In-place Switch: Reprogramming based SLC Cache Design for Hybrid 3D SSDs

Recently, 3D SSDs are widely adopted in PCs, data centers, and cloud storage systems. To increase capacity, high bit-density cells, such as Triple-Level Cell (TLC), are utilized within 3D SSDs. However, due to the inferior performance of TLC, a portion of TLCs is configured to operate as Single-Level Cell (SLC) to provide high performance, with host data initially directed to the SLCs. In SLC/TLC hybrid 3D SSDs, a portion of the TLC space is designated as an SLC cache to achieve high SSD performance by writing host data at the SLC speed. Given the limited size of the SLC cache, block reclamation is necessary to free up the SLC cache during idle periods. However, our preliminary studies indicate that the SLC cache can lead to a performance cliff if filled rapidly and cause significant write amplification when data migration occurs during idle times. In this work, we propose leveraging a reprogram operation to address these challenges. Specifically, when the SLC cache is full or during idle periods, a reprogram operation is performed to switch used SLC pages to TLC pages in place (termed In-place Switch, IPS). Subsequently, other free TLC space is allocated as the new SLC cache. IPS can continuously provide sufficient SLC cache within SSDs, significantly improving write performance and reducing write amplification. Experimental results demonstrate that IPS can reduce write latency and write amplification by up to 0.75 times and 0.53 times, respectively, compared to state-of-the-art SLC cache technologies.