Early Dirty Buffer Flush with Second Chance for SSDs

As high-performance server-based applications become more prevalent, there is a growing demand for high-performance storage solutions. In response, SSDs that use NAND flash memory as storage media are quickly replacing hard disks in the high-performance storage market. One way to improve SSD performance is to use an internal large-capacity memory as a buffer cache for NAND. Previous studies have shown that early flushing, which ensures sufficient clean buffers by flushing dirty buffers to NAND in advance when the ratio of dirty buffers exceeds a threshold, significantly reduces the average response time of I/O requests. However, the early flush can also have a negative side effect, namely an increase in NAND write operations. To address this problem, this study proposes a selective early flush policy. This policy evaluates the likelihood of a candidate dirty buffer being rewritten upon the early flush, and delays flushing if the candidate has a high rewrite likelihood. Through this selective early flush, the proposed policy reduces NAND write operations by up to 18.0% compared to the existing early flush policy in the mixed trace. Additionally, the response time of I/O requests is also improved in most of the considered configurations.