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File Systems

File Concept

A file is a named collection of related information stored on secondary storage.

Attributes: - Name - Type - Location - Size - Protection (permissions) - Time, date, and user identification

File Operations

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

File Types

  • Regular files (text or binary)
  • Directories
  • Special files (devices)

Directory Structure

Single-Level Directory

All files in one directory. Simple but no organization for multiple users.

Two-Level Directory

Separate directory for each user. Solves name collision problem.

Tree-Structured Directory

Most common. Hierarchical structure with arbitrary depth.

Path names: - Absolute path: /home/user/file.txt - Relative path: ../other/file.txt

File Allocation Methods

1. Contiguous Allocation

Each file occupies a set of contiguous blocks on disk.

Advantages: - Simple - only starting location and length needed - Fast sequential access - Fast random access

Disadvantages: - External fragmentation - Difficulty in growing files

2. Linked Allocation

Each file is a linked list of disk blocks scattered anywhere on disk.

Advantages: - No external fragmentation - Files can grow easily

Disadvantages: - Slow random access (must traverse list) - Space overhead for pointers - Reliability (if one link breaks, file is lost)

3. Indexed Allocation

Brings all pointers together into an index block.

Advantages: - Fast random access - No external fragmentation

Disadvantages: - Overhead of index block - What if file is larger than index block?

Solution for large files: Multi-level indexing (similar to multi-level page tables)

Inodes (Unix/Linux)

An inode (index node) contains file metadata and pointers to data blocks.

Inode structure: - File metadata (permissions, owner, timestamps, size) - Direct pointers (typically 12) - point directly to data blocks - Single indirect pointer - points to a block of pointers - Double indirect pointer - points to a block of single indirect pointers - Triple indirect pointer - points to a block of double indirect pointers

Maximum file size: Determined by number of pointers and block size.

Example with 4KB blocks: 

12 direct pointers: 12 × 4KB = 48KB
1 single indirect: 1K pointers × 4KB = 4MB
1 double indirect: 1K × 1K pointers × 4KB = 4GB
1 triple indirect: 1K × 1K × 1K pointers × 4KB = 4TB

Free Space Management

Bit Vector (Bitmap)

Each block represented by 1 bit. 0 = free, 1 = allocated (or vice versa).

Advantage: Simple, easy to find contiguous blocks
Disadvantage: Requires extra space

Linked List

Free blocks linked together.

Advantage: No waste of space
Disadvantage: Slow to traverse

Grouping

Store addresses of n free blocks in first free block. Last address points to another block containing addresses of free blocks.

Counting

Store address of first free block and count of contiguous free blocks.

File System Commands

# List files with inodes
ls -li

# Display file metadata
stat filename

# Disk space usage
df -h              # File system usage
du -sh directory/  # Directory size

# File permissions
chmod 755 file     # rwxr-xr-x
chown user file    # Change owner

File Permissions (Unix/Linux)

rwxr-xr--
|||
||└─ Others: read only
|└── Group: read + execute
└─── Owner: read + write + execute

Octal notation:
r = 4, w = 2, x = 1
rwx = 7, r-x = 5, r-- = 4
Example: 754 = rwxr-xr--

For files: x means executable For directories: x means can enter/traverse the directory

DMA (Direct Memory Access)

Device controller transfers data directly between device buffer and main memory without CPU copying each byte.

How CPU knows operation is complete: Device controller generates an interrupt when block transfer completes.