This manual is for Plzip (version 1.7, 7 February 2018).

Copyright © 2009-2018 Antonio Diaz Diaz.

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

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 (0.4 to 2 percent larger compressed files). Note that the number of usable threads is limited by file size; 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 (see Minimum file sizes).

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 data sharing and long-term 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. See Data safety (lziprecover).

  * The lzip format is as simple as possible (but not simpler). The lzip manual provides the source 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 the lzip reference implementation is copylefted, which guarantees that it will remain free forever.

A nice feature of the lzip format is that a corrupt byte is easier to repair the nearer it is from the beginning of the file. Therefore, with the help of lziprecover, losing an entire archive just because of a corrupt byte near the beginning is a thing of the past.

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 other programs like 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 (see Memory requirements).

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 decompressed files. Integrity testing of concatenated compressed files is also supported.

Meaning of plzip's output
*************************

The output of plzip looks like this:

     plzip -v foo
       foo:  6.676:1, 14.98% ratio, 85.02% saved, 450560 in, 67493 out.

     plzip -tvv foo.lz
       foo.lz:  6.676:1, 14.98% ratio, 85.02% saved.  ok

The meaning of each field is as follows:

N:1
     The compression ratio (uncompressed_size / compressed_size), shown as N to 1.

ratio
     The inverse compression ratio (compressed_size / uncompressed_size), shown as a percentage. A decimal ratio is easily obtained by moving the decimal point two places to the left; 14.98% = 0.1498.

saved
     The space saved by compression (1 - ratio), shown as a percentage.

in
     The size of the uncompressed data. When decompressing or testing, it is shown as decompressed. Note that plzip always prints the uncompressed size before the compressed size when compressing, decompressing, testing or listing.

out
     The size of the compressed data. When decompressing or testing, it is shown as compressed.

When decompressing or testing at verbosity level 4 (-vvvv), the dictionary size used to compress the file is also shown.

LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never have been compressed. Decompressed is used to refer to data which have undergone the process of decompression.

Invoking plzip
**************

The format for running plzip is:

     plzip [OPTIONS] [FILES]

- used as a FILE argument means standard input. It can be mixed with other FILES and is read just once, the first time it appears in the command line.

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.

-a
--trailing-error
     Exit with error status 2 if any remaining input is detected after decompressing the last member. Such remaining input is usually trailing garbage that can be safely ignored. See concat-example.

-B BYTES
--data-size=BYTES
     When compressing, set the size of the input data blocks 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, except for option -0 where it defaults to 1 MiB. Plzip will reduce the dictionary size if it is larger than the chosen data size.

-c
--stdout
     Compress or decompress to standard output; keep input files unchanged. If compressing several files, each file is compressed independently. This option is needed when reading from a named pipe (fifo) or from a device.

-d
--decompress
     Decompress the specified files. If a file does not exist or can't be opened, plzip continues decompressing the rest of the files. If a file fails to decompress, or is a terminal, plzip exits immediately without decompressing the rest of the files.

-f
--force
     Force overwrite of output files.

-F
--recompress
     When compressing, force re-compression of files whose name already has the .lz or .tlz suffix.

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

-l
--list
     Print the uncompressed size, compressed size and percentage saved of the specified files. Trailing data are ignored. The values produced are correct even for multimember files. If more than one file is given, a final line containing the cumulative sizes is printed. With -v, the dictionary size, the number of members in the file, and the amount of trailing data (if any) are also printed. With -vv, the positions and sizes of each member in multimember files are also printed. -lq can be used to verify quickly (without decompressing) the structural integrity of the specified files. (Use --test to verify the data integrity). -alq additionally verifies that none of the specified files contain trailing data.

-m BYTES
--match-length=BYTES
     When compressing, 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, overriding the system's default. 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. When compressing on a 32 bit system, plzip tries to limit the memory use to under 2.22 GiB (4 worker threads at level -9) by reducing the number of threads below the system's default. 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 (see Minimum file sizes), 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, or a file named FILE.lz when compressing. A second .lz extension is not added if FILE already ends in .lz or .tlz.

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

-s BYTES
--dictionary-size=BYTES
     When compressing, set the dictionary size limit in bytes. Plzip will use the smallest possible dictionary size for each file without exceeding this limit. Valid values range from 4 KiB to 512 MiB. Values 12 to 29 are interpreted as powers of two, meaning 2^12 to 2^29 bytes. 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 / 8) 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 files, 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 files. If a file does not exist, can't be opened, or is a terminal, plzip continues checking the rest of the files. If a file fails the test, plzip may be unable to check the rest of the files.

-v
--verbose
     Verbose mode.

     When compressing, show the compression ratio and size for each file processed.

     When decompressing or testing, further -v's (up to 4) increase the verbosity level, showing status, compression ratio, dictionary size, decompressed size, and compressed size.

     Two or more -v options show the progress of (de)compression, except for single-member files.

-0 .. -9
     Set the compression parameters (dictionary size and match length limit) as shown in the table below. The default compression level is -6. Note that -9 can be much slower than -0. These options have no effect when decompressing, testing or listing.

     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 --dictionary-size and --match-length options directly to achieve optimal performance.

     Level   Dictionary size   Match length limit
     -0   64 KiB    16 bytes
     -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.

--loose-trailing
     When decompressing, testing or listing, allow trailing data whose first bytes are so similar to the magic bytes of a lzip header that they can be confused with a corrupt header. Use this option if a file triggers a "corrupt header" error and the cause is not indeed a corrupt header.

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.

Program design
**************

When compressing, plzip divides the input file into chunks and compresses as many chunks simultaneously as worker threads are chosen, creating a multimember compressed file.

When decompressing, plzip decompresses as many members simultaneously as worker threads are chosen. Files that were compressed with lzip will not be decompressed faster than using lzip (unless the -b option was used) because lzip usually produces single-member files, which can't be decompressed in parallel.

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.

                             ,------------,
                         ,-->| worker   0 |--,
                         |   `------------'  |
,-------,   ,----------, |   ,------------,  |   ,-------,   ,--------,
| input |-->| splitter |-+-->| worker   1 |--+-->| muxer |-->| output |
| file  |   `----------' |   `------------'  |   `-------'   |  file  |
`-------'                |        ...        |               `--------'
                         |   ,------------,  |
                         `-->| worker N-1 |--'
                             `------------'

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, the use of RAM is greatly reduced and the decompression speed of large files with many members is only limited by the number of processors available and by I/O speed.

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 (the "magic" bytes)
     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)
     The dictionary size is calculated by taking a power of 2 (the base size) and substracting from it a fraction between 0/16 and 7/16 of the base size.

     Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).

     Bits 7-5 contain the numerator of the fraction (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 Stream format (lzip), for a complete 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 multimember files.

Memory required to compress and decompress
******************************************

The amount of memory required per thread for decompression or testing is approximately the following:

  * For decompression of a regular (seekable) file to another regular file, or for testing of a regular file; the dictionary size.

  * For testing of a non-seekable file or of standard input; the dictionary size plus up to 5 MiB.

  * For decompression of a regular file to a non-seekable file or to standard output; the dictionary size plus up to 32 MiB.

  * For decompression of a non-seekable file or of standard input; the dictionary size plus up to 35 MiB.

The amount of memory required per thread for compression is approximately the following:

  * For compression at level -0; 1.5 MiB plus 3.375 times the data size (see --data-size). Default is 4.875 MiB.

  * For compression at other levels; 11 times the dictionary size plus 3.375 times the data size. Default is 142 MiB.

The following table shows the memory required per thread for compression at a given level, using the default data size for each level:

Level   Memory required
-0     4.875 MiB
-1    17.75 MiB
-2    26.625 MiB
-3    35.5 MiB
-4    53.25 MiB
-5    71 MiB
-6   142 MiB
-7   284 MiB
-8   426 MiB
-9   568 MiB

Minimum file sizes required for full compression speed
******************************************************

When compressing, plzip divides the input file into chunks and compresses as many chunks simultaneously as worker threads are chosen, creating a multimember compressed file.

For this to work as expected (and roughly multiply the compression speed by the number of available processors), the uncompressed file must be at least as large as the number of worker threads times the chunk size (see --data-size). Else some processors will not get any data to compress, and compression will be proportionally slower. The maximum speed increase achievable on a given file is limited by the ratio (file_size / data_size). For example, a tarball the size of gcc or linux will scale up to 8 processors at level -9.

The following table shows the minimum uncompressed file size needed for full use of N processors at a given compression level, using the default data size for each level:

Processors   2   4   8   16   64   256
Level
-0     2 MiB     4 MiB     8 MiB    16 MiB    64 MiB   256 MiB
-1     4 MiB     8 MiB    16 MiB    32 MiB   128 MiB   512 MiB
-2     6 MiB    12 MiB    24 MiB    48 MiB   192 MiB   768 MiB
-3     8 MiB    16 MiB    32 MiB    64 MiB   256 MiB     1 GiB
-4    12 MiB    24 MiB    48 MiB    96 MiB   384 MiB   1.5 GiB
-5    16 MiB    32 MiB    64 MiB   128 MiB   512 MiB     2 GiB
-6    32 MiB    64 MiB   128 MiB   256 MiB     1 GiB     4 GiB
-7    64 MiB   128 MiB   256 MiB   512 MiB     2 GiB     8 GiB
-8    96 MiB   192 MiB   384 MiB   768 MiB     3 GiB    12 GiB
-9   128 MiB   256 MiB   512 MiB     1 GiB     4 GiB    16 GiB

Extra data appended to the file
*******************************

Sometimes extra data are found appended to a lzip file after the last member. Such trailing data may be:

  * Padding added to make the file size a multiple of some block size, for example when writing to a tape. It is safe to append any amount of padding zero bytes to a lzip file.

  * Useful data added by the user; a cryptographically secure hash, a description of file contents, etc. It is safe to append any amount of text to a lzip file as long as none of the first four bytes of the text match the corresponding byte in the string "LZIP", and the text does not contain any zero bytes (null characters). Nonzero bytes and zero bytes can't be safely mixed in trailing data.

  * Garbage added by some not totally successful copy operation.

  * Malicious data added to the file in order to make its total size and hash value (for a chosen hash) coincide with those of another file.

  * In rare cases, trailing data could be the corrupt header of another member. In multimember or concatenated files the probability of corruption happening in the magic bytes is 5 times smaller than the probability of getting a false positive caused by the corruption of the integrity information itself. Therefore it can be considered to be below the noise level. Additionally, the test used by plzip to discriminate trailing data from a corrupt header has a Hamming distance (HD) of 3, and the 3 bit flips must happen in different magic bytes for the test to fail. In any case, the option --trailing-error guarantees that any corrupt header will be detected.

Trailing data are in no way part of the lzip file format, but tools reading lzip files are expected to behave as correctly and usefully as possible in the presence of trailing data.

Trailing data can be safely ignored in most cases. In some cases, like that of user-added data, they are expected to be ignored. In those cases where a file containing trailing data must be rejected, the option --trailing-error can be used. See --trailing-error.

A small tutorial with examples
******************************

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 are important, give the --keep option to plzip and don't remove the original file until you verify the compressed file with a command like plzip -cd file.lz | cmp file -.

Example 1: Replace a regular file with its compressed version file.lz and show the compression ratio.

     plzip -v file

Example 2: Like example 1 but the created file.lz has a block size of 1 MiB. The compression ratio is not shown.

     plzip -B 1MiB file

Example 3: Restore a regular file from its compressed version file.lz. If the operation is successful, file.lz is removed.

     plzip -d file.lz

Example 4: Verify the integrity of the compressed file file.lz and show status.

     plzip -tv file.lz

Example 5: Compress a whole device in /dev/sdc and send the output to file.lz.

     plzip -c /dev/sdc > file.lz

Example 6: The right way of concatenating the decompressed output of two or more compressed files. See Trailing data.

     Don't do this
       cat file1.lz file2.lz file3.lz | plzip -d
     Do this instead
       plzip -cd file1.lz file2.lz file3.lz

Example 7: Decompress file.lz partially until 10 KiB of decompressed data are produced.

     plzip -cd file.lz | dd bs=1024 count=10

Example 8: Decompress file.lz partially from decompressed byte 10000 to decompressed byte 15000 (5000 bytes are produced).

     plzip -cd file.lz | dd bs=1000 skip=10 count=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.