|Developer(s)||Phillip Lougher, Robert Lougher|
|Introduced||2002 with Linux|
|Max. volume size||16 EiB (264) bytes|
|Max. file size||16 EiB (264) bytes|
|Attributes||POSIX and extended attributes|
|Transparent compression||gzip LZMA LZO LZMA2 LZ4 Zstd|
|Supported operating systems||Linux|
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes up to 1 MB for greater compression. Several compression algorithms are supported. Squashfs is also the name of free software, licensed under the GPL, for accessing Squashfs filesystems.
Squashfs is used by the Live CD versions of Arch Linux, Debian, Fedora, Gentoo Linux, Linux Mint, Salix, Ubuntu and on embedded distributions such as the OpenWrt and DD-WRT router firmware. It is also used in Chromecast  and for the system partitions of Android Nougat. It is often combined with a union mount filesystem, such as UnionFS, OverlayFS, or aufs, to provide a read-write environment for live Linux distributions. This takes advantage of both the Squashfs's high speed compression abilities and the ability to alter the distribution while running it from a live CD. Distributions such as Debian Live, Mandriva One, Puppy Linux, Salix Live and Slax use this combination. AppImage project which aims to create portable linux applications, uses squashfs for creating appimages.
Squashfs was initially maintained as an out-of-tree Linux patch. The initial version 1.0 was released on 23 October 2002. In 2009 Squashfs was merged into Linux mainline as part of Linux 2.6.29. In that process, the backward-compatibility code for older formats was removed. Since then the Squashfs kernel space code has been maintained in the Linux mainline tree, while the user space tools remain on the project's Sourceforge page.
The original version of Squashfs used gzip compression, although Linux kernel 2.6.34 added support for LZMA and LZO compression, Linux kernel 2.6.38 added support for LZMA2 compression (which is used by xz), and Linux kernel 3.19 added support for LZ4 compression. Support for Zstandard was added in Linux Kernel 4.14.
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