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1. What is SquashFS

1.1. Introduction

When creating tiny-sized and embedded Linux systems, every byte of the storage device (floppy, flash disk, etc.) is very important, so compression is used everywhere possible. Also, compressed file systems are frequently needed for archiving purposes. For huge public archives, as well as for personal media archives, this is essential.

SquashFS brings all this to a new level. It is a read-only file system that lets you compress whole file systems or single directories, write them to other devices/partitions or to ordinary files, and then mount them directly (if a device) or using a loopback device (if it is a file). The modular, compact system design of SquashFS is bliss. For archiving purposes, SquashFS gives you a lot more flexibility and performance speed than a tarball archive.

SquashFS is distributed as a Linux kernel source patch (which enables SquashFS read support in your kernel), the mksquashfs tool, which creates squashed file systems (in a file or on a block device) and the unsquashfs tool, which extracts multiple files from an existing squashed file system.

The latest SquashFS release tree is 3.x, the former one was 2.x. This document describes both these releases with proper notes given. For example, if some feature or parameter is different in these release trees, it will be written as follows: new value (3.x) or old value (2.x)

1.2. Overview of SquashFS

  • Data, inodes and directories are compressed

  • SquashFS stores full uid/gids (32 bits), and file creation time

  • Files up to 2^64 bytes are supported; file systems can be up to 2^64 bytes

  • Inode and directory data are highly compacted, and packed on byte boundaries; each compressed inode is on average 8 bytes in length (the exact length varies on file type, i.e. regular file, directory, symbolic link, and block/character device inodes have different sizes)

  • SquashFS can use block sizes up to up to 64 Kb (2.x) and 1Mb (3.x). The default size is 128Kb (3.x), which achieves greater compression ratios than the normal 4K block size

  • By the 2.x release it was introduced the concept of fragment blocks: an ability to join multiple files smaller than block size into a single block, achieving greater compression ratios

  • File duplicates are detected and removed

  • Both big and little endian architectures are supported; SquashFS can mount file systems created on different byte-order machines

1.3. Making it clear

Now let's make sure any further discussions will be clearer for you to understand. The procedure of getting SquashFS working basically consists of the following steps:

  1. Patching and recompiling the target Linux kernel to enable SquashFS support

  2. Compiling the mksquashfs and the unsquashfs tools

  3. Creating a compressed file system with mksquashfs

  4. Testing: mounting a squashed file system to a temporary location

  5. Modifying the /etc/fstab or startup scripts of your target Linux system to mount the new squashed file system when needed