Extending flash lifetime in secondary storage

Unlike magnetic disks, NAND flashes can be written a limited number of times. As flash memory densities increase and cell sizes shrink, further decreases in write endurance is expected. Although some mitigation is achieved by wear leveling, write endurance remains a concern for write intensive applications. In this research, we use a DRAM cache to filter write traffic to flash by coalescing and merging overwrites. To handle integrity of data upon power failure, we use a supercapacitor to provide short duration backup power during which DRAM data can be retired to flash memory. The effectiveness of such a mechanism is not obvious considering that a large file-system cache already exists which also merges overwrites. We investigated: (i) a DRAM and a flash disk cache combo within a magnetic disk controller and (ii) a DRAM only cache when flash is a full secondary storage. Our simulations show that using a medium sized DRAM cache, flash lifetime doubles with lazy updates compared to early update policy. Moreover, miss ratio and average response times decrease as well. With little effort, our technique can be extended to improve the usable life of other emerging non-volatile memories, such as PCM and MRAM.