Simplifying and Accelerating NOR Flash I/O Stack for RAM-Restricted Microcontrollers

NOR flash has been increasingly popular for RAM-restricted microcontrollers due to its small package, high reliability, etc. To satisfy RAM restrictions, existing NOR flash file systems migrate their functionalities, i.e., block-level data organization and wear leveling (WL), from RAM to NOR flash. However, such fine-grained block-level management introduces frequent index updates and NOR flash scanning, leading to severe I/O amplification, which further deteriorates as they are decoupled in existing NOR flash file systems. To address the problem, we propose NF2FS, a NOR flash-friendly file system. Our key insight is that applications running on NOR flash usually have (1) small file sizes, therefore block-based data organization can be converted to flat file layout (for fast file/dir scanning); (2) deterministic I/O patterns, thereby WL can be achieved through coarse file swapping. As a result, NF2FS relaxes data organization and WL to a coarse-grained file level, which are then cooperated within the file system. We implement NF2FS in FreeRTOS using a range of techniques, including the all-logging layout, along with efficient layout management approaches such as dual bitmap space allocator and soft-update-like crash consistency. Experiments suggest that NF2FS significantly outperforms existing works and can prevent quick NOR flash wear-out.