SQUASHFS Files Explained

Squashfs Filesystem

Overview

Squashfs is a compressed read-only filesystem for Linux and other Unix-like operating systems. It compresses files, inodes, and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Squashfs is widely used in embedded systems, live CDs/DVDs, and container images due to its excellent compression ratio and fast random access capabilities.

Technical Specifications

Format Details

• Filesystem Type: Compressed read-only
• Block Sizes: 4KB to 1MB (default: 128KB)
• Compression: Multiple algorithms supported
• Endianness: Little-endian and big-endian support
• Maximum File Size: 2^63 bytes

Compression Algorithms

• GZIP: Default, good balance of speed and compression
• LZMA/XZ: Better compression but slower
• LZO: Faster decompression
• LZ4: Very fast compression/decompression
• ZSTD: Modern algorithm with good speed/ratio balance

History and Development

Squashfs was created by Phillip Lougher and first released in 2002:

• 2002: Initial release (Squashfs 1.0)
• 2004: Squashfs 2.0 with improved compression
• 2006: Squashfs 3.0 included in Linux kernel
• 2009: Squashfs 4.0 with major format changes
• 2014: XZ compression support added
• 2017: LZ4 compression support
• 2020: ZSTD compression support

Common Use Cases

Live Operating Systems

# Creating a live CD/DVD filesystem
mksquashfs /source/directory filesystem.squashfs -comp xz -Xbcj x86

# Mounting the filesystem
sudo mount -t squashfs filesystem.squashfs /mnt/squashfs

Container Images

# Docker layers often use squashfs
# Extract Docker image layer
docker save image:tag | tar -xvf -
unsquashfs layer.tar

Embedded Systems

# Creating firmware image
mksquashfs /rootfs /firmware/rootfs.squashfs -comp lzo -b 64K

# Optimizing for space
mksquashfs /system system.squashfs -comp xz -Xdict-size 100%

Tools and Software

Creation Tools

# Basic squashfs creation
mksquashfs source_directory output.squashfs

# Advanced options
mksquashfs /source /output.squashfs \
    -comp xz \
    -Xbcj x86 \
    -b 1M \
    -processors 4 \
    -no-recovery

Extraction Tools

# Extract entire filesystem
unsquashfs filesystem.squashfs

# Extract to specific directory
unsquashfs -d /target/dir filesystem.squashfs

# List contents without extracting
unsquashfs -l filesystem.squashfs

Advanced Tools

• squashfs-tools: Standard utilities (mksquashfs, unsquashfs)
• squashfs-tools-ng: Next generation tools
• squashfuse: FUSE implementation for user-space mounting

Technical Features

Compression Options

# GZIP compression (default)
mksquashfs /source output.squashfs -comp gzip

# XZ compression for maximum compression
mksquashfs /source output.squashfs -comp xz -Xdict-size 100%

# LZ4 for fastest access
mksquashfs /source output.squashfs -comp lz4 -Xhc

# ZSTD balanced approach
mksquashfs /source output.squashfs -comp zstd -Xcompression-level 15

Block Size Optimization

# Small block size for better compression
mksquashfs /source output.squashfs -b 4K

# Large block size for better I/O performance
mksquashfs /source output.squashfs -b 1M

# Auto-detect optimal block size
mksquashfs /source output.squashfs -b 0

File System Features

• Hard Links: Preserved and shared
• Symbolic Links: Fully supported
• Extended Attributes: Preserved
• File Holes: Detected and optimized
• Duplicate Detection: Automatic deduplication

Performance Characteristics

Compression Comparison

Algorithm	Compression Ratio	Compression Speed	Decompression Speed
GZIP	Good	Medium	Fast
XZ/LZMA	Excellent	Slow	Medium
LZO	Fair	Fast	Very Fast
LZ4	Fair	Very Fast	Very Fast
ZSTD	Very Good	Fast	Fast

Optimization Examples

# For maximum compression (slow creation, normal access)
mksquashfs /source output.squashfs -comp xz -Xdict-size 100% -b 1M

# For fastest access (larger size)
mksquashfs /source output.squashfs -comp lz4 -b 64K

# Balanced approach
mksquashfs /source output.squashfs -comp zstd -Xcompression-level 10

Best Practices

Creation Guidelines

• Choose compression based on use case
• Use larger block sizes for sequential access
• Use smaller block sizes for random access
• Consider target hardware capabilities

Optimization Strategies

# Remove unnecessary files before compression
mksquashfs /source output.squashfs -e dev proc sys tmp

# Sort files for better compression
mksquashfs /source output.squashfs -sort sort_file.txt

# Use wildcards for exclusions
mksquashfs /source output.squashfs -wildcards -e "*.tmp" "*.log"

Mounting Options

# Standard mounting
mount -t squashfs -o loop image.squashfs /mnt

# With specific options
mount -t squashfs -o loop,ro,noatime image.squashfs /mnt

# User-space mounting with squashfuse
squashfuse image.squashfs /mnt

Security Considerations

Read-Only Nature

• Inherently secure against modification
• Cannot be corrupted by runtime errors
• Suitable for trusted boot environments

Integrity Checking

• Built-in checksums for data integrity
• Filesystem metadata is checksummed
• Corruption detection during access

Use in Secure Environments

# Create filesystem with verification
mksquashfs /source output.squashfs -comp xz -check-data

# Verify filesystem integrity
unsquashfs -fsck output.squashfs

Integration Examples

Docker and Containers

# Using squashfs in container builds
FROM alpine:latest
RUN apk add --no-cache squashfs-tools
COPY create_layer.sh /
RUN ./create_layer.sh

Embedded Linux

# Buildroot integration
BR2_TARGET_ROOTFS_SQUASHFS=y
BR2_TARGET_ROOTFS_SQUASHFS4_XZ=y

# OpenWrt usage
make menuconfig # Enable squashfs for rootfs

Live Systems

# Create live system
mksquashfs /chroot livefs.squashfs -comp xz
# Add to ISO or boot media

File Format Structure

Filesystem Layout

1. Superblock: Filesystem metadata
2. Compression Options: Algorithm parameters
3. Datablocks: Compressed file data
4. Metadata Blocks: Compressed metadata
5. Inode Table: File system objects
6. Directory Table: Directory entries
7. Fragment Table: Small file fragments
8. Export Table: NFS export support

Advantages

• Excellent compression ratios
• Fast random access
• Low memory usage during access
• Cross-platform compatibility
• Atomic filesystem image

Limitations

• Read-only filesystem
• Cannot be modified after creation
• Requires kernel support for mounting
• Creation can be CPU intensive

Squashfs remains the preferred choice for read-only filesystems where space efficiency and fast access are critical, particularly in embedded systems, live operating systems, and containerized environments.