Editing Cloop

{{Short description|Linux kernel module}}
{{lowercase}}
{{Infobox software
| name                   = cloop
| title                  = cloop
| logo                   = <!-- [[File: ]] -->
| screenshot             = <!-- [[File: ]] -->
| caption                = 
| collapsible            = 
| author                 = [[Rusty Russell]]
| developer              = [[Klaus Knopper]]
| released               = <!-- {{Start date|YYYY|MM|DD|df=yes/no}} -->
| discontinued           = 
| latest release version = 2.637-1
| latest release date    = {{Start date and age|2011|01|12|df=yes/no}}
| latest preview version = 
| latest preview date    = <!-- {{Start date and age|YYYY|MM|DD|df=yes/no}} -->
| programming language   = 
| operating system       = 
| platform               = 
| size                   = 338 KB
| language               = 
| status                 = 
| genre                  = [[Loadable kernel module]]
| license                = [[GNU GPL]] v2<ref>License stated in Readme in the [http://debian-knoppix.alioth.debian.org/packages/cloop/cloop_2.639-2.tar.gz tarball] {{Webarchive|url=https://web.archive.org/web/20120307122032/http://debian-knoppix.alioth.debian.org/packages/cloop/cloop_2.639-2.tar.gz |date=2012-03-07}}.</ref>
| website                = {{URL|http://knoppix.net/wiki/Cloop}}
}}
The '''compressed loop device''' ('''cloop''') is a [[Loadable kernel module|module]] for the [[Linux kernel]].  It adds support for transparently decompressed, read-only [[Device file#Block devices|block devices]]. It is not a compressed [[file system]]: cloop is mostly used as a convenient way to compress conventional file systems onto [[Live CD]]s.

Cloop was originally written for the [[Levanta]] Bootable Business Card by [[Rusty Russell]], but is now maintained by [[Klaus Knopper]], the author of [[Knoppix]].  

A compression ratio of about 2.5:1 is common for software. The Knoppix cloop image, for example, is 700 MB compressed and around 1.8 GB uncompressed.

== Design ==
cloop images contain:

* A [[shell script]] (with mount commands for the image)
* A header with the number of blocks and the uncompressed block size
* A seek index with compressed and uncompressed block sizes in pairs
* [[zlib]]-compressed data blocks, packed end-to-end

The data blocks are compressed separately; this makes it possible to seek to individual blocks without having to decompress the entire image from the start, but at the cost of slightly reducing the compression ratio.  Live CD images typically use a block size of 256&nbsp;KB as a compromise between decompression speed and space-efficiency.

Apple uses a similar file format in the compressed variant of its [[.dmg|DMG]] disk images.

== Limitations ==
The design of the cloop driver requires that compressed blocks be read whole from disk.  This makes cloop access inherently slower when there are many scattered reads, which can happen if the system is low on memory or when a large program with many shared libraries is starting. A big issue is the seek time for CD-ROM drives (~80 ms), which exceeds that of hard disks (~10 ms) by a large factor. On the other hand, because files are packed together, reading a compressed block may thus bring in more than one file into the cache. The effects of tail packing are known to improve seek times (cf. [[reiserfs]], [[btrfs]]), especially for small files. Some performance tests related to cloop have been conducted.<ref>{{Citation|title=Cloop performance testing on a ThinkPad T61 |url=http://www-etud.iro.umontreal.ca/~jaegerch/div/cloop_performance_tests }}{{dead link|date=June 2016|bot=medic}}{{cbignore|bot=medic}}<sup> 31-Dec-2011</sup></ref>

== See also ==
* [[Loop device]]
* [[Cramfs]]
* [[SquashFS]]
* [[e2compr]]

== References ==
{{reflist}}

==External links==
{{Wiktionary}}

*cloop [https://web.archive.org/web/20120501230646/http://debian-knoppix.alioth.debian.org/packages/cloop/ sources] against the mainline Linux kernels and a [http://hg.slitaz.org/wok/raw-file/tip/linux-cloop/stuff/cloop.u patch] to support any known cloop format. Note: versions 0.xx are for kernel 2.2; 1.xx are for kernel 2.4; 2.xx are for kernel 2.4 and 2.6.
*[http://www.knoppix.net/wiki/Cloop cloop at Knoppix Linux Wiki] (installation instructions are here)
*Slides from a LinuxTag [http://www.knopper.net/knoppix-info/cloop-chlt4-screen.pdf presentation] by Klaus Knopper on the implementation of cloop (in German).
*A [http://fusecloop.sourceforge.net/ fuse] driver for cloop with a [http://hg.slitaz.org/wok/raw-file/tip/fusecloop/stuff/fusecloop.u patch] ([http://hg.slitaz.org/wok/file/tip/fusecloop/description.txt#l1 description]) to support any known cloop format and the [http://cook.slitaz.org/cooker.cgi?download=../wok/fusecloop/taz/fusecloop-0.20.1/fs/usr/bin/fusecloop binary].

[[Category:Compression file systems]]
[[Category:Knoppix]]
[[Category:Third-party Linux kernel modules]]