path: root/doc/plzip.info

This is plzip.info, produced by makeinfo version 4.13+ from plzip.texi.

INFO-DIR-SECTION Data Compression
START-INFO-DIR-ENTRY
* Plzip: (plzip).               Parallel compressor compatible with lzip
END-INFO-DIR-ENTRY


File: plzip.info,  Node: Top,  Next: Introduction,  Up: (dir)

Plzip Manual
************

This manual is for Plzip (version 1.2-rc2, 1 July 2014).

* Menu:

* Introduction::           Purpose and features of plzip
* Program design::         Internal structure of plzip
* Invoking plzip::         Command line interface
* File format::            Detailed format of the compressed file
* Problems::               Reporting bugs
* Concept index::          Index of concepts


   Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Antonio Diaz Diaz.

   This manual is free documentation: you have unlimited permission to
copy, distribute and modify it.


File: plzip.info,  Node: Introduction,  Next: Program design,  Prev: Top,  Up: Top

1 Introduction
**************

Plzip is a massively parallel (multi-threaded), lossless data compressor
based on the lzlib compression library, with a user interface similar to
the one of lzip, bzip2 or gzip.

   Plzip can compress/decompress large files on multiprocessor machines
much faster than lzip, at the cost of a slightly reduced compression
ratio. Note that the number of usable threads is limited by file size,
so on files larger than a few GB plzip can use hundreds of processors,
but on files of only a few MB plzip is no faster than lzip.

   Plzip uses the lzip file format; the files produced by plzip are
fully compatible with lzip-1.4 or newer, and can be rescued with
lziprecover.

   The lzip file format is designed for long-term data archiving, taking
into account both data integrity and decoder availability:

   * The lzip format provides very safe integrity checking and some data
     recovery means. The lziprecover program can repair bit-flip errors
     (one of the most common forms of data corruption) in lzip files,
     and provides data recovery capabilities, including error-checked
     merging of damaged copies of a file.

   * The lzip format is as simple as possible (but not simpler). The
     lzip manual provides the code of a simple decompressor along with
     a detailed explanation of how it works, so that with the only help
     of the lzip manual it would be possible for a digital
     archaeologist to extract the data from a lzip file long after
     quantum computers eventually render LZMA obsolete.

   * Additionally lzip is copylefted, which guarantees that it will
     remain free forever.

   The member trailer stores the 32-bit CRC of the original data, the
size of the original data and the size of the member. These values,
together with the value remaining in the range decoder and the
end-of-stream marker, provide a 4 factor integrity checking which
guarantees that the decompressed version of the data is identical to
the original. This guards against corruption of the compressed data,
and against undetected bugs in plzip (hopefully very unlikely). The
chances of data corruption going undetected are microscopic. Be aware,
though, that the check occurs upon decompression, so it can only tell
you that something is wrong. It can't help you recover the original
uncompressed data.

   Plzip uses the same well-defined exit status values used by lzip and
bzip2, which makes it safer than compressors returning ambiguous warning
values (like gzip) when it is used as a back end for tar or zutils.

   Plzip will automatically use the smallest possible dictionary size
for each file without exceeding the given limit. Keep in mind that the
decompression memory requirement is affected at compression time by the
choice of dictionary size limit.

   When compressing, plzip replaces every file given in the command line
with a compressed version of itself, with the name "original_name.lz".
When decompressing, plzip attempts to guess the name for the
decompressed file from that of the compressed file as follows:

filename.lz    becomes   filename
filename.tlz   becomes   filename.tar
anyothername   becomes   anyothername.out

   (De)compressing a file is much like copying or moving it; therefore
plzip preserves the access and modification dates, permissions, and,
when possible, ownership of the file just as "cp -p" does. (If the user
ID or the group ID can't be duplicated, the file permission bits
S_ISUID and S_ISGID are cleared).

   Plzip is able to read from some types of non regular files if the
'--stdout' option is specified.

   If no file names are specified, plzip compresses (or decompresses)
from standard input to standard output. In this case, plzip will
decline to write compressed output to a terminal, as this would be
entirely incomprehensible and therefore pointless.

   Plzip will correctly decompress a file which is the concatenation of
two or more compressed files. The result is the concatenation of the
corresponding uncompressed files. Integrity testing of concatenated
compressed files is also supported.

   WARNING! Even if plzip is bug-free, other causes may result in a
corrupt compressed file (bugs in the system libraries, memory errors,
etc).  Therefore, if the data you are going to compress is important,
give the '--keep' option to plzip and do not remove the original file
until you verify the compressed file with a command like
'plzip -cd file.lz | cmp file -'.


File: plzip.info,  Node: Program design,  Next: Invoking plzip,  Prev: Introduction,  Up: Top

2 Program design
****************

For each input file, a splitter thread and several worker threads are
created, acting the main thread as muxer (multiplexer) thread. A "packet
courier" takes care of data transfers among threads and limits the
maximum number of data blocks (packets) being processed simultaneously.

   The splitter reads data blocks from the input file, and distributes
them to the workers. The workers (de)compress the blocks received from
the splitter. The muxer collects processed packets from the workers, and
writes them to the output file.

   When decompressing from a regular file, the splitter is removed and
the workers read directly from the input file. If the output file is
also a regular file, the muxer is also removed, and the workers write
directly to the output file. With these optimizations, decompression
speed of large files with many members is only limited by the number of
processors available and by I/O speed.


File: plzip.info,  Node: Invoking plzip,  Next: File format,  Prev: Program design,  Up: Top

3 Invoking plzip
****************

The format for running plzip is:

     plzip [OPTIONS] [FILES]

   Plzip supports the following options:

'-h'
'--help'
     Print an informative help message describing the options and exit.

'-V'
'--version'
     Print the version number of plzip on the standard output and exit.

'-B BYTES'
'--data-size=BYTES'
     Set the input data block size in bytes. The input file will be
     divided in chunks of this size before compression is performed.
     Valid values range from 8 KiB to 1 GiB. Default value is two times
     the dictionary size. Plzip will reduce the dictionary size if it
     is larger than the chosen data size.

'-c'
'--stdout'
     Compress or decompress to standard output. Needed when reading
     from a named pipe (fifo) or from a device.

'-d'
'--decompress'
     Decompress.

'-f'
'--force'
     Force overwrite of output files.

'-F'
'--recompress'
     Force recompression of files whose name already has the '.lz' or
     '.tlz' suffix.

'-k'
'--keep'
     Keep (don't delete) input files during compression or
     decompression.

'-m BYTES'
'--match-length=BYTES'
     Set the match length limit in bytes. After a match this long is
     found, the search is finished. Valid values range from 5 to 273.
     Larger values usually give better compression ratios but longer
     compression times.

'-n N'
'--threads=N'
     Set the number of worker threads. Valid values range from 1 to "as
     many as your system can support". If this option is not used,
     plzip tries to detect the number of processors in the system and
     use it as default value. 'plzip --help' shows the system's default
     value.

     Note that the number of usable threads is limited to
     ceil( file_size / data_size ) during compression (*note
     --data-size::), and to the number of members in the input during
     decompression.

'-o FILE'
'--output=FILE'
     When reading from standard input and '--stdout' has not been
     specified, use 'FILE' as the virtual name of the uncompressed
     file. This produces a file named 'FILE' when decompressing, and a
     file named 'FILE.lz' when compressing.

'-q'
'--quiet'
     Quiet operation. Suppress all messages.

'-s BYTES'
'--dictionary-size=BYTES'
     Set the dictionary size limit in bytes. Valid values range from 4
     KiB to 512 MiB. Plzip will use the smallest possible dictionary
     size for each file without exceeding this limit. Note that
     dictionary sizes are quantized. If the specified size does not
     match one of the valid sizes, it will be rounded upwards by adding
     up to (BYTES / 16) to it.

     For maximum compression you should use a dictionary size limit as
     large as possible, but keep in mind that the decompression memory
     requirement is affected at compression time by the choice of
     dictionary size limit.

'-t'
'--test'
     Check integrity of the specified file(s), but don't decompress
     them.  This really performs a trial decompression and throws away
     the result.  Use it together with '-v' to see information about
     the file.

'-v'
'--verbose'
     Verbose mode.
     When compressing, show the compression ratio for each file
     processed. A second '-v' shows the progress of compression.
     When decompressing or testing, further -v's (up to 4) increase the
     verbosity level, showing status, compression ratio, decompressed
     size, and compressed size.

'-1 .. -9'
     Set the compression parameters (dictionary size and match length
     limit) as shown in the table below. Note that '-9' can be much
     slower than '-1'. These options have no effect when decompressing.

     The bidimensional parameter space of LZMA can't be mapped to a
     linear scale optimal for all files. If your files are large, very
     repetitive, etc, you may need to use the '--match-length' and
     '--dictionary-size' options directly to achieve optimal
     performance. For example, '-9m64' usually compresses executables
     more (and faster) than '-9'.

     Level   Dictionary size   Match length limit
     -1      1 MiB             5 bytes
     -2      1.5 MiB           6 bytes
     -3      2 MiB             8 bytes
     -4      3 MiB             12 bytes
     -5      4 MiB             20 bytes
     -6      8 MiB             36 bytes
     -7      16 MiB            68 bytes
     -8      24 MiB            132 bytes
     -9      32 MiB            273 bytes

'--fast'
'--best'
     Aliases for GNU gzip compatibility.


   Numbers given as arguments to options may be followed by a multiplier
and an optional 'B' for "byte".

   Table of SI and binary prefixes (unit multipliers):

Prefix   Value                     |   Prefix   Value
k        kilobyte  (10^3 = 1000)   |   Ki       kibibyte (2^10 = 1024)
M        megabyte  (10^6)          |   Mi       mebibyte (2^20)
G        gigabyte  (10^9)          |   Gi       gibibyte (2^30)
T        terabyte  (10^12)         |   Ti       tebibyte (2^40)
P        petabyte  (10^15)         |   Pi       pebibyte (2^50)
E        exabyte   (10^18)         |   Ei       exbibyte (2^60)
Z        zettabyte (10^21)         |   Zi       zebibyte (2^70)
Y        yottabyte (10^24)         |   Yi       yobibyte (2^80)


   Exit status: 0 for a normal exit, 1 for environmental problems (file
not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused plzip to panic.


File: plzip.info,  Node: File format,  Next: Problems,  Prev: Invoking plzip,  Up: Top

4 File format
*************

Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.
-- Antoine de Saint-Exupery


   In the diagram below, a box like this:
+---+
|   | <-- the vertical bars might be missing
+---+

   represents one byte; a box like this:
+==============+
|              |
+==============+

   represents a variable number of bytes.


   A lzip file consists of a series of "members" (compressed data sets).
The members simply appear one after another in the file, with no
additional information before, between, or after them.

   Each member has the following structure:
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | Lzma stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All multibyte values are stored in little endian order.

'ID string'
     A four byte string, identifying the lzip format, with the value
     "LZIP" (0x4C, 0x5A, 0x49, 0x50).

'VN (version number, 1 byte)'
     Just in case something needs to be modified in the future. 1 for
     now.

'DS (coded dictionary size, 1 byte)'
     Lzip divides the distance between any two powers of 2 into 8
     equally spaced intervals, named "wedges". The dictionary size is
     calculated by taking a power of 2 (the base size) and substracting
     from it a number of wedges between 0 and 7. The size of a wedge is
     (base_size / 16).
     Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).
     Bits 7-5 contain the number of wedges (0 to 7) to substract from
     the base size to obtain the dictionary size.
     Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB
     Valid values for dictionary size range from 4 KiB to 512 MiB.

'Lzma stream'
     The lzma stream, finished by an end of stream marker. Uses default
     values for encoder properties. See the lzip manual for a full
     description.

'CRC32 (4 bytes)'
     CRC of the uncompressed original data.

'Data size (8 bytes)'
     Size of the uncompressed original data.

'Member size (8 bytes)'
     Total size of the member, including header and trailer. This field
     acts as a distributed index, allows the verification of stream
     integrity, and facilitates safe recovery of undamaged members from
     multi-member files.



File: plzip.info,  Node: Problems,  Next: Concept index,  Prev: File format,  Up: Top

5 Reporting bugs
****************

There are probably bugs in plzip. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

   If you find a bug in plzip, please send electronic mail to
<lzip-bug@nongnu.org>. Include the version number, which you can find
by running 'plzip --version'.


File: plzip.info,  Node: Concept index,  Prev: Problems,  Up: Top

Concept index
*************

[index]
* Menu:

* bugs:                                  Problems.              (line 6)
* file format:                           File format.           (line 6)
* getting help:                          Problems.              (line 6)
* introduction:                          Introduction.          (line 6)
* invoking:                              Invoking plzip.        (line 6)
* options:                               Invoking plzip.        (line 6)
* program design:                        Program design.        (line 6)
* usage:                                 Invoking plzip.        (line 6)
* version:                               Invoking plzip.        (line 6)



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Node: Introduction873
Node: Program design5442
Node: Invoking plzip6496
Ref: --data-size6941
Node: File format12090
Node: Problems14595
Node: Concept index15124

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