File System Structure and File Concepts

File System (FS) Structure

• A file system consists of a collection of files, a directory structure, and resides on secondary storage devices (disks).
• The directory structure provides information about all files in the system.

File Concept

• A file is a collection of data stored on a secondary storage device.
• File types classify the content of the file, such as:
  • Data (numeric, character, binary)
  • Program (contents defined by the file's creator)
  • Many types (e.g., text file, source file, executable file)

File Attributes

• A file has several attributes, which vary from one operating system (OS) to another, but typically include:
  • Name (only information kept in human-readable form)
  • Type (needed for systems that support different types)
  • Location (pointer to file location on device)
  • Size (current file size)
  • Protection (controls who can read, write, execute)
  • Time, date, and user identification (useful for protection, security, and usage monitoring)

File Operations

• An OS must provide a number of operations associated with files, including:
  • Create (find space in the file system and make an entry for the new file in the directory)
  • Write (make a system call specifying the file name and information to be written)
  • Read (use a system call specifying the file's name and where to put the next block)
  • Seek (repositioning within a file, searching the directory for the appropriate entry)
  • Delete (search the directory for the named file and erase the directory entry)
  • Truncate (erase the contents of a file but keep its attributes)
  • Open (search the directory structure on disk for the entry and move the content to memory)
  • Close (move the content of the entry in memory to the directory structure on disk)

Open Files

• Most file operations involve searching the directory for the entry associated with the named file.
• To avoid searching, the OS keeps a table containing information about open files, including:
  • Open-file table
  • File pointer (tracks the last read/write location)
  • File-open count (counts the number of times a file is open by multiple processes)
  • Disk location of the file (info needed to locate the file on a disk)

File Structure

• Files must conform to a required structure understood by the OS.
• File structures can be:
  • None (sequence of words, bytes)
  • Simple record structure
  • Lines
  • Fixed length
  • Variable length
  • Complex structures (e.g., formatted document, relocatable load file)

Access Methods

• Information stored in files must be accessed and read into memory.
• Access methods include:
  • Sequential Access (information is processed in order, one record after another)
  • Direct Access / Relative Access (file is made up of fixed-length logical records, allowing rapid access to specific records)

Directory Structure

• A collection of nodes containing information about all files.
• Directory files contain information about files, including:
  • Name
  • Location
  • Size
  • Type
• Both the directory structure and the files reside on disk.

Directory Organization

• Efficiency (locating a file quickly)
• Naming (convenient to users)
• Two users can have the same name for different files.
• The same file can have several different names.
• Grouping (logical grouping of files by properties)

Operations Performed on Directory

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

Protection

• Protection is needed to prevent accidental or deliberate destruction of someone else's information.
• Information is private and should not be read by other users.
• The OS must provide means to protect the information in the file system.
• Many types of operations can be controlled, including:
  • Read
  • Write
  • Execute
  • Append
  • Delete
  • List