How Much Can Data Compressibility Help to Improve NAND Flash Memory Lifetime?

Although data compression can benefit flash memory lifetime, little work has been done to rigorously study the full potential of exploiting data compressibility to improve memory lifetime. This work attempts to fill this missing link. Motivated by the fact that memory cell damage strongly depends on the data content being stored, we first propose an implicit data compression approach (i.e., compress each data sector but do not increase the number of sectors per flash memory page) as a complement to conventional explicit data compression that aims to increase the number of sectors per flash memory page. Due to the runtime variation of data compressibility, each flash memory page almost always contains some unused storage space left by compressed data sectors. We develop a set of design strategies for exploiting such unused storage space to reduce the overall memory physical damage. We derive a set of mathematical formulations that can quantitatively estimate flash memory physical damage reduction gained by the proposed design strategies for both explicit and implicit data compression. Using 20nm MLC NAND flash memory chips, we carry out extensive experiments to quantify the content dependency of memory cell damage, based upon which we empirically evaluate and compare the effectiveness of the proposed design strategies under a wide spectrum of data compressibility characteristics.