An MFS is a routine that stands between a file operation, such as READ or WRITE, and the filing system itself. The MFS routine has an opportunity to examine the data being used for a file operation, and to modify it before passing the data on to the filing system, or back to the program that requested the operation. In effect, an MFS is a shell around an existing filing system, one that monitors all input and output to that filing system.
Another way of defining an MFS is to think of it as a filter through which all file operation requests must pass before the file can be accessed. This functionality provides access to data at the filing system level. Through use of an MFS, a developer can monitor or "interrupt" all file I/O activity to a file, and modify the data being read or written, or otherwise take action based on the file activity.
