File System

1
File System

• Most visible aspect of any Operating System to
  the User ( Users interact most frequently).
• Provides the following facilities
• Mechanism for permanent / non volatile storage of
  information in various secondary medias.
• Presents an uniform logical view of all these
  storage medias to the user O.S.

2
Stored Information

• Programs
• User Programs Source, Object , Executable.
• System Programs Packages Executable.
• Data
• a) Text.
• b) Documents.
• c) Multi Media.

3
Storage Medias (Filing Cabinets)

• Magnetic Tape Sequential Access.
• Disks
• 1. Magnetic Hard Disk (Direct Access
  Outer to Inner
• R/W Head floats inside an air
  cushion on the Disk surface).
• 2. Winchester Disk ( Hermitically
  Sealed Magnetic Hard
• Disk ).
• 3. Floppy Disk (Flexible magnetic
  Disk R/W Head
• comes into direct contact with
  Disk surface).
• 4. Optical Disks (CD_ROM,
  DVD_ROM,DVD_RAM,
• WORM ) Hard Disks, R/W Head
  LASER Beam ,
• Direct Access Inner to Outer.
• Pen Drive Electrically Alterable ROMs.

4
File System (Key Features)

• Collection of Files stored on secondary medias.
  File A named collection of related
  information (Records) .
• A directory structure helps in the following
• - File Organization.
• - Access Rights (Protection
  Mechanism) .
• One or more Partitions ( Provides separation of
  directories useful for better manageability of
  large filing cabinet. )
• Implementation (Mapping from Logical viewpoint
  to Physical Media) .

5
File System Vs. Virtual Memory

• Virtual Memory helps to support a very large
  address space in executable memory to a
  particular process. File system is a global
  feature of managing the Filing Cabinets
  (Secondary Media).
• File Management predates virtual memory by
  several years.
• Files stored in secondary media is accessible
  even outside the relevant process provided proper
  access rights are allowed.
• Modern O.S. employ both virtual memory (paging )
  as well as file systems as separate interfaces to
  secondary memory where pages (virtual memory)
  being prevalent together with Disk Swap space
  within the realms of process memory management
  whereas all files are treated as a separate
  resource (Back Up Store).
• However the same architectural interface and
  technology employed to support File Systems are
  used to support Virtual Memory as well.

6
A typical File Descriptor

• Name/External Name A string of alphanumeric
  characters understood by the user (Human readable
  form).
• Identifier An unique tag, a number helps to
  identify the file in a file system used by the
  O.S. (non human accessible).
• Type Helps to identify the area of use of the
  file (To be illustrated later) (Human readable
  form).
• Size/Length in bytes /Blocks . Maximum allowed
  size is specified sometimes (Human readable
  form).
• Protection Accessibility Control Based on
  user Group (Partly Human readable form).
• Owner Identification Related to user id.
  (Human readable form).
• Time Date (Human readable form) constitutes
  of one or more of the following
• - File Creation Date Time.
• - Last modification Date Time.
• - Last access Date Time.
• Location Pointer to the start (sometimes also
  the Last) physical block of the file as stored on
  the secondary media (Non Human accessible but
  O.S. readable).
• State Open / Closed.
• Sharing Attribute
• Storage Device Details System accessibility.
• N.B
• Any Directory is also a File.
• Each File is also a structured Abstract Data
  Type (ADT).

7
Basic File Operations

• Creation of a new File CREATE FILE.
• Deletion of an Existing File DELETE FILE.
• Opening an Existing File OPEN FILE.
• All types of file element (Record) accesses
  (to be illustrated shortly) must be preceded by
  OPEN FILE. This is necessary for activating the
  file access mechanism.
• Closing an Opened File CLOSE FILE. (After all
  references / accesses to that file are finished).

8
File Operations - 1 (Tasks at hand)

• A. CREATE FILE
• Allocate space for the File in the designated
  media (if space exists).
• After allocating space, create a directory entry
  with all relevant information. (File is still
  empty i.e. does not contain any record).
• B.DELETE FILE
• Locate the File Blocks in the media as well as
  check whether the file is closed or not.
• Release all spaces occupied by that File in the
  designated media .
• After freeing space, remove the directory entry
  for that file.

9
File Operations - 2 (Tasks at hand)

• C. OPEN an Existing FILE (Salient Features)
• Activates the File Access Mechanism.
• Helps to eliminate repeated searching of the
  file in the directory.
• Several Processes may Open the Same File.
• Associated Data Structures

P 1
Process Open File Table
P 2
System Open File Table
Secondary Store
10
Informations associated with an Open File

• Access Rights Obtained from the directory
  matched with Access Requesting Process Id . Used
  to regulate the file accessibility from each of
  the requesting processes. Contained in the
  System File Table.
• Start Pointer Address of the starting block of
  the File on Secondary Media. Contained in the
  System File Table.
• Current File Position Pointer Unique to each
  Reader processes accessing the file contained in
  the Process File Table.. Assuming a single file
  writer process and no other access during
  writing.
• Open Count Non zero Integer counter indicating
  number of processes currently accessing the file.
  Contained in the Process File Table as well as a
  part of the System File Table. Each open call by
  a process of a particular file increments this
  counter.
• Current Access Date Directory entry used
  updated by the System File Table.
• File Size Directory entry used updated by the
  System File Table.

11
File Operations 3(File Element / Record Access)

• Read (Reading a record) from the current file
  pointer position.
• Write (Writing a record) after/at the current
  file pointer position.
• N.B There may be two(2) separate file pointers
  namely a Read Pointer and a Write Pointer
  associated with any opened file.
• Append Add a new record AFTER the last record of
  the file.
• Reposition the Current File Pointer (Read/Write)
  Dictated by the File Organization as well as
  access mechanism as supported by the O.S.
  /Storage Media.
• Close (File) Inverse of OPEN File.

12
Type of a File (The type extension)

• File type associated with the file name itself
  in the following manner
• ltFile Name gt.ltExtension) File Type Specifier
  Stringgt
• ? Wild Card (ALL Types that share the same
  name)

13
File Types (Usage associated access)

• File Type used Extension Usage
  Accesibility
• --------------------------------------------------
  -------------
• Document doc, tex, pdf, ps Typed
  Documents Direct
• --------------------------------------------------
  ---------------------------------------
• Source c , c, asm , java
  Program Source Direct
• -------------------------------------------------
  ---------------------------------------
• archive arc, tar, zip
  Related files in Direct / Seq
• 
  Grouped , Compressed
• --------------------------------------------------
  ----------------------------------------
• Library lib , dll
  Support Routines Direct
• --------------------------------------------------
  ----------------------------------------
• Data Text txt, dat
  Textual Data Direct / Seq.
• --------------------------------------------------
  ------------------------------------------
• Multimedia mpeg, avi, jpeg Binary
  File containing Direct
• 
  Audio Video , Picture
• --------------------------------------------------
  -------------------------------------------
• Executable exe, com, bin
  Executable Module Direct / Seq.
• --------------------------------------------------
  --------------------------------------------
• Batch bat , sh
  Sequence of Commands Seq Preferable

14
File Organization Criteria

• Access time of File Elements / Logical Records
  (The faster the better).
• Ease of Update.
• Economy of storage i.e. ease of secondary memory
  access.
• Simple to maintain.
• Reliability factor.

15
File Organization vs Access

• Pile Records are collected as they arrive.
  Variable length records, Variable Set of Fields,
  Chronological Order. Accessed by exhaustive
  search.
• Sequential Fixed length Records with fixed
  number of fields arranged sequentially based on
  some key field accessed strictly sequentially.
• Indexed Sequential Single Index field searched
  randomly followed by sequential access of
  records.
• Indexed Multi Level Index based Random access.
• Hashed (Random ) Direct Access search the start
  of the relevant record by employing some hashed
  key.

16
Directory Structure

• Linear ( Rarely Used).
• Hierarchical
• a) Tree / Acyclic Graph Mostly Used.
• b) Cyclic Graph Management difficult.

17
File System Software Architecture

• User Program
• File Organization

Logical I/O
Basic I/O Supervisor
Basic File System
Device Interface
18
Typical Directory Structure

Root Directory
User 1
User 2
User N
.
Sub_ Directories
Different Files
19
Information Storage Structure(Logical Vs.
Physical View)

Applications
Records
Structured Record Files / Logical Files
Record ? Byte Stream Translation
Byte Stream Files
Stream ? Block Translation
Storage Devices
Storage Blocks