OS-File System

An important component of an operating system is the file system. File systems generally contain

• Access Methods – these are concerned with the manner in which data stored in files is accessed.
• File Management – This is concerned with providing the mechanisms for files to be stored, referenced, shared and secured.
• Auxiliary storage Management – This is concerned with allocating space for files on secondary storage devices.
• File integrity mechanisms – These are concerned with guaranteeing that the information in a file is uncorrupted.

The file system is primarily concerned with managing secondary storage space, particularly disk storage. Let us assume an environment of a large-scale timesharing system supporting approximately 100 active terminals accessible to a user community of several thousand users. It is common for user accounts to contain between 10 and 100 files. Thus with a user community of several thousand users, a system disks might contain 50,000 to 1,00,000 or more separate files. These files need to be accessed quickly to keep response times small.

A file system for this type of environment may be organized as follows. A root is used to indicate where on disk the root directory begins. The root directory points to the various user directories. A user directory contains an entry for each of a user’s files; each entry points to where the corresponding file is stored on disk.

Files names should be unique within a given user directory. In hierarchically structured file systems, the system name of a file is usually formed as pathname from the root directory to the file. For eg., in a two-level file system with users A,B and C and in which A has files PAYROLL and INVOICES, the pathname for file  PAYROLL is A:PAYROLL.