Squashfs
Squashfs is a compressed read only file system. It supports all the modern compression algorithms, including gzip, lzma, lzo and zstd compression. It can be used together with aufs to provide a fast writable filesystem for desktop and embedded systems.
Contents
Requirements
The Linux kernel must be built with CONFIG_SQUASHFS. It can be found in make menuconfig under ▸ . Several sub-options for it become visible once you select it. The default selection is usually fine.
Utilities
There are two competing sets of user-space utilities for squashfs:
squashfs-tools, the original squashfs tools. These tools are usually the ones that are available in GNU/Linux distributions. Maintained at github.com/AgentD/squashfs-tools-ng.squashfs-tools-ng, a completely different set of user-space tools with no relation tosquashfs-tools. Maintained at github.com/plougher/squashfs-tools.
squashfs-tools development looked dead to those who looked at the SourceForge repository when the developers moved from SourceForge to GitHub in the mid-2010s. A few developers who assumed it was dead decided to fork it and actively maintain that fork. They looked at the code, scratched their heads and decided to just scrap everything and start over. Those efforts became squashfs-tools-ng. The current squashfs-tools-ng utilities share no code with squashfs-tools, they are entire different implementations.
The original developers squashfs-tools kept actively developing the original set of utilities at GitHub where it is still, as of March 2021, being actively maintained.
mksquashfs (squashfs-utils)
squashfs filesystems are made using mksquashfs. -b specifies the block size. 131072 (128k) is the default size. A larger block size will usually result in slightly better compression, but the read speed can be worse. The default block size is a good choice.
| Shell command(s): |
nice -n 19 mksquashfs /usr /squashed/backup/usr.sfs.`date --iso` -comp xz -b 131072
|
gzip compression is used if no -comp parameter is provided. gzip, lzma, lzo, lz2, xz and zstd are supported. See Comparison of Compression Algorithms to get an idea how they compare in general. You may want to refer to mksquashfs --help to see what options are available for each; there are algorithm-specific options (-Xcompression-level, for example, is only available for gzip, lzo and zstd).
Compression comparison
These are the results of a /usr tree on a very big Gentoo Linux installation with a few games being compressed using gzip and xz.
|
19090 Mbytes filesystem |
gzip, 128k bs |
gzip, 256k bs |
xz, 128k bs |
xz, 256k bs |
|
Compressed size |
8992.04 Mbytes |
8955.73 Mbytes |
8384.18 Mbytes |
8247.87 Mbytes |
|
% of uncompressed filesystem size |
48.23% |
48.04% |
44.97% |
44.24% |
|
% uncompressed inode table size |
28.59% |
28.54% |
22.32% |
22.17% |
|
% uncompressed dir size |
36.37% |
36.26% |
33.46% |
33.40% |
The difference between xz and gzip is huge, the difference between 128k and 256k block sizes is not.
aufs
It is possible to combine read-only squashfs file systems with a writable file system on top that behind-the-scenes writes changes to another location.
Gentoo Linux users can use squashfs+aufs to create a fast root filesystem.[1]
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