Introduction

The UNIX file system layout defines how data is physically organized on a disk (or disk partition). Understanding the layout is crucial for comprehending how the kernel stores and retrieves files.

A UNIX file system on disk is divided into distinct sections, each with a specific purpose.

Disk Partitioning

• A physical disk can be divided into multiple partitions.
• Each partition can contain an independent file system.
• The kernel treats each partition as a logical device.

File System Layout on Disk

A typical UNIX file system partition has the following structure:

┌────────────┬─────────────┬────────────┬──────────────────────┐
│ Boot Block │ Super Block │ Inode List │     Data Blocks      │
└────────────┴─────────────┴────────────┴──────────────────────┘
  Block 0       Block 1      Blocks 2-N    Remaining Blocks

Detailed Description of Each Section

1. Boot Block (Block 0):

• The first block of every file system.
• Contains the bootstrap code — a small program that loads the operating system kernel into memory during system startup.
• Only the boot block of the root file system is actually used for booting. Boot blocks of other partitions are typically empty but still reserved.
• Size: Usually 1 block (512 bytes to 1 KB).

2. Super Block (Block 1):

• Contains critical metadata about the entire file system:
• Total size of the file system
• Number of inodes
• Number of data blocks
• Size of each block
• Number of free inodes and free data blocks
• Pointers to the free inode list and free block list
• Flags indicating if the file system has been modified
• The super block is loaded into memory when the file system is mounted.
• If the super block is corrupted, the entire file system becomes inaccessible.

3. Inode List (Inode Table):

• A linear array of inodes (index nodes).
• Each inode has a unique inode number within the file system.
• The inode list starts right after the super block.
• The number of inodes is determined when the file system is created (mkfs command) and cannot be changed later.
• Inode 0 is reserved (unused).
• Inode 1 is reserved for bad blocks.
• Inode 2 is always the root directory (/) of the file system.

4. Data Blocks:

• The remaining blocks on the disk, used to store:
• File contents (actual data).
• Directory entries (filename-to-inode mappings).
• Indirect blocks (pointers to more data blocks for large files).
• Data blocks occupy the majority of the disk space.

How File Access Works (Simplified)

1. User requests a file by path (e.g., /home/user1/file.txt).
2. Kernel starts at the root inode (inode 2).
3. Reads the root directory data blocks to find the entry for home → gets its inode number.
4. Reads home inode → finds data blocks → looks for user1 → gets its inode.
5. Reads user1 inode → finds file.txt → gets its inode number.
6. Reads file.txt inode → gets pointers to data blocks → reads the actual data.

Summary

• A UNIX file system is divided into: Boot Block, Super Block, Inode List, and Data Blocks.
• The Boot Block contains bootstrap code for OS loading.
• The Super Block stores metadata about the entire file system.
• The Inode List holds all file metadata (one inode per file).
• Data Blocks store the actual file contents and directory entries.