A Selective Cache For SSD Controller

Solid-state disks based on NAND flash have the advantages of non-volatile, well random read performance, shock-resistant and low energy consumption. But NAND flash blocks need to be erased before re-write, which lead to high latency due to executing garbage collection operations. Moreover, the chips with running garbage collection cannot respond to any read or write requests, resulting in the performance degradation for the overall solid-state disk.We analyzed the problem above through experiments and found that only a small part of chips executing garbage collection at the same time, but these chips block I/O requests from the controller. The controller has to wait for some suspended requests, even if other chips still are idle. Based on the analysis, we proposed a selective cache named S_Cache. When chips are running garbage collection, S_Cache holds the write requests on these chips under a write-back mode, thus separating garbage collection and write operations as well as effectively reducing idle time of other chips. Once the completion of garbage collection on chips, the buffered data will be immediately written back to the chip, thereby reducing cache requirement. S_Cache is implemented based on Micro-Add emulator for two storage layouts RAID0 and RAID5.The experiments of three typical workloads FIN1, DTRS, LVMP show that S_Cache obtains the performance effects in both storage layouts. Regardless of cache capacity limits, the average utilization rate of chips can be increased by 7.72% and 5.18% respectively. Solid-state disk throughput can be increased by 29.97% and 14.45% respectively. We do the further experiments in RAID0 layout. The further results from the RAID0 layout show that when the cache capacity is only 512 KB, S_Cache increases the chip utilization rate by 4.55% and solid disk throughput by 13.38%.