In-Field Testing of NAND Flash Storage: Why and How?

NAND Flash memories have rapidly emerged as a storage class memory such as SSD (Solid State Disk), CF (Compact Flash) Card, SD (Secure Digital Memory) Card. Due to its distinct operation mechanisms, NAND Flash memory suffers from erase/program endurance, data retention and program/read disturbance problems. Specifically, erase and program operation keeps in developing bad blocks during the lifetime of memory chips. Bad blocks are blocks that contain faulty bits but the ECC (Error Correction Code) algorithm cannot correct them. Although wear leveling tries to balance the erase/program operations on different blocks so that all blocks can wear out at a similar pace, new bad blocks still inevitably occur.We propose an in-field testing technique which takes some pages in a block as predictors. Due to wear out faster than the other pages, the predictors will become bad before the other pages in the block become bad. The further questions are (1) how to detect those wearing fast pages so as to use them as predictors, (2) how many predictors are needed to achieve a satisfactory prediction accuracy, (3) misprediction will result in what negative impact on performance and endurance.