SquashFS

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Squashfs (.sfs) is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes up to 1024 KB for greater compression. Squashfs is also free software (licensed under the GPL) for accessing Squashfs filesystems.

Squashfs is intended for general read-only file system use and in constrained block device/memory systems (e.g. embedded systems) where low overhead is needed. The standard version of Squashfs uses gzip compression.

Uses

Squashfs is used by the Live CD versions of DD-WRT, Debian, Finnix, Gentoo Linux and Ubuntu. It is also combined with an union mount filesystem, such as UnionFS or aufs, to provide a read-write environment for live Linux distributions. This takes advantage of both the SquashFS's high speed compression abilities with the ability to alter the distribution while running it from a live CD. Distributions such as Slax and Debian Live use this combo.

As the on-disk format of SquashFS have not stabilized yet[1], it is not ready for long-term archival use (i.e. in cases where a tar file may be used).