Update Frequency-Directed Subpage Management for Mitigating Garbage Collection and DRAM Overheads.

The increased flash page sizes cause a large number of subpage requests due to the difference in host and flash I/O units. The subpage requests may degrade the space utilization, response time, and lifetime of NAND flash memories. Addressing the subpage issue, a few studies in the past have proposed merging subpages to generate full pages. Although these subpage schemes may improve performance and lifetime, they incur immense DRAM space for storing the sector information of merged pages. Moreover, they do not consider the update frequencies of the subpages for merging and thus an update request to a subpage causes partial page invalidation, which leads to garbage collection (GC) overhead. To address these issues, our proposed scheme considers the update frequencies of the subpages for merging in order to avoid partial page invalidation, which in turn improves the GC efficiency. Further, the proposed scheme uses a sector information table (SIT) in flash pages to store the fine-grained sector information of merged subpages. In the light of experiment results, our scheme, on average, reduces DRAM footprint, flash writes, and block erasures by 29%, 18%, and 13%, respectively.