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The compressed ROM/RAM file system (or cramfs) is a free ( GPL'ed) read-only Linux file system designed for simplicity and space-efficiency. It is mainly used in embedded and small-footprint systems.

Unlike a compressed image of a conventional file system, a cramfs image can be used as it is, i.e. without first decompressing it. For this reason, some Linux distributions use cramfs for initrd images ( Debian 3.1 in particular) and installation images ( SUSE Linux in particular), where there are constraints on memory and image size.

In 2013, Linux maintainers indicated that cramfs was made obsolete by squashfs, ^[1] but the file system got rehabilitated in 2017 for use in low-memory devices where using squashfs may not be viable. ^[2]

Design

Files on cramfs file systems are zlib-compressed one page at a time to allow random read access. The metadata is not compressed, but is expressed in a terse representation that is more space-efficient than conventional file systems.

The file system is intentionally read-only to simplify its design; random write access for compressed files is difficult to implement. cramfs ships with a utility (mkcramfs) to pack files into new cramfs images.

File sizes are limited to less than 16MB.

Maximum file system size is a little under 272MB. (The last file on the file system must begin before the 256MB block, but can extend past it.)

See also

• List of file systems
• Comparison of file systems
• SquashFS, a read-only compressed file system
• Util-linux contains the cramfs utilities

References

External links

• Official website
• "Cramfs - cram a filesystem onto a small ROM". kernel.org. Retrieved July 21, 2017.