Cloop

Poll without page-refresh

Article on other languages:

Cloop
Cloop

	del.icio.us
	Digg
	Furl
	Reddit
	Rojo
	Add to OnlyWire

The compressed loopback device or cloop is a module for the Linux kernel. It adds support for transparently decompressed, read-only block devices. It is not a compressed file system in itself.

It was originally written for the Levanta Bootable Business Card by Rusty Russell, but is now maintained by Klaus Knopper, the author of Knoppix. cloop is mostly used as a convenient way to compress conventional file systems onto Live CDs.

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

1 Design
2 Limitations
3 See also
4 External links

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 256k as a compromise between decompression speed and space-efficiency.

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

Limitations

The design of the cloop driver requires that compressed blocks be read whole from disk. This makes cloop access very slow when there are many small scattered reads, which can happen if the system is low on memory or when a large program with many shared libraries is starting up.

External links

cloop sources and patches for cloop against the mainline Linux kernels. 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.
cloop at Knoppix Linux Wiki (installation instructions are here)
Slides from a LinuxTag presentation by Klaus Knopper on the implementation of cloop (in German).