PACA: A Page Type Aware Read Cache Scheme in QLC Flash-based SSDs

QLC flash-based SSDs are gaining increasing attention and are expected to be widely used in read-intensive application scenarios, since they provide high density and low cost but suffer from poor write endurance and performance. QLC flash has four types of pages, between which read latency variation is as large as 1.6 to 4.8 times. This raises a critical concern for QLC SSDs to provide adequate and stable read performance. Notice that the SSD-internal cache (built with DRAM or non-volatile RAM) has long been utilized to improve write performance and lifetime. In this paper, we argue that the cache also plays an important role in read performance optimization of QLC SSDs. We design a novel flash page type aware read cache scheme, called PACA. It exploits read latency variation of QLC pages to prioritize caching data stored in high-latency QLC pages in a workload-adaptive manner. We verified PACA in FEMU, a popular SSD emulator. Experimental results show that PACA can reduce the average SSD read latency by up to 44.5%, compared with a baseline read cache scheme being unaware of flash page types.