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File management systems

The file management system, FMS is the subsystem of an operating system that manages the data storage organization on disk, and provides services to processes related to file access. In this sense, it interfaces the application programs with the low-level media-I/O (e.g. disk I/O) subsystem, freeing on the application programmers from having to deal with low-level intricacies and allowing them to implement I/O using convenient data-organizational abstractions like files and records. On the other end, the FMS services often is the only way thorough which applications can access the data stored in the files, thus achieving an encapsulation of the data themselves which can be usefully exploited for the purposes of data protection, maintenance and control.

We can summarize the aims of a FMS as follows:

• Data Management. An FMS should provide data management services to the applications through convenient abstractions, simplifying and making device-independant the common operations involved in data access and modification.
• Generality with respect to storage devices. The FMS data abstractions and access methods should remain unchanged irrespective of the devices involved in data storage.
• Validity. An FMS should guarantee that at any given moment the stored data reflect the operations performed on them, regardless of the time delays involved in actually performing those operations. Appropriate access synchronization mechanism should be used to enforce validity when multiple accesses from independant processes are possible.
• Protection. Illegal or potentially dangerous operations on the data should be controlled by the FMS, by enforcing a well defined data protection policy.
• Concurrency. In multiprogramming systems, concurrent access to the data should be allowed with minimal differences with respect to single-process access, save for access synchronization enforcement.
• Performance. The above functionalities should be offered achieving at the same a good compromise in terms of data access speed and data transfering rate.

From the point of view of an end user (or application) an FMS typically provides the following functionalities:

• File creation, modification and deletion.
• User's (or user groups') ownership of files, and access control on the basis of ownership permissions.
• Facilities to structure data within files (predefined record formats, etc).
• Facilities for maintaining data redundance against technical failure (back-ups, disk mirroring, etc.).
• Logical identification and structuring of the data, via file names and hyerarchical directory structures.

 
• FMS Architecture

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