File Systems

File Concept

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

Attributes: Name, Type, Location, Size, Protection, Timestamps, Owner

Operations: Create, Write, Read, Seek, Delete, Truncate

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Directory Structure

flowchart TD
    R["/\n(root)"] --> H["/home"]
    R --> E["/etc"]
    R --> U["/usr"]
    H --> S["/home/sabrina"]
    H --> B["/home/bob"]
    S --> D["docs/"]
    S --> F["notes.txt"]
    D --> F2["report.pdf"]

    style R fill:#7c4dff,color:#fff
    style S fill:#1565c0,color:#fff

• Absolute path: /home/sabrina/notes.txt
• Relative path: ../bob/file.txt

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File Allocation Methods

1. Contiguous Allocation

flowchart LR
    F["File\n(start=2, length=4)"] --> B2["Block 2"]
    B2 --> B3["Block 3"]
    B3 --> B4["Block 4"]
    B4 --> B5["Block 5"]

    style F fill:#1565c0,color:#fff
    style B2 fill:#00897b,color:#fff
    style B3 fill:#00897b,color:#fff
    style B4 fill:#00897b,color:#fff
    style B5 fill:#00897b,color:#fff

• Fast sequential and random access
• External fragmentation
• Hard to grow files

2. Linked Allocation

flowchart LR
    F["File\n(start=2)"] --> B2["Block 2\ndata | next→5"]
    B2 --> B5["Block 5\ndata | next→8"]
    B5 --> B8["Block 8\ndata | next→NULL"]

    style F fill:#1565c0,color:#fff

• No external fragmentation
• Slow random access (must traverse list)
• Pointer overhead

3. Indexed Allocation

flowchart LR
    F["File\n(index block=3)"] --> IB["Index Block 3\n[5, 8, 12, 17, ...]"]
    IB --> B5["Block 5\ndata"]
    IB --> B8["Block 8\ndata"]
    IB --> B12["Block 12\ndata"]

    style IB fill:#f57c00,color:#fff
    style F fill:#1565c0,color:#fff

• Fast random access
• No external fragmentation
• Overhead of index block

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Inodes (Unix/Linux)

flowchart TD
    I["Inode\n(metadata + pointers)"] --> DP["12 Direct Pointers\n→ data blocks directly\n(48KB with 4KB blocks)"]
    I --> SP["Single Indirect\n→ block of pointers\n(4MB)"]
    I --> DP2["Double Indirect\n→ block of single indirect\n(4GB)"]
    I --> TP["Triple Indirect\n→ block of double indirect\n(4TB)"]

    DP --> DB1["Data Block"]
    SP --> PB["Pointer Block"] --> DB2["Data Block"]

    style I fill:#7c4dff,color:#fff
    style DP fill:#00897b,color:#fff
    style SP fill:#1565c0,color:#fff
    style DP2 fill:#f57c00,color:#fff
    style TP fill:#ef5350,color:#fff

Example with 4KB blocks:

12 direct:        12 × 4KB       =  48KB
Single indirect:  1K × 4KB       =   4MB
Double indirect:  1K × 1K × 4KB  =   4GB
Triple indirect:  1K³ × 4KB      =   4TB

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Free Space Management

Method	How	Pros	Cons
Bitmap	1 bit per block (0=free, 1=used)	Simple, find contiguous blocks	Extra space
Linked list	Free blocks linked together	No waste	Slow traversal
Grouping	Addresses of n free blocks stored in first free block	Efficient	Complex
Counting	First free block + count of contiguous free blocks	Good for contiguous	More complex

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File Permissions (Unix/Linux)

rwxr-xr--
|||
||└─ Others:  r-- = 4
|└── Group:   r-x = 5
└─── Owner:   rwx = 7

Octal: 754

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

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Useful 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

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DMA (Direct Memory Access)

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

flowchart LR
    CPU(["CPU\ninitializes transfer"]) --> DC["Device Controller"]
    DC -->|"Direct transfer\n(no CPU involved)"| MEM["Main Memory"]
    DC -->|"Interrupt when\ncomplete"| CPU

    style CPU fill:#7c4dff,color:#fff
    style MEM fill:#00897b,color:#fff