CSAL: the Next-Gen Local Disks for the Cloud

Cloud local disks are attractive for their affordable price and high performance. The recent advancement in CPUs motivates cloud vendors to further multiplex the computing resources to serve more users. Unfortunately, such proposals are constrained by the limited offerings of cloud local disks per server as the underlying storage devices are either large but slow (e.g., HDDs) or fast yet small (e.g., NVMe SSDs).

In this paper, we explore the possibility of leveraging highcapacity QLC-based SSDs for cloud local disks. However, the three preliminary unsuccessful attempts indicate that QLC SSDs cannot simply work as drop-in replacement. The root cause is the two levels of write amplification caused by device-level address mapping with Indirection Unit and NAND-level garbage collection.

With these lessons learned, we propose CSAL, the nextgen local disks in Alibaba Cloud. CSAL includes a highperformance SSD as write buffers and a large-capacity QLC SSD for persistence. With a two-level Logical to Physical (L2P) address mapping table, CSAL achieves fine-grained (4KB) data accessing and significantly alleviates the two levels of write amplification. Results show that CSAL always prevails with superior performance and can achieve up to 2.22×, 1.82×, and 2.03× speedups against the second-best peers in micro, application, and deployment benchmarking, respectively. As of now, we have deployed CSAL on thousands of servers and made CSAL open-source to the public.