cloop

Cloop (compressed loop device ) is a module for the Linux kernel. It is a kind of extension of the loop devices, creates the support for transparent decompressed block devices. However, it is not a compressed file system, but applies only to loop devices, for example, files that have a file system.

Cloop was originally written by Rusty Russell for the Linux Bootable Business Card Care, but is now managed by Klaus Knopper, who used it for the live Linux distribution Knoppix. Because the average compression ratio of about 2.5 to 1 (for normal software), the method is very good for LiveCDs. The Knoppix Cloop image, for example, 700 MB and comprises about 1.8 GB of compressed software.

Appearance of a cloop image

A cloop image contains:

• A shell script (mount commands for the image )
• A header with the size of an uncompressed block and the number of blocks of the medium cloop
• An array of offsets of the compressed data blocks ( the size of a compressed data block, the difference between the offset of the subsequent block and the offset of the current block. )
• The compressed data blocks

The data blocks are compressed separately. Thus, it is possible to read out individual blocks, without the entire image to be decompressed in advance. This saves a lot of memory, but at the expense of access speed and the total compression rate. LiveCD images usually have a block size of 256 KB, which is a good compromise between access time and storage efficiency.

As compression methods zlib ( LZ77 Deflate ) and 7zip ( LZMA ) are supported.