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File System in Operating System


Computers can store information on various storage media

The operating system abstracts from the physical properties of its storage devices to define a logical storage unit called a file.

Files are mapped by the operating system onto non volatile physical devices.

A file is a named collection of related information that is recorded on secondary storage.

For a user's perspective, a file is the smallest allotment of logical secondary storage (i.e., data supposedly cannot be written to secondary storage unless written in a file).

Files commonly represent programs (both source and object code) and data.

Data file contents may be numeric, alphabetic, alphanumeric, or binary.

Files may be free form (e.g., text files) or rigidly formatted.

In general, a file is a sequence of bits, bytes, lines, or records, whose meaning is defined by the file's creator and user.

Many important applications need to store more information then have in virtual address space of a process.
  • Must store large amounts of data
  • Gigabytes -> terabytes -> petabytes
The information must survive the termination of the process using it.
  • Lifetime can be seconds to years
  • Must have some way of finding it !
Multiple processes must be able to access the information concurrently.

Disks are used to store files

Information is stored in blocks on a disks

Can read and write blocks

We use file system as an abstraction to deal with accessing the information kept in blockks on a disk

Files are created by a process.

Thousands of them on a disk

Managed by the Operating System (OS)

OS structures them, names them, protects them

Two ways of looking at a file system 
  • User point of view:
             How do we name a file, protect it, organize the files
  • Implementation point of view
             How are they organized on a disk

Start with user, then go to implementation.

The user point of view
  • Naming
  • Structure
  • File Type (directories)
  • File Access

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