Introduction

The inode (index node) is the most critical data structure in the UNIX file system. Every file in UNIX is internally represented by an inode. The inode contains all the information the kernel needs to manage a file — except the file's name.

What is an Inode?

• An inode is a fixed-size data structure stored on disk in the inode list.
• Each file has exactly one inode.
• Each inode has a unique inode number within its file system.
• The inode number serves as the file's internal identity.

Inode Contents (Fields)

Block Addressing in Inodes

A key challenge is: how does an inode locate the data blocks of a file, especially for large files?

UNIX uses a combination of direct and indirect pointers:

1. Direct Blocks (10 pointers):

• The first 10 entries in the block address array point directly to data blocks.
• For a block size of 1 KB, direct blocks can address up to 10 KB of data.

2. Single Indirect Block (1 pointer):

• Points to a block that contains pointers to data blocks.
• If a block can hold 256 pointers (1 KB block / 4-byte pointer), this adds 256 KB.

3. Double Indirect Block (1 pointer):

• Points to a block of pointers, each pointing to another block of pointers to data blocks.
• Adds 256 × 256 = 65,536 blocks = 64 MB.

4. Triple Indirect Block (1 pointer):

• Three levels of indirection.
• Adds 256 × 256 × 256 = 16,777,216 blocks = 16 GB.

Block Addressing Diagram

Inode Block Address Array:
┌────────────┐
│ Direct 0   │──→ Data Block
│ Direct 1   │──→ Data Block
│ ...        │
│ Direct 9   │──→ Data Block
│ Single Ind │──→ [Ptr Block] ──→ Data Blocks
│ Double Ind │──→ [Ptr Block] ──→ [Ptr Block] ──→ Data Blocks
│ Triple Ind │──→ [Ptr Block] ──→ [Ptr Block] ──→ [Ptr Block] ──→ Data Blocks
└────────────┘

Maximum File Size Calculation (Example: 1 KB block, 4-byte pointer)

What an Inode Does NOT Contain

• File name — stored in the directory (as filename-to-inode mapping).
• File data — stored in data blocks (inode only contains pointers to them).

Summary

• An inode stores all file metadata except the filename.
• Block addressing uses 10 direct, 1 single indirect, 1 double indirect, and 1 triple indirect pointer.
• This scheme allows UNIX to support very large files efficiently.
• Understanding inodes is essential for understanding how UNIX manages files internally.