Study On Nand Flash File System Of The On-board Data Recorder

With the progress of wireless communication technology, the reliability and availability of wireless communication are highly improved. Communication Based Train Control (CBTC) is the trend of urban railway traffic control system in the future.As key equipment in CBTC system, Vehicle On Board Controller (VOBC) has complicated functions and high safety requirments. Therefore, mass status information and operation logs of Vehicle On Board Controller (VOBC) are required to be recorded as the basis of debuging for the developers. In order to reduce the complexity of searching certain information, so as to facilitate fault location and fault recurrence, it is required to introduce a file system into the On-board Data Recorder for the effective data management.According to the requirement of the On-board Data Recorder, Nand Flash was adopted as the mass storage unit, and then a design of the structure and the algorithm for Nand Flash System was presented in this paper. In the file management level, based on the concept of the i-node structure and the log-structured file system, a tree structure and "write-before-erase" strategy was introduced to guarantee the rapidity of performance and the reliability of data. And in the space management level, an index structure of space managment table was proposed in order to improve the efficiency of searching and allocating some certain blocks.In the garbage collection,a dirtiest-list method was designed, which took both the high reclaim efficiency and the erasure cycles into consideration.For the uneven wearing of the flash caused by the cold data which had not been updated for a long time, a wear-leveling algorithm was implemented to move the cold data into the free block with the maximum erasure cycles and make it possible for blocks with minimal erasure cycles to go into the wear-cycle.A crash recovery mechanism was proposed to improve the data reliability, and the data recovery was implemented by judging the status flag and the recorded object number in the spare area.On the basis of the design, a formal verification method - Colored Petri Nets(CPN) was broght in to establish the model of the space allocation algorithm for the Flash file system. The "top to bottom" idea was adopted to build the top model and submodels including Write Model, Delete Model, Garbage Collection Model and Wearleveling Model. In the paper, Write Model was mainly studied, and the completeness of the model was verified by the method of state space analysis.This provided an available verification method for the whole model.Tests and analyses about the efficiency of garbage collection and effect of wear-leveling were implemented by the method of computer simulation. Through the analyses, it was founded that there was a bug in grabage colleciton mechanism. And aiming this bug, an improved method was proposed and had been tested with a better result.