Plzip Manual

This manual is for Plzip (version 1.11, 21 January 2024).

• Introduction: Purpose and features of plzip
• Output: Meaning of plzip's output
• Invoking plzip: Command-line interface
• Program design: Internal structure of plzip
• Memory requirements: Memory required to compress and decompress
• Minimum file sizes: Minimum file sizes required for full speed
• File format: Detailed format of the compressed file
• Trailing data: Extra data appended to the file
• Examples: A small tutorial with examples
• Problems: Reporting bugs
• Concept index: Index of concepts

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

1 Introduction

Plzip is a massively parallel (multi-threaded) implementation of lzip, compatible with lzip 1.4 or newer. Plzip uses the compression library lzlib.

Lzip is a lossless data compressor with a user interface similar to the one of gzip or bzip2. Lzip uses a simplified form of the 'Lempel-Ziv-Markov chain-Algorithm' (LZMA) stream format to maximize interoperability. The maximum dictionary size is 512 MiB so that any lzip file can be decompressed on 32-bit machines. Lzip provides accurate and robust 3-factor integrity checking. Lzip can compress about as fast as gzip (lzip -0) or compress most files more than bzip2 (lzip -9). Decompression speed is intermediate between gzip and bzip2. Lzip is better than gzip and bzip2 from a data recovery perspective. Lzip has been designed, written, and tested with great care to replace gzip and bzip2 as the standard general-purpose compressed format for Unix-like systems.

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.

For creation and manipulation of compressed tar archives tarlz can be more efficient than using tar and plzip because tarlz is able to keep the alignment between tar members and lzip members.

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 program lziprecover 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 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, 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 automatically uses for each file the largest dictionary size that does not exceed neither the file size nor the limit given. 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, if you have appropriate privileges, 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 either the option -c or the option -o is specified.

Plzip refuses to read compressed data from a terminal or write compressed data to a terminal, as this would be entirely incomprehensible and might leave the terminal in an abnormal state.

Plzip correctly decompresses 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.

2 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 -tvvv foo.lz
       foo.lz:  6.676:1, 14.98% ratio, 85.02% saved.  450560 out,  67493 in. 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

Size of the input data. This is the uncompressed size when compressing, or the compressed size when decompressing or testing. Note that plzip always prints the uncompressed size before the compressed size when compressing, decompressing, testing, or listing.

out

Size of the output data. This is the compressed size when compressing, or the decompressed size when decompressing or testing.

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.

3 Invoking plzip

The format for running plzip is:

     plzip [options] [files]

If no file names are specified, plzip compresses (or decompresses) from standard input to standard output. A hyphen '-' 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. Remember to prepend ./ to any file name beginning with a hyphen, or use '--'.

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. This version number should be included in all bug reports.

-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 in bytes of the input data blocks. The input file is 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 reduces the dictionary size if it is larger than the data size specified. See Minimum file sizes.

-c

--stdout

Compress or decompress to standard output; keep input files unchanged. If compressing several files, each file is compressed independently. (The output consists of a sequence of independently compressed members). This option (or -o) is needed when reading from a named pipe (fifo) or from a device. Use 'lziprecover -cd -i' to recover as much of the decompressed data as possible when decompressing a corrupt file. -c overrides -o. -c has no effect when testing or listing.

-d

--decompress

Decompress the files specified. The integrity of the files specified is checked. If a file does not exist, can't be opened, or the destination file already exists and --force has not been specified, plzip continues decompressing the rest of the files and exits with error status 1. If a file fails to decompress, or is a terminal, plzip exits immediately with error status 2 without decompressing the rest of the files. A terminal is considered an uncompressed file, and therefore invalid.

-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 files specified. 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.

If any file is damaged, does not exist, can't be opened, or is not regular, the final exit status is > 0. -lq can be used to check quickly (without decompressing) the structural integrity of the files specified. (Use --test to check the data integrity). -alq additionally checks that none of the files specified 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 maximum 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.

Plzip starts the number of threads required by each file without exceeding the value specified. 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. You can find the number of members in a lzip file by running 'plzip -lv file.lz'.

-o file

--output=file

If -c has not been also specified, write the (de)compressed output to file, automatically creating any missing parent directories; keep input files unchanged. If compressing several files, each file is compressed independently. (The output consists of a sequence of independently compressed members). This option (or -c) is needed when reading from a named pipe (fifo) or from a device. -o - is equivalent to -c. -o has no effect when testing or listing.

In order to keep backward compatibility with plzip versions prior to 1.9, when compressing from standard input and no other file names are given, the extension '.lz' is appended to file unless it already ends in '.lz' or '.tlz'. This feature will be removed in a future version of plzip. Meanwhile, redirection may be used instead of -o to write the compressed output to a file without the extension '.lz' in its name: 'plzip < file > foo'.

-q

--quiet

Quiet operation. Suppress all messages.

-s bytes

--dictionary-size=bytes

When compressing, set the dictionary size limit in bytes. Plzip uses for each file the largest dictionary size that does not exceed neither the file size nor 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. Dictionary sizes are quantized so that they can be coded in just one byte (see coded-dict-size). If the size specified does not match one of the valid sizes, it is 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 files specified, 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 fails the test, does not exist, can't be opened, or is a terminal, plzip continues testing the rest of the files. A final diagnostic is shown at verbosity level 1 or higher if any file fails the test when testing multiple 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

Compression level. Set the compression parameters (dictionary size and match length limit) as shown in the table below. The default compression level is -6, equivalent to -s8MiB -m36. 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 options --dictionary-size and --match-length directly to achieve optimal performance.

If several compression levels or -s or -m options are given, the last setting is used. For example -9 -s64MiB is equivalent to -s64MiB -m273

Level	Dictionary size (-s)	Match length limit (-m)
-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.

--in-slots=n

Number of 1 MiB input packets buffered per worker thread when decompressing from non-seekable input. Increasing the number of packets may increase decompression speed, but requires more memory. Valid values range from 1 to 64. The default value is 4.

--out-slots=n

Number of 1 MiB output packets buffered per worker thread when decompressing to non-seekable output. Increasing the number of packets may increase decompression speed, but requires more memory. Valid values range from 1 to 1024. The default value is 64.

--check-lib

Compare the version of lzlib used to compile plzip with the version actually being used at run time and exit. Report any differences found. Exit with error status 1 if differences are found. A mismatch may indicate that lzlib is not correctly installed or that a different version of lzlib has been installed after compiling plzip. Exit with error status 2 if LZ_API_VERSION and LZ_version_string don't match. 'plzip -v --check-lib' shows the version of lzlib being used and the value of LZ_API_VERSION (if defined).

Numbers given as arguments to options may be expressed in decimal, hexadecimal, or octal (using the same syntax as integer constants in C++), and 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)
R	ronnabyte (10^27)	|	Ri	robibyte (2^90)
Q	quettabyte (10^30)	|	Qi	quebibyte (2^100)

4 Internal structure of plzip

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. Each chunk is compressed in-place (using the same buffer for input and output), reducing the amount of RAM required.

When decompressing, plzip decompresses as many members simultaneously as worker threads are chosen. Files that were compressed with lzip are not decompressed faster than using lzip (unless the option -b 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.

5 Memory required to compress and decompress

The amount of memory required per worker 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 1 MiB plus up to the number of 1 MiB input packets buffered (4 by default).
• For decompression of a regular file to a non-seekable file or to standard output; the dictionary size plus up to the number of 1 MiB output packets buffered (64 by default).
• For decompression of a non-seekable file or of standard input; the dictionary size plus 1 MiB plus up to the number of 1 MiB input and output packets buffered (68 by default).

The amount of memory required per worker 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

6 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 do not get any data to compress, and compression is 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 scales up to 10 or 14 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

7 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

+---+
|   | <-- the vertical bars might be missing
+---+

represents one byte; a box like this:

+==============+
|              |
+==============+

represents a variable number of bytes.

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 subtracting 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 subtract 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 for a complete description.

'CRC32 (4 bytes)'

Cyclic Redundancy Check (CRC) of the original uncompressed data.

'Data size (8 bytes)'

Size of the original uncompressed data.

'Member size (8 bytes)'

Total size of the member, including header and trailer. This field acts as a distributed index, improves the checking of stream integrity, and facilitates the safe recovery of undamaged members from multimember files. Lzip limits the member size to 2 PiB to prevent the data size field from overflowing.

8 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; an "End Of File" string (to check that the file has not been truncated), 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 matches 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 is 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.

9 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 option --keep to plzip and don't remove the original file until you check the compressed file with a command like 'plzip -cd file.lz | cmp file -'. Most RAM errors happening during compression can only be detected by comparing the compressed file with the original because the corruption happens before plzip compresses the RAM contents, resulting in a valid compressed file containing wrong data.

       tar -xf foo.tar.lz
     or
       plzip -cd foo.tar.lz | tar -xf -

     plzip -v file

     plzip -B 1MiB file

     plzip -d file.lz

     plzip -tv file.lz

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

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

     plzip -cd file.lz | dd bs=1000 skip=10 count=5

       plzip -c /dev/sdc > file.lz
     or
       plzip /dev/sdc -o file.lz

10 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' and 'plzip -v --check-lib'.

Concept index

• bugs: Problems
• examples: Examples
• file format: File format
• getting help: Problems
• introduction: Introduction
• invoking: Invoking plzip
• memory requirements: Memory requirements
• minimum file sizes: Minimum file sizes
• options: Invoking plzip
• output: Output
• program design: Program design
• trailing data: Trailing data
• usage: Invoking plzip
• version: Invoking plzip