File Management

File Concept
Since main memory is usually too small to contain all the data and programs permanently, the computer system must provide secondary storage to back up main memory. The file system provides the mechanism for on-line storage of and access to both data and programs residing on the disks.
The operating system implements the abstract concept of a file by managing mass-storage media, such as tapes and disks, and the devices that control them. Also, files are normally organized into directories to make them easier to use. Finally, when multiple users have access to files, it may be desirable to control by whom and in what ways (for example, read, write, append) files may be accessed.
The files are mapped by the operating system onto physical devices. The devices that attach to a computer vary in many aspects. Some devices transfer a character or a block of characters at a time. Some can be accessed only sequentially, others randomly. Some transfer data synchronously, others asynchronously. They can be read-only or read-write. They vary greatly in speed. In many ways, they are also the slowest major component of the computer. Because of all this device variation, the operating system needs to provide a wide range of functionality to applications, to allow them to control all aspects of the devices.