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CHAPTER 18: FILE MANAGEMENT

Overview

• Introduction to File Management

• Logical and Physical View of Files

• Role of the File Management System

• Logical File Access Methods

• Physical File Storage

• File Systems, Volumes, Disks, Partitions, and Storage Pools

• Directory Structure

• Network File Access

• File Protection

• Journaling File Systems

Introduction to Files

• Definition: A collection of related data.

• Block: The basic unit of file storage, typically between 256 and 4096 bytes; generally requires a minimum of one block for storage.

• Cluster: Grouping of one or more blocks corresponding to one or more sectors on a disk, allowing efficient access.

• File Extension: Suffix at the end of a file name that identifies the file type, which helps the operating system determine how to handle the file.

• File Association: Specifies the program that utilizes the file type based on its extension, linking it to appropriate applications.

Terminology

• Logical View: User's perspective of file contents and attributes, which may differ from the actual storage method.

• Physical View: The actual method used to store files within the system, which may include various storage strategies.

• Access Methods:

  • Sequential Access: Requires retrieval of data from the beginning of the file, useful for reading through entire datasets.

  • Random/Relative Access: Allows data retrieval at any point in the file, enabling quick access to specific information.

• Storage Types:

  • Contiguous: Blocks of data are stored together in sequence, enhancing reading speed.

  • Noncontiguous: Blocks are scattered across the device, which can slow access times but allows more flexible storage.

File Management System

• Purpose: Facilitates a logical view of storage and hides the complexities of physical implementations.

• Commands: Translates user commands into appropriate device actions, streamlining user interactions with the file system.

• File Manipulation: Handles operations within files and directory structures, allowing for the creation, deletion, and modification of files.

• Security: Ensures the integrity and protection of files through access controls and other security measures.

File Operations:

Whole File Operations

• Copy: Duplicates the file in a specified location.

• Move: Transfers the file to a new location.

• List: Displays the contents of directories or file attributes.

• Print: Sends the file to a printer for a hard copy.

• Load and Execute Programs: Runs executable files, typically in application environments.

• Append Data: Adds information to the end of an existing file.

• Compile Files: Converts source code into executable programs, enabling usage.

Within a File Operations

• Open & Close file: Methods to access and release file resources.

• Read/Write bytes to/from files: Allows the transfer of data between files and the computing environment.

• Move file pointer within the file: Adjusts the location for the next read/write operation, crucial for data management.

Record Storage Operations

• Retrieve Records: Allows access to specific data entries for processing.

• Store Records: Saves data entries into the file for future retrieval.

• Add/Delete Records: Manages the contents of files dynamically.

• Modify Records: Makes changes to existing entries as required.

File Directory Operations

• Create new file: Establishes a new file entry within the file system.

• Move/Rename/Delete files: Modifies the organizational structure of files within the directory.

• Append one file to another: Combines file contents into a single file entity for data management.

File Management and I/O Functions

• Separation Benefits:

  • Allows for independent changes to I/O devices, enhancing system flexibility.

  • Simplifies data redirection for various application needs.

File Access Methods

• Sequential Access: Data is read in order, making it common for most file types.

• Random Access: Uses fixed-length records, with locations calculated typically through hashing.

• Indexed Access: Employs key indexes for quick record access, seen in systems like Indexed Sequential Access Method (ISAM).

Physical File Storage

• Categorized as: Contiguous, Non-contiguous, Linked, and Indexed.

• File System Examples: DOS/Windows FAT, UNIX i-nodes, Windows NTFS provide various methods of organizing and accessing files effectively.

Contiguous Storage Allocation

• Description: Assigns all blocks to a file in sequence.

• Challenges: Must predict and allocate space for potential file growth while managing fragmentation that impacts performance.

Linked Allocation

• Description: Allows noncontiguous storage with each block linking to the next, optimizing space use.

• Advantages: No fragmentation occurs, and it simplifies file growth.

• Disadvantages: Can result in slower random access and difficulties in recovery of defective blocks.

File Allocation Table (FAT)

• Structure: A linked allocation structure with links stored in a table.

• Disadvantages: High space requirements which may lead to potential integrity issues if corrupted.

Indexed Allocation

• Description: All links stored in a single index block for quick access.

• Advantages: Avoids fragmentation and facilitates efficient random access.

• Disadvantages: Slower access times due to index referencing, and recovery from corruption can be complex.

UNIX I-NODES

• Structure: Contains file attributes and pointers to data blocks in an indexed setup.

• Benefits: Rapid access for small files, efficiently handles large file sizes through well-structured pointers.

Windows NTFS

• Features: Supports dynamic volume sizes and utilizes a Master File Table (MFT) for management.

• MFT Attributes: Includes file attributes, metadata, and provisions for system file management.

Microsoft Resilient File System (REFS)

• Characteristics: Enhanced file management features, including advanced metadata handling and improved data integrity measures.

Free Space Management

• Methods:

  • Bitmap: Uses one bit per block to indicate usage and availability.

  • Linked List: A pointer tracks first free block and subsequent free blocks for space allocation.

Other Secondary Storage Allocation

• Tape Allocation: Contiguous allocation when feasible; for files that grow, rewrites may be required.

• Optical Drives: Utilizes mechanisms akin to hard disk storage for organization and access.

Directory Structure

• Purpose: Organizes files for efficient access, abstracting physical device details for user interface.

• Components: Encompasses partitions and volumes with defined mounting requirements for logical access.

Tree-Structure Directory

• Description: Hierarchical organization of files, stemming from a root directory, illustrating relationships among files.

• Pathnames:

  • Absolute Path: Full path from the root (e.g., C:\FINANCE\QUICKEN\Q.EXE).

  • Relative Path: Describes the path from the current directory context.

Acyclic Directory Structures

• Functionality: Allows for linked directories in a non-cyclic organization.

• Challenges: May lead to cycles or dangling links if not managed properly.

Network File Access

• Protocols: Includes FTP and NFS, allowing remote file operations across networks.

• Configuration: Typically requires user processes to interact with remote file managers guided by network protocols for effective access.

File Protection

• Methods:

  • Logins and Passwords: Basic authentication to protect file access.

  • Access Control List (ACL): Details user permissions for specific files, enhancing security hierarchies.

  • UNIX/Linux Owner/Group/Everyone Protection: Classifies permissions at three levels to protect file integrity based on user groupings.

Journaling File Systems

• Functionality: Maintains a log of all write transactions to uphold data integrity during operable conditions.

• Examples: NTFS, Linux ext3/ext4, and IBM JFS represent systems employing journaling techniques for reliability.