FlexHMB: A Flexible HMB Design Towards Bufferless Mobile Flash

Since the last decade, NAND flash has been widely adopted in mobile devices (e.g., smartphones) as the main storage media. Unlike enterprise solid-state drives and hard disk drives, mobile devices organize NAND flash in a DRAMless form, called mobile flash, which lacks an internal DRAM buffer to accommodate write requests due to the space constraints of mobile devices. Instead, it allocates Single-Level-Cell (SLC) flash blocks as a write buffer to accelerate write I/O bursts. However, we observe that this well-known design not only fails to consistently deliver high write performance but also compromises mobile flash capacity and longevity. The former is caused by the severe SLC reclamation interference, while the latter stems from its low density and the occupation of scarce over-provisioning blocks. To address these challenges, we propose FlexHMB, which utilizes the mobile flash controller to manage a high-performance and flexible write buffer allocated from the host-side main memory. Specifically, FlexHMB leverages the host memory buffer (HMB) feature to replace the traditional SLC buffer with a DRAM-based one. By doing so, FlexHMB shifts mobile flash towards bufferless architecture and eliminates penalties from the SLC buffer. To avoid the potential reliability issue arising from the volatility of DRAM, we design a write transaction mechanism to guarantee order consistency. While a large buffer can deliver higher performance, it may lead to competition with mobile applications for memory resources. Taking this into consideration, we design a flexible and dynamic resizing mechanism for the write buffer to make a balance between efficient request handling and memory utilization. The evaluation results show that FlexHMB reduces write latency by 93.54% compared to the SLC write buffer when memory usage is constrained.