Cramfs - AbsoluteAstronomy.com

The compressed ROM file system (or cramfs) is a free

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

(GPL

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

'ed) read-only Linux

Linux kernel

The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....

file system

File system

A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

designed for simplicity and space-efficiency. It is mainly used in embedded system

Embedded system

An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

s 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 the need to decompress the image first. For this reason, some Linux distribution

Linux distribution

A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...

s also use cramfs for initrd

Initrd

In computing, initrd is a scheme for loading a temporary file system into memory in the boot process of the Linux kernel. initrd and initramfs refer to slightly different methods of achieving this...

images (Debian

Debian

Debian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...

3.1 in particular) and installation images (SUSE Linux in particular), where there are constraints on memory and image size.

Design

Files on cramfs file systems are zlib

Zlib

zlib is a software library used for data compression. zlib was written by Jean-Loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. Zlib is also a crucial component of many software platforms including Linux, Mac OS X,...

-compressed one page at a time to allow random read access. The meta-data is not compressed, but is expressed in a terse representation that is designed to be 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.)

Design

See also