is executed in a chip, it is not effective if write operations are concentrated into specified chips. This paper proposes wear-leveling and error control method by using redundant flash memories in order to improve reliability and lifetime of the SSD. In the proposed method, error control is usually executed, and wear-leveling among the chips is executed when the bias in the erasure cycles is large. The execution frequency of wear-leveling is adjusted considering deterioration of the cell. Evaluations of bit error rate and lifetime show that the proposed method has high reliability and durability.