Lesson 7 File System

• File

  Contiguous logical address space (physical may not be contiguous)
• Types of files

  • Data (numeric, character, binary)
  • Program
• File Attributes

  • Name
  • Identifier
  • Type
  • Location
  • Size
  • Protection
  • Time, date, and user identification

  
• Information about files are kept in the directory structure, which is maintained on the disk
• Many variations of file attributes, including extended file attributes (e.g. checksum)
• File Operations

  • Create
  • Write
  • Read
  • Reposition within file (seek)
  • Delete
  • Truncate
  • Open
  • Close
• Open File


  • Locate the file on the disk according to its file name (path)
  • Establish data structures to share file among different processes
  • Allocate file descriptor(ID-used by read/write requests)
• Data structures for managing open files

  • Open-file table
  • File pointer
  • File-open count
  • Disk location of the file
  • Access rights

  
• Classical Unix data structures for open files

  • Per Process Table of Open Files
  • System-wide File Table
  • System-wide V-Node Table (Linux –no V-node, only I-node)

  
• Per Process Table of Open Files

  • File descriptor Flag
  • Ptr to entry in the System Wide File Table

  
• System Wide Open File Table

  • File Status Flags
  • File offset(position of next read/write)
  • V-node ptr (entry in V-node table) or I-node ptr (entry in I-node table)

  
• 1. Node Table


  • Type of file (regular, pipe, directory, etc)
  • Ptr to function operates such type of file
  • I-node info

  
• 1. node


  • file type
  • access permissions
  • Owner and group owner
  • Size of the file
  • number of links
  • device number of the device where the file is stored
  • time of last access
  • time of last modification
  • time of last change to the i-node
  • block numbers of the blocks in which file is stored
• Unix kernel, file is identified by its i-node number (not filename)
• Unix directory file contains names of the files, their i-node numbers
• Open File Locking


  • Shared lock
  • Exclusive lock
  • Mandatory
  • Advisory
• File Locking Example - Java API

  • Shared lock
  • Exclusive lock
• File Types


  • None (no structure) - sequence of words, bytes
  • Simple record structure (Lines, Fixed length, Variable length)
  • Complex Structures (Formatted document, Relocatable load file)
• Access Methods


  • Sequential Access
  • Direct Access
• Other Access Methods


  • Index for the file which contains pointer to the actual disk block
  • IBM indexed sequential-access method (ISAM)
  • VMS operating system provides index and relative files
• Directory Structure


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


  • Efficiency
  • Naming
  • Grouping
• Single-Level Directory


  • Naming problem
  • Grouping problem
• Two-Level Directory


  • Path name
  • Efficient searching
  • No grouping capability
• Tree-Structured Directories


  • Efficient searching
  • Grouping Capability
  • Current directory (working directory)
  • Absolute or relative path name
  • Creating a new file
  • Deleting a file
  • Creating a new subdirectory
• Acyclic-Graph Directories


  Have shared subdirectories and files
• General Graph Directory


  • Cycles
  • Garbage collection
  • Cycle detection algorithm
• File System Mounting


  Mount point
• File Protection


  • Read
  • Write
  • Execute
  • Append
  • Delete
  • List
• Access Lists and Groups

  • Mode of access: read, write, execute
  • Three classes of users: owner, group, public