The compressed ROM/RAM file system (or cramfs) is a
free (
GPL
The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general us ...
'ed) read-only
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
file system
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one lar ...
designed for simplicity and space-efficiency. It is mainly used in
embedded
Embedded or embedding (alternatively imbedded or imbedding) may refer to:
Science
* Embedding, in mathematics, one instance of some mathematical object contained within another instance
** Graph embedding
* Embedded generation, a distributed ge ...
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 distribution
A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading on ...
s use cramfs for
initrd
In Linux systems, initrd (''initial ramdisk'') is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are comm ...
images (
Debian
Debian (), also known as Debian GNU/Linux, is a Linux distribution composed of free and open-source software, developed by the community-supported Debian Project, which was established by Ian Murdock on August 16, 1993. The first version of De ...
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
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported. Squashfs is als ...
, but the file system got rehabilitated in 2017 for use in low-memory devices where using squashfs may not be viable.
cramfs: rehabilitate it
/ref>
Design
Files on cramfs file systems are zlib
zlib ( or "zeta-lib", ) 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 ...
-compressed one page
Page most commonly refers to:
* Page (paper), one side of a leaf of paper, as in a book
Page, PAGE, pages, or paging may also refer to:
Roles
* Page (assistance occupation), a professional occupation
* Page (servant), traditionally a young ma ...
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
The following lists identify, characterize, and link to more thorough information on Computer file systems.
Many older operating systems support only their one "native" file system, which does not bear any name apart from the name of the operating ...
* Comparison of file systems
The following tables compare general and technical information for a number of file systems.
General information
Limits
Metadata
Features File capabilities
Block capabilities
Note that in addition to the below table, blo ...
* SquashFS
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported. Squashfs is als ...
, a read-only compressed file system
* Util-linux
is a standard package distributed by the Linux Kernel Organization for use as part of the Linux operating system. A fork, (with meaning "next generation"), was created when development stalled, but has been renamed back to , and is the off ...
contains the cramfs utilities
References
External links
*
*
{{Filesystem
Free special-purpose file systems
Compression file systems
Read-only file systems supported by the Linux kernel