Refresh frequency reduction of data stored in SSDs based on A-timer and timestamps

Increasing the number of bits per cell and technology scaling are ways to reduce the cost per gigabyte of flash memories and solid-state drives (SSDs). Unfortunately, this trend has a negative impact on data retention capability and cycling endurance. Periodic data refresh allows dealing with a reduced retention time and, indirectly, may be used to improve cycling endurance. A worst case data refresh frequency is not optimal in the presence of important temperature variations as it may become unnecessarily pessimistic and alter the SSD response latency and energy consumption. Here, a flexible data refresh methodology is proposed based on approximations of the Arrhenius-curves employed to describe the temperature impact on the retention capability of flash memories. These approximations may be implemented with the help of a small module called A-timer. For an asymmetric temperature distribution between 30°C and 70°C, it is estimated that the refresh frequency can be reduced by more than 63× and almost 3× for respectively charge detrapping and SILC failure mechanisms.