Basic Concepts of Computer File Systems

Basic Concept and Role of a File System

A computer file system is the method and data structure an operating system uses to store, organize and retrieve data on storage devices. It decides how files are named, arranged and accessed on media such as hard disks and solid-state drives.

In simple terms, the file system acts as the bridge between user applications and the physical storage medium. It helps the operating system keep data orderly, secure and accessible.

Core Components of a File System

• Files: Named collections of related information recorded on secondary storage.
• Directories: Hierarchical structures that contain files and subdirectories to organise data.
• Metadata: Data about data, including file size, creation date, modification time, owner and access permissions.

Key Functions of File Systems

• Storage allocation: Assigns space on storage media for new files and manages free space.
• Security control: Enforces permissions and access rules to restrict unauthorised file access.
• Address translation: Converts logical file requests from applications into physical disk block addresses.
• Directory management: Maintains directory structures for quick navigation and retrieval.
• Data organisation: Keeps records structured so that files can be stored and found efficiently.

Common File System Types

File Allocation Methods

• Contiguous allocation: Each file occupies a set of adjacent blocks on the storage disk.
• Advantage: Supports fast sequential and direct access to file data.
• Limitation: Suffers from external fragmentation and difficulty in expanding file size.
• Linked allocation: Each file is a linked list of disk blocks scattered across storage.
• Advantage: Eliminates external fragmentation and allows files to grow dynamically.
• Limitation: Direct access is slow because pointers must be read sequentially.
• Indexed allocation: Stores all pointers for a file in a dedicated index block.
• Advantage: Enables efficient direct access without external fragmentation.
• Limitation: Requires extra storage overhead for maintaining index blocks.

Important Facts to Remember

• FAT: The File Allocation Table was introduced by Microsoft in 1977 for floppy disks.
• NTFS: New Technology File System was introduced by Microsoft in 1993 with Windows NT 3.1.
• Root directory: The topmost directory in a hierarchical file system structure.
• ext4: The Linux ext4 file system supports journaling to prevent data corruption during power failures.
• Journaling file systems: Maintain a log of changes before writing them to the main storage disk.

Key Prelims Takeaways

• File system: Controls how data is stored, named, organised and retrieved by an operating system.
• Files, directories and metadata: These are the basic building blocks of any file system.
• Permissions: File systems help enforce security and user access control.
• Logical-to-physical mapping: A major function of a file system is translating file requests into disk block addresses.
• FAT32: Supports a maximum file size of 4 GB and a maximum volume size of 2 TB.
• NTFS and APFS: Both are designed for large-scale storage needs and advanced file management.
• Journaling: Helps reduce corruption by logging changes before they are committed to storage.