Making Garbage Collection Wear Conscious for Flash SSD

Since NAND flash memory has certain inherent limitations like out-of-place updates and a coarse granularity of erase unit, a NAND flash memory based solid state disk needs a garbage collector to erase and recycle previously used blocks from time to time. Garbage collection, however, can significantly decrease flash SSD performance as it incurs high overhead. Various techniques have been proposed to reduce the cost of garbage collection. Unfortunately, most of them only take performance into consideration while ignoring block wear status when choosing victim blocks. As a result, some blocks could be frequently erased and recycled due to garbage collection, and thus, prematurely fail because of a high concentration of erasure cycles. In this research, we propose a wear conscious garbage collection mechanism named WECO (wear conscious). To demonstrate the effectiveness of WECO, we extend a well-known SSD simulator FlashSim so that WECO can be integrated into typical FTLs (flash translation layers) such as DFTL (demand-based FTL) and PM (page mapping). Experimental results show that WECO-DFTL and WECO-PM substantially improve wear-leveling while maintaining a performance similar to the two original FTLs.