extract_compressed_fs - Online in the Cloud

PROGRAM:

NAME

create_compressed_fs, extract_compressed_fs - convert and extract a filesystem to/from a
cloop volume

SYNOPSIS

create_compressed_fs [ OPTIONS ] image outfile

extract_compressed_fs filename

DESCRIPTION

create_compressed_fs compresses a filesystem image to a compressed image suitable for
mounting with the cloop driver.

extract_compressed_fs uncompresses a filesystem image created by create_compressed_fs.

OPTIONS

Run the create_compressed_fs command with "-h" to get a list of available options and
description of data storages modes and advanced usage (eg. with a network cluster).
Blocksize must be a multiple of 512 bytes.

There is an alternative set of options which is supported for compatibility with the
original version of create_compressed_fs. When it's called with a pure number as output,
it assumes the old syntax and maps the default options to memory mode and the block size
to the number. -b for best mode is recognized as well.

EXAMPLES

create_compressed_fs image.ext2 image.ext2.cloop

extract_compressed_fs image.ext2.cloop | cmp image.ext2 -

mkcmd="genisoimage -joliet-long -D -r data"

$mkcmd | create_compressed_fs -s $($mkcmd --print-size)i - file.cloop

create_compressed_fs image.iso -S image.cloop.a0 - | split -b 1024m - image.cloop.

cat image.cloop.* | cdrskin -

NOTES

For optimal performance, create_compressed_fs should be used with real input and output
files.

Operation with STDIN or STDOUT is complicated. create_compressed_fs cannot operate as a
simple pipe filter because it needs to know some things in advance: a) the data length, b)
the block lengths calculated during the compression. This meta data needs to be stored in
the header AFTER the compression is trough, therefore it is not possible to push all
generated data to STDOUT on-the-fly.

And so compromises need to be made. The simpliest method (used by the ancient
create_compressed_fs) is a big temporary memory buffer (option -m) to keep compressed data
in RAM. However, the overall system performance suffers from such usage and this may drive
other applications into OOM conditions.

The next alternative to this is storing the temprorary data in an extra file aside (the -f
option). This improves the overall system performance but create_compressed_fs operation
is slower because it needs to copy data back when it's finished. Plus, double disk space
for the output is required. If disk space is limited, the -r option could be used (reusing
output file for temporary data), however the speed of the copy-back step may decrease
significantly.

There are strategies to avoid the temporary data storage completely. First, only files
should be used. If that is not possible, then the output should be a (seekable) file and
the input data length should be known (either specified with the -s option or detected
from the input file size).

Second, it is possible to store the header of the cloop volume in a separate file. After
the work, header can be merged together with the compressed data with the cat command. The
output data can be written to stdout. Since the header is stored too late, this method is
not very usefull for immediate data transfer. However, it allows to do some tricks like
piping to the split utility (to store DVD images in smaller files on VFAT) or piping data
trough ssh if the local system has no disk space (and copying the header separately).

MOUNTING

You can pass an option to the cloop module on loading, or use losetup or losetup
functionality of the mount command. Examples:

insmod cloop.o file=/path/to/compressed/image

losetup /dev/cloop1 /path/to/compressed/image

mount -o ro -t whatever /dev/cloop /mnt/compressed

mount cl /mnt/misc -oloop=/dev/cloop0

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