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ext4 (Fourth Extended File System)

ext4 is a modern journaling file system for Linux, introduced as an improvement over ext3. It offers enhanced performance, scalability, and reliability while remaining backward compatible with ext3. Here's a detailed look at the ext4 file system and its features:

Structure of the ext4 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 ext4 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 with nanosecond precision.

  • Pointers to Data Blocks:

    • Includes direct, indirect, double indirect, and triple indirect pointers to data blocks.

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 ext4

  • Backward Compatibility:

    • Fully compatible with ext3; ext3 file systems can be converted to ext4.

  • Delayed Allocation:

    • Optimizes block allocation to reduce fragmentation and improve performance.

  • Extents:

    • Replaces traditional block mapping with extents, which describe a range of contiguous blocks.

  • Journal Checksumming:

    • Protects the integrity of the journal by adding checksums.

  • Large File and Volume Support:

    • Supports volumes up to 1 EB and files up to 16 TB (with appropriate block sizes).

  • 64-bit Storage:

    • Allows for larger files and file systems.

  • Multiblock Allocation:

    • Allocates multiple blocks at once, improving performance for large files.

Journaling Modes

  1. Writeback:

    • Only metadata is journaled, offering high 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, providing maximum reliability.

Tools and Commands for ext4

  • fsck.ext4:

    • Checks and repairs ext4 file systems.

  • mkfs.ext4:

    • Creates an ext4 file system.

  • tune2fs:

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

  • mount:

    • Mounts the ext4 file system, with options to specify journaling modes and features.

Example

# Check and repair an ext4 file system fsck.ext4 /dev/sdX # Create an ext4 file system mkfs.ext4 /dev/sdX # Mount an ext4 file system with journaling mode mount -o data=journal /dev/sdX /mnt # Convert an ext3 file system to ext4 tune2fs -O extents,uninit_bg,dir_index /dev/sdX

ext4 combines reliability with modern performance optimizations, making it a default choice for many Linux distributions.

Last modified: 19 January 2025
ext3 (Third Extended File System)FAT (File Allocation Table)