You’ve probably heard about the endurance limitations of Flash–particularly MLC–and the hoops manufacturers jump through to lengthen life expectancy. If you really want to understand what this issue is all about and how SSD vendors handle it, check out Eric Slack’s Storage Switzerland post entitled Why Flash Wears Out and How to Make it Last Longer.
Slack provides a very thorough explanation of how and why NAND Flash degrades, why MLC degrades faster than SLC, what actually happens during that degrading process, and all the tricks SSD manufacturers employ to slow it down. Techniques include sophisticated error correction, spare blocks of NAND flash that take over when one block degrades, and wear leveling, which distributes write operations across available blocks to ensure that a single block doesn’t wear out prematurely. Vendors also embed advanced technologies, such as digital signal processing, in their SSD controllers to reduce bit errors and reduce the workload on the error correcting (ECC) engine, and employ sophisticated read level adjustments to recognize data on a degraded Flash block. Some SSD controllers can also make sophisticated adjustments to the way a cell is read and written to minimize wear.