OS Notes

File System Introduction

Introduction to file systems — what they are, why we need them, basic concepts of files and directories, and how file systems organize data on storage devices.

Introduction

When you save a document, close your laptop, and open it again the next day, your document is still there. This persistence — data surviving beyond a single program execution or power cycle — is provided by the file system. Without it, every byte of data would vanish the moment you shut down your computer.

A file system is the OS component that manages how data is stored, organized, and retrieved on persistent storage devices (hard drives, SSDs, USB drives). It provides the abstraction of "files" and "directories" — hiding the raw complexity of disk sectors and blocks behind a clean, intuitive interface. Think of it as the librarian that organizes millions of books (data blocks) into a system where you can find any book by its title (filename) and location (directory path).

Why Do We Need File Systems?

Without a file system, a disk is just a flat array of billions of bytes with no structure. You would need to remember exact byte positions for every piece of data. The file system solves several problems:

Naming: Give data meaningful names instead of sector numbers
Organization: Group related files into directories (folders)
Access control: Control who can read, modify, or delete each file
Persistence: Ensure data survives reboots and power failures
Sharing: Allow multiple programs to access the same data
Space management: Track which disk blocks are free and which are used
Integrity: Protect against data corruption from crashes

Basic File System Concepts

Files

A file is a named collection of related data stored on secondary storage. From the user perspective, it is the smallest unit of persistent storage. Files can contain anything — text, images, executables, databases.

Directories (Folders)

A directory is a special file that contains a list of other files and directories. It provides hierarchical organization — grouping related files and enabling navigation through the file system.

Path

A path specifies the location of a file in the directory hierarchy:

Absolute path: Full path from root — /home/student/documents/report.pdf
Relative path: Path from current directory — documents/report.pdf

Metadata

Information about a file (not the data itself): name, size, creation date, permissions, owner, location on disk. Stored separately from file content.

File System Architecture

User Application ↓ (open, read, write, close) ┌─────────────────────────────────────┐ │ Virtual File System (VFS) │ ← Uniform API for all file systems ├────────┬──────────┬─────────────────┤ │ ext4 │ NTFS │ FAT32 │ ← File system implementations ├────────┴──────────┴─────────────────┤ │ Block I/O Layer │ ← Manages disk block operations ├─────────────────────────────────────┤ │ Device Drivers │ ← Hardware-specific code ├─────────────────────────────────────┤ │ Physical Storage (HDD/SSD) │ └─────────────────────────────────────┘

Common File Systems

File System	OS	Features
ext4	Linux	Journaling, extents, 16TB file limit
NTFS	Windows	ACLs, encryption, compression, journaling
APFS	macOS/iOS	Copy-on-write, snapshots, encryption
FAT32	Universal	Simple, no permissions, 4GB file limit
exFAT	Universal	Like FAT32 but no 4GB limit
XFS	Linux	High-performance, large file support
Btrfs	Linux	Copy-on-write, snapshots, RAID

File Operations

The OS provides these basic operations on files:

Create: Allocate space, add directory entry
Open: Find file, load metadata, return file descriptor
Read: Copy data from file to process memory
Write: Copy data from process memory to file
Seek: Move the read/write position within the file
Close: Release file descriptor, flush buffers
Delete: Remove directory entry, free allocated blocks

c example

// Basic file operations in C
int fd = open("data.txt", O_RDWR | O_CREAT, 0644);  // Open/create
write(fd, "Hello", 5);                                // Write
lseek(fd, 0, SEEK_SET);                              // Seek to beginning
read(fd, buffer, 5);                                  // Read
close(fd);                                            // Close
unlink("data.txt");                                   // Delete

Real-World Analogy

A file system is like a filing cabinet system in an office. The cabinet (disk) has drawers (partitions), folders (directories), and papers (files). Each paper has a label (filename) and is organized by category (directory structure). The filing clerk (file system) knows where everything is, can find papers quickly (indexing), and ensures only authorized people access sensitive documents (permissions). Without this system, papers would be piled randomly and finding anything would be nearly impossible.

Key Takeaways

File systems provide persistent, organized, named storage on disk devices
They abstract raw disk blocks into files and directories for user convenience
Key operations: create, open, read, write, seek, close, delete
Different file systems (ext4, NTFS, APFS) offer different features and tradeoffs
The Virtual File System (VFS) layer provides a uniform API across different file system types
File systems manage space allocation, metadata, access control, and data integrity
Understanding file systems helps you make informed choices about storage and data management

Exam Focus

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Search Terms

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Continue learning this concept

File SystemDirectory StructureUnderstanding directory structures — single-level, two-level, tree-structured, acyclic graph, and general graph directories with their advantages and disadvantages.File SystemFile Allocation MethodsHow file systems allocate disk space — contiguous allocation, linked allocation, and indexed allocation with FAT and inode-based approaches, advantages and disadvantages of each.File SystemFile ConceptsUnderstanding file concepts — file attributes, file types, file structure, file access methods (sequential, direct, indexed), and internal file representation.File SystemFile ProtectionFile protection mechanisms in operating systems — access control types, Unix permissions (rwx), Access Control Lists (ACLs), capabilities, and protection domain concepts.File SystemFile System ImplementationHow file systems are implemented — on-disk structures, in-memory structures, VFS layer, inode-based implementation, and the journey of a file operation from user call to disk access.File SystemFree Space ManagementHow operating systems track and manage free disk space — bitmap method, linked list, grouping, counting approaches, and their performance characteristics.

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