Wireless Router Anti-power-down File System Design And Implementation

Flash memory is evolving in the embedded systems due to its attractive features such as large capacity, fast access speed and low power consumption. Flash is a form of EEPROM, available mainly in two types, the traditional NOR flash and the newer NAND flash. The majority applications of flash for file storage have involved using the flash to emulate a block device with 512 bytes sectors, on top of which using standard file systems. This approach provides no wear leveling, and is extremely unsafe with the potential power loss between the erase and subsequent rewrite of the data. JFFS/JFFS2 are the newly emerging file systems specifically designed for flash memory, which provide wear leveling directly on the chip and are crash-proof because they are log-structured. However, JFFS/JFFS2 only supports limited number of types of flashes.The mainboard of the Wireless Router in our project only supports the Disk On Chip and the Compact Flash as the on-board external storage device. DOC is a type of NAND flash device and emulated by the NFTL as a normal block device, on top of which you may put the normal file systems. Experiments on the WR show that using the normal file systems on DOC is not crash-proof. CF is a type of NOR flash and it cannot be directly supported by JFFS/JFFS2. It supports the normal file systems by introducing the FTL, or with MTD emulation using block device, it is feasible to put the JFFS/JFFS2 file systems on CF. Experiments show that such emulation improves the crash-proof performance of the system, but it also greatly reduces the life cycle of flash chip.Considering the characteristics of most embedded Linux systems, that though the system writes miscellaneous files at runtime, most of them need not be persistent across reboots, this paper proposed a new file system technology, which moves all the write operations on the files into RAM at runtime and only saves the modified files back to the flash within a short time when the persistent storage is needed. It also has adopted some fault-tolerance mechanisms to detect and recovery from accidental power losses even when the seldom saving process is right in progress. To make the most efficient use of the flash chip, the read-only files are crammed into a compressed file system; as for the rewritable files, it uses the normal file systems with the chip builtin FTL/NFTL to provide wear leveling. The technology presented in the paper makes the best combination of the file system formats that the Linux kernel supports and it is especially suitable for embedded Linux systems.