Second Chance for End-of-Life Flash Memory Blocks via Page Isolation

Flash memories are widely preferred for their high bit density despite their relatively low endurance limitations. One of the dominant reasons for flash memory’s low reliability is cell-to-cell program interference during page operations. Programming a target cell also alters the threshold voltages of neighboring cells depending on the distance. These effects become more prominent as blocks age, resulting in them being abandoned after reaching their end of life due to unrecoverable errors. To manage and increase endurance, techniques such as overprovisioning, wear-leveling, proactive recovery, and architectural design alternatives are proposed. We propose the Page Isolation technique to reuse some pages of aged blocks that are otherwise abandoned regardless of techniques used during the standard lifetime. The key idea is to isolate some pages to enable others to continue to be used, reaching at least $7.7 \times$ more endurance than their projected lifetime. The technique suggests skipping every other page (more pages for higher ages) in the process of declaring free page addresses after the garbage collection for the blocks that are reaching an uncorrectable error count. The simplicity of the technique allows for easy implementation in the Flash Translation Layer without increasing computational complexity. The Page isolation lowers penalty of capacity loss due to the retirement of the aged-blocks by enabling to use some pages of these blocks beyond their anticipated lifetime.