Graceful Space Degradation: An Uneven Space Management for Flash Storage Devices

The high cell density, multilevel-cell programming, and manufacturing process variance force the new coming flash memory to have large bit-error-rate variance among blocks and pages, where a flash chip consists of multiple blocks and each block consists of a fixed number of pages. In order to avoid storing the crucial user data in more fragile pages, conventional flash management software tends to aggressively discard the high bit-error-rate area in the unit of a block. However, together with the aggressive discarding strategies and the enlarging sizes of pages/blocks of next generation flash memory, the available space of flash devices might encounter a very sharp degradation and therefore result in rapidly-shortened device lifespan. Thus, we advocate the concept of “graceful space degradation” to mitigate this problem by discarding the high bit-error-rate (or worn-out) area in the unit of pages (instead of blocks). To furthermore realize this concept, we are the pioneer to put forward an “uneven space management” to manage flash blocks containing different number of bad pages. Our design especially focuses on placing data with different access behaviors to make the best uses of blocks with different available space so as to ultimately prolong the device lifespan with good access performance. The experiments were conducted based on representative realistic workloads, and the results reveal that the proposed design can extend the device lifetime by at least 2.38 times of that of existent approaches, with very limited performance overheads.