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ext3 (Third Extended File System)

ext3 is a journaling file system for Linux, introduced as an extension of ext2. It provides enhanced reliability and recovery features while maintaining backward compatibility with ext2. Here's a detailed look at the ext3 file system and its features:

Structure of the ext3 File System

1. Boot Sector:

The boot sector contains metadata about the file system, including:

  • Inode Count:

    • Total number of inodes in the file system.

  • Block Count:

    • Total number of blocks in the file system.

  • Reserved Block Count:

    • Number of blocks reserved for the root user.

  • Free Block Count:

    • Number of free blocks available.

  • Free Inode Count:

    • Number of free inodes available.

  • First Data Block:

    • Block number of the first data block.

  • Block Size:

    • Size of each block (commonly 1024, 2048, or 4096 bytes).

  • Journal Information:

    • Metadata related to the journaling feature.

2. Inode Structure:

Each file or directory in ext3 is represented by an inode, which contains:

  • File Type and Permissions:

    • Indicates if it's a file, directory, or special type.

  • File Size:

    • Size of the file in bytes.

  • Timestamps:

    • Tracks file creation, modification, and access times.

  • Pointers to Data Blocks:

    • Direct and indirect pointers to the blocks storing file content.

3. Directory Structure:

Directories are implemented as files containing a list of directory entries. Each entry includes:

  • Inode Number:

    • Points to the inode representing the file or subdirectory.

  • Entry Length:

    • Length of the directory entry.

  • File Name Length:

    • Length of the file name.

  • File Name:

    • The actual name of the file or subdirectory.

Features of ext3

  • Journaling:

    • Provides three journaling modes: Writeback, Ordered, and Journal.

    • Helps ensure file system consistency after crashes.

  • Backward Compatibility:

    • Fully compatible with ext2; ext2 file systems can be converted to ext3 without data loss.

  • Improved Reliability:

    • Prevents corruption by journaling metadata and optionally file data.

  • Large File and Volume Support:

    • Supports volumes up to 32 TB and files up to 2 TB (with appropriate block sizes).

Journaling Modes

  1. Writeback:

    • Only metadata is journaled, providing the highest performance but lower reliability.

  2. Ordered (default):

    • Metadata is journaled, and file data is written to disk before metadata updates.

  3. Journal:

    • Both metadata and file data are journaled, offering maximum reliability.

Tools and Commands for ext3

  • fsck.ext3:

    • Checks and repairs ext3 file systems.

  • mkfs.ext3:

    • Creates an ext3 file system.

  • tune2fs:

    • Adjusts tunable file system parameters, such as enabling or disabling journaling.

  • mount:

    • Mounts the ext3 file system, with options to specify journaling modes.

Example

# Check and repair an ext3 file system fsck.ext3 /dev/sdX # Create an ext3 file system mkfs.ext3 /dev/sdX # Mount an ext3 file system with journaling mode mount -o data=journal /dev/sdX /mnt # Convert an ext2 file system to ext3 tune2fs -j /dev/sdX

ext3 brought significant improvements in reliability and recovery while maintaining the simplicity of ext2.

Last modified: 19 January 2025
ext2 (Second Extended File System)ext4 (Fourth Extended File System)