Why Your File System Is Just a More Complex Folder

From Physical Disk to Logical Files

Physical disk = empty warehouse
    ↓
    Partition = divided into rooms
    ↓
    Formatting = installing shelving system
    ↓
    File system = warehouse manager software

Without a file system a disk is only a collection of 0s and 1s; with one the operating system can locate directories such as "My Documents".

Common File Systems

FAT32 – typical on USB drives and older devices; compatible widely but limits single files to <4 GB.

NTFS – default for Windows system drives; supports large files and permission management.

exFAT – used on large‑file USB drives; removes the 4 GB limit of FAT32.

ext4 – common on Linux; efficient and stable.

APFS – macOS modern file system; provides encryption, snapshots, and merged partitions.

XFS – enterprise storage; handles very large files with high performance.

How a File System Works

1. Superblock – Warehouse Records

Superblock records:
- File system size
- Number of free blocks
- Inode table location
- File system state

2. Inode – File "ID Card"

Inode contains:
- File size
- Creation and modification timestamps
- File permissions
- File type
- Data block pointers

Each file has an inode and is accessed via its inode number.

# Show a file's inode number
ls -i document.txt
# Output example: 1234567 document.txt

# Show detailed file info (including inode)
stat document.txt

3. Data Blocks – Storage Cabinets

inode structure:
┌─────────────┐
│  File size  │
│  Permissions│
│  Timestamps │
├─────────────┤
│  Block ptr1 ──→ Data block 1
│  Block ptr2 ──→ Data block 2
│  Block ptr3 ──→ Data block 3
│  ...         │
└─────────────┘

A single inode can point to multiple data blocks, forming a chain for large files.

Tree Structure of a File System

/
├── home/
│   ├── user1/
│   │   ├── documents/
│   │   │   ├── report.docx
│   │   │   └── notes.txt
│   │   └── pictures/
│   │       └── photo.jpg
│   └── user2/
├── etc/
│   └── config.conf
└── var/
    └── log/
        └── system.log

Each directory is a folder.

Each file resides at a leaf node.

Path separator: / on Linux/macOS, \ on Windows.

Why the C Drive Fills Quickly

Windows C: (system drive)
    ↓
    Default installation location
    ↓
    Install software → C:
    Download files → C:
    WeChat cache → C:
    ↓
    Disk usage grows over time

Solutions

Change default install location to another drive (e.g., D: or E:).

Move large user folders such as WeChat or QQ to another drive.

Clean temporary files regularly (e.g., Disk Cleanup).

Expand storage by adding a larger disk or deleting unnecessary large files.

Linux ext4 File System

# Show disk usage
df -h

# Show directory size
du -sh /home/user/documents

# Show file type
file document.txt

ext4 Features

Journaling – prevents corruption after power loss and records operation logs for fast recovery.

Delayed allocation – collects data before writing it in a single operation, improving performance.

Dynamic inode allocation – inodes are allocated on demand, eliminating the need for pre‑allocation.

File System Permission Management

Linux Permissions

# ls -l shows permissions
-rwxr-xr-x 1 user group 4096 Jan 15 10:30 document.txt

# Explanation of fields:
# - First character: file type (- for regular file, d for directory)
# - rwx: owner permissions (read, write, execute)
# - r-x: group permissions (read, execute)
# - r-x: others permissions (read, execute)

Windows (NTFS) Permissions

Full control

Modify

Read & execute

Read

Write

Virtual File System (VFS)

Programs can operate files through a single set of APIs regardless of the underlying file system.

Application
    ↓
Virtual File System layer (VFS)
    ↓
┌─────┬─────┬─────┬─────┐
│ext4 │NTFS │FAT32│APFS │ ← actual file systems
└─────┴─────┴─────┴─────┘

VFS provides a unified interface that abstracts away the differences between ext4, NTFS, FAT32, etc.

Summary: File System Family

Core concepts:
1. Superblock – records basic file‑system information.
2. Inode – stores metadata (size, timestamps, permissions, type, block pointers).
3. Data blocks – hold the actual file content.
4. Directory entry – points to an inode (acts like a shortcut).

Common systems:
Windows → NTFS (system drive), exFAT (large‑file USB)
Linux   → ext4 (default), XFS (large‑scale)
macOS   → APFS (modern), HFS+ (legacy)

Choosing the appropriate file system affects how efficiently small files, large files, and permissions are handled.