cloop

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cloop ( c ompressed l oop d evice ) is a module for the Linux kernel . It is a kind of extension of the loop device that provides support for transparently decompressed block devices. However, it is not a compressed file system , it only refers to loop devices, for example files that contain a file system.

cloop was originally written by Rusty Russell for the Linuxcare Bootable Business Card , but is now managed by Klaus Knopper , who uses it for the live Linux distribution Knoppix . Because of the average compression rate of around 2.5 to 1 (for normal software ), the process is very suitable for LiveCDs. For example, the Knoppix Cloop image is 700 MB and includes approximately 1.8 GB of compressed software.

Appearance of a cloop image

A cloop image contains:

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

The data blocks are compressed separately. This makes it possible to read out individual blocks without previously decompressing the entire image. This saves a considerable amount of RAM , but at the expense of the access speed and the overall compression rate. LiveCD images usually have a block size of 256 kB, which is a good compromise between access time and storage efficiency.

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

Web links

Individual evidence

  1. ↑ Man page of create_compressed_fs