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What is File Management System?
A File Management System is that set of system software that provides services to users and applications in the use of files. Typically, a file management consists of system utility programs that run as privileged applications. However, at the very least, a File Management System can need special services from the operating system; at the most the entire file management system is considered part of the operating system. Typically, the only way that a user or application may access files is through the file management system. This relives the user or programmer of the necessity of developing special purpose software for each application and provides the system with a means of controlling its most important asset.
File Management System Requirement
For an interactive, general-purpose system, the following constitute a minimal set of requirements:
- Each user should be able to create, delete, read, and change files.
- Each user may have controlled access to other user’s files.
- Each user may control what type of accesses is allowed to the user’s files.
- Each user should be able to restructure the user’s files in a form appropriate to the problem.
- Each user should be able to move data between files.
- Each user should be able to back up and recover the user’s files in case of damage.
File Management System Architecture
If one looks at the typical file system software architecture, at the lowest level, device drivers communicate directly with peripheral devices or their controller or channels. A device driver is responsible for starting the i/o operations on a device and processing the completion of an i/o request. For file operations, the typical devices controlled are disk and tape drives.
Next level, the basic file system, or the physical i/o level is the primary interface with the environment outside of the computer system. It deals with blocks of data that are exchanged with disk or tape systems. Thus it is concerned with the placement of those blocks on the secondary storage device.and on the buffering of those blocks in main memory. The basic file system is often considered part of the operating system.
The basic i./o supervisor is responsible for all file i/o initiation and termination. At this level control structures are maintained that deal with device i/o, scheduling and file status. The basic i./o supervisor is concerned with the selection of the device on which file i/o is to be performed, on the basis of which file has been selected. It is also concerned with scheduling disk and tape accesses to optimize performance. Input/Output buffers are assigned and secondary memory is allocated at this level. The basic i/o supervisor is part of the operating system.
Logical i/o enable users and applications to access records. Thus, whereas the basic file system deals with blocks of data, the logical i/o module deals with file records. Logical i/o provides a general-purpose record i/o capability and maintains basic data about files. The level of the file system closest to the user is often termed the access method. It provides a standard interface between applications and the file systems and devices that hold the data.
File Management Function
Users and application programs interact with the file system by means of commands for creating and deleting files and for performing operations on files. Before performing any operation, the file system must identify and locate the selected file. This requires the use of some sort of directory that serves to describe the location of all files, plus their attributes. In addition, most shared systems enforce user access control; only authorized users are allowed to access particular files in particular ways. The basic operations that a user or application may perform on a file are performed at the record level. The user or application views the file as having some structure that organizes the records such as a sequential structure. Thus to translate the user commands into specific file manipulation commands, the access method appropriate to this file structure must be employed.
While users and applications are concerned with records, i/o is done on a block basis. Thus the record of a file must be blocked for o/p and unblocked for after i/p. To support block i/o of files, several functions are needed. The Secondary storage must be managed. This involves allocating files to free blocks on secondary storage and managing free storage so as to know what blocks are available for new files and growth in existing files. In addition individual block i/o requests must be scheduled and this entire scheduling and file allocation is concerned with performance optimization.
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