View source for Cramfs

{{Short description|File system}}
{{refimprove|date=August 2016}}
{{DISPLAYTITLE:cramfs}}
{{ infobox file system
| name                    = Cramfs
| developer               = 
| full_name               = 
| variants                = 
| introduction_date       = 2001
| introduction_os         = Linux
| partition_id            = 
| directory_struct        = 
| file_struct             = 
| file_types              = 
| bad_blocks_struct       = 
| bootable                = 
| min_volume_size         = 
| max_volume_size         = <272 MiB
| max_file_size           = 16 MiB
| file_size_granularity   = 
| max_files_no            = 
| max_filename_size       =
| max_dirname_size        = 
| max_directory_depth     = 
| filename_character_set  = 
| dates_recorded          = 
| date_range              = 
| date_resolution         = 
| forks_streams           = 
| attributes              = 
| file_system_permissions = 
| compression             = 
| encryption              = 
| data_deduplication      = 
| copy_on_write           = 
| OS                      = [[Linux]], [[BSD]]
| website                 = {{url|https://sourceforge.net/projects/cramfs/}}
}}
The '''compressed ROM/RAM file system''' (or '''cramfs''') is a [[Free software|free]] ([[GNU General Public License|GPL]]'ed) read-only [[Linux kernel|Linux]] [[file system]] designed for simplicity and space-efficiency. It is mainly used in [[embedded system|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 distribution]]s 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]],<ref>[https://github.com/torvalds/linux/commit/54886a7153353ea2bf21ebfc1b8e030e71d151d7 github/torvalds/linux: cramfs: mark as obsolete]</ref> but the file system got rehabilitated in 2017 for use in low-memory devices where using squashfs may not be viable.<ref>[https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/cramfs?id=8d59598c35dc1071e6c36f86c9a95f26dd08b4e5 cramfs: rehabilitate it]</ref>

==Design==
Files on cramfs file systems are [[zlib]]-compressed one [[Page (computer memory)|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 (<code>mkcramfs</code>) 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==
{{Portal|Free and open-source software}}
* [[List of file systems]]
* [[Comparison of file systems]]
* [[SquashFS]], a read-only compressed file system
* [[Util-linux]] contains the cramfs utilities

==References==
{{Reflist}}

==External links==
* {{official website|https://sourceforge.net/projects/cramfs/}}
* {{cite web|title=Cramfs - cram a filesystem onto a small ROM|url=https://www.kernel.org/doc/Documentation/filesystems/cramfs.txt|website=kernel.org|access-date=July 21, 2017}}

{{Filesystem}}

[[Category:Free special-purpose file systems]]
[[Category:Compression file systems]]
[[Category:Read-only file systems supported by the Linux kernel]]