src/third_party/zstd-1.2.0/programs/zstd.1.md

In zstdr: R Bindings to the 'Zstandard' Compression Library

zstd(1) -- zstd, zstdmt, unzstd, zstdcat - Compress or decompress .zst files

SYNOPSIS

zstd [OPTIONS] [-|INPUT-FILE] [-o OUTPUT-FILE]

zstdmt is equivalent to zstd -T0

unzstd is equivalent to zstd -d

zstdcat is equivalent to zstd -dcf

DESCRIPTION

zstd is a fast lossless compression algorithm and data compression tool, with command line syntax similar to gzip (1) and xz (1). It is based on the LZ77 family, with further FSE & huff0 entropy stages. zstd offers highly configurable compression speed, with fast modes at > 200 MB/s per code, and strong modes nearing lzma compression ratios. It also features a very fast decoder, with speeds > 500 MB/s per core.

zstd command line syntax is generally similar to gzip, but features the following differences :

• Source files are preserved by default. It's possible to remove them automatically by using the --rm command.
• When compressing a single file, zstd displays progress notifications and result summary by default. Use -q to turn them off.
• zstd does not accept input from console, but it properly accepts stdin when it's not the console.
• zstd displays a short help page when command line is an error. Use -q to turn it off.

zstd compresses or decompresses each file according to the selected operation mode. If no files are given or file is -, zstd reads from standard input and writes the processed data to standard output. zstd will refuse to write compressed data to standard output if it is a terminal : it will display an error message and skip the file. Similarly, zstd will refuse to read compressed data from standard input if it is a terminal.

Unless --stdout or -o is specified, files are written to a new file whose name is derived from the source file name:

• When compressing, the suffix .zst is appended to the source filename to get the target filename.
• When decompressing, the .zst suffix is removed from the source filename to get the target filename

Concatenation with .zst files

It is possible to concatenate .zst files as is. zstd will decompress such files as if they were a single .zst file.

OPTIONS

Integer suffixes and special values

In most places where an integer argument is expected, an optional suffix is supported to easily indicate large integers. There must be no space between the integer and the suffix.

• KiB: Multiply the integer by 1,024 (2\^10). Ki, K, and KB are accepted as synonyms for KiB.
• MiB: Multiply the integer by 1,048,576 (2\^20). Mi, M, and MB are accepted as synonyms for MiB.

Operation mode

If multiple operation mode options are given, the last one takes effect.

• -z, --compress: Compress. This is the default operation mode when no operation mode option is specified and no other operation mode is implied from the command name (for example, unzstd implies --decompress).
• -d, --decompress, --uncompress: Decompress.
• -t, --test: Test the integrity of compressed files. This option is equivalent to --decompress --stdout except that the decompressed data is discarded instead of being written to standard output. No files are created or removed.
• -b#: Benchmark file(s) using compression level #
• --train FILEs: Use FILEs as a training set to create a dictionary. The training set should contain a lot of small files (> 100).

Operation modifiers

• -#: # compression level [1-19] (default: 3)
• --ultra: unlocks high compression levels 20+ (maximum 22), using a lot more memory. Note that decompression will also require more memory when using these levels.
• -T#, --threads=#: Compress using # threads (default: 1). If # is 0, attempt to detect and use the number of physical CPU cores. This modifier does nothing if zstd is compiled without multithread support.
• -D file: use file as Dictionary to compress or decompress FILE(s)
• --nodictID: do not store dictionary ID within frame header (dictionary compression). The decoder will have to rely on implicit knowledge about which dictionary to use, it won't be able to check if it's correct.
• -o file: save result into file (only possible with a single INPUT-FILE)
• -f, --force: overwrite output without prompting, and (de)compress symbolic links
• -c, --stdout: force write to standard output, even if it is the console
• --[no-]sparse: enable / disable sparse FS support, to make files with many zeroes smaller on disk. Creating sparse files may save disk space and speed up decompression by reducing the amount of disk I/O. default : enabled when output is into a file, and disabled when output is stdout. This setting overrides default and can force sparse mode over stdout.
• --rm: remove source file(s) after successful compression or decompression
• -k, --keep: keep source file(s) after successful compression or decompression. This is the default behavior.
• -r: operate recursively on dictionaries
• -h/-H, --help: display help/long help and exit
• -V, --version: display version number and exit
• -v: verbose mode
• -q, --quiet: suppress warnings, interactivity, and notifications. specify twice to suppress errors too.
• -C, --[no-]check: add integrity check computed from uncompressed data (default : enabled)
• --: All arguments after -- are treated as files

DICTIONARY BUILDER

zstd offers dictionary compression, useful for very small files and messages. It's possible to train zstd with some samples, the result of which is saved into a file called a dictionary. Then during compression and decompression, reference the same dictionary. It will improve compression ratio of small files. Typical gains range from 10% (at 64KB) to x5 better (at <1KB).

• --train FILEs: Use FILEs as training set to create a dictionary. The training set should contain a lot of small files (> 100), and weight typically 100x the target dictionary size (for example, 10 MB for a 100 KB dictionary).

  Supports multithreading if zstd is compiled with threading support. Additional parameters can be specified with --train-cover. The legacy dictionary builder can be accessed with --train-legacy. Equivalent to --train-cover=d=8,steps=4. -o file: Dictionary saved into file (default name: dictionary). --maxdict=#: Limit dictionary to specified size (default: 112640). --dictID=#: A dictionary ID is a locally unique ID that a decoder can use to verify it is using the right dictionary. By default, zstd will create a 4-bytes random number ID. It's possible to give a precise number instead. Short numbers have an advantage : an ID < 256 will only need 1 byte in the compressed frame header, and an ID < 65536 will only need 2 bytes. This compares favorably to 4 bytes default. However, it's up to the dictionary manager to not assign twice the same ID to 2 different dictionaries. --train-cover[=k#,d=#,steps=#]: Select parameters for the default dictionary builder algorithm named cover. If d is not specified, then it tries d = 6 and d = 8. If k is not specified, then it tries steps values in the range [50, 2000]. If steps is not specified, then the default value of 40 is used. Requires that d <= k.

  Selects segments of size k with highest score to put in the dictionary. The score of a segment is computed by the sum of the frequencies of all the subsegments of size d. Generally d should be in the range [6, 8], occasionally up to 16, but the algorithm will run faster with d <= 8. Good values for k vary widely based on the input data, but a safe range is [2 * d, 2000]. Supports multithreading if zstd is compiled with threading support.

  Examples:

  zstd --train-cover FILEs

  zstd --train-cover=k=50,d=8 FILEs

  zstd --train-cover=d=8,steps=500 FILEs

  zstd --train-cover=k=50 FILEs

• --train-legacy[=selectivity=#]: Use legacy dictionary builder algorithm with the given dictionary selectivity (default: 9). The smaller the selectivity value, the denser the dictionary, improving its efficiency but reducing its possible maximum size. --train-legacy=s=# is also accepted.

  Examples:

  zstd --train-legacy FILEs

  zstd --train-legacy=selectivity=8 FILEs

BENCHMARK

• -b#: benchmark file(s) using compression level #
• -e#: benchmark file(s) using multiple compression levels, from -b# to -e# (inclusive)
• -i#: minimum evaluation time, in seconds (default : 3s), benchmark mode only
• -B#: cut file into independent blocks of size # (default: no block)
• --priority=rt: set process priority to real-time

ADVANCED COMPRESSION OPTIONS

--zstd[=options]:

zstd provides 22 predefined compression levels. The selected or default predefined compression level can be changed with advanced compression options. The options are provided as a comma-separated list. You may specify only the options you want to change and the rest will be taken from the selected or default compression level. The list of available options:

• strategy=strat, strat=strat: Specify a strategy used by a match finder.

  There are 8 strategies numbered from 0 to 7, from faster to stronger: 0=ZSTD_fast, 1=ZSTD_dfast, 2=ZSTD_greedy, 3=ZSTD_lazy, 4=ZSTD_lazy2, 5=ZSTD_btlazy2, 6=ZSTD_btopt, 7=ZSTD_btopt2.

• windowLog=wlog, wlog=wlog: Specify the maximum number of bits for a match distance.

  The higher number of increases the chance to find a match which usually improves compression ratio. It also increases memory requirements for the compressor and decompressor. The minimum wlog is 10 (1 KiB) and the maximum is 27 (128 MiB).

• hashLog=hlog, hlog=hlog: Specify the maximum number of bits for a hash table.

  Bigger hash tables cause less collisions which usually makes compression faster, but requires more memory during compression.

  The minimum hlog is 6 (64 B) and the maximum is 26 (128 MiB).

• chainLog=clog, clog=clog: Specify the maximum number of bits for a hash chain or a binary tree.

  Higher numbers of bits increases the chance to find a match which usually improves compression ratio. It also slows down compression speed and increases memory requirements for compression. This option is ignored for the ZSTD_fast strategy.

  The minimum clog is 6 (64 B) and the maximum is 28 (256 MiB).

• searchLog=slog, slog=slog: Specify the maximum number of searches in a hash chain or a binary tree using logarithmic scale.

  More searches increases the chance to find a match which usually increases compression ratio but decreases compression speed.

  The minimum slog is 1 and the maximum is 26.

• searchLength=slen, slen=slen: Specify the minimum searched length of a match in a hash table.

  Larger search lengths usually decrease compression ratio but improve decompression speed.

  The minimum slen is 3 and the maximum is 7.

• targetLen=tlen, tlen=tlen: Specify the minimum match length that causes a match finder to stop searching for better matches.

  A larger minimum match length usually improves compression ratio but decreases compression speed. This option is only used with strategies ZSTD_btopt and ZSTD_btopt2.

  The minimum tlen is 4 and the maximum is 999.

• overlapLog=ovlog, ovlog=ovlog: Determine overlapSize, amount of data reloaded from previous job. This parameter is only available when multithreading is enabled. Reloading more data improves compression ratio, but decreases speed.

  The minimum ovlog is 0, and the maximum is 9. 0 means "no overlap", hence completely independent jobs. 9 means "full overlap", meaning up to windowSize is reloaded from previous job. Reducing ovlog by 1 reduces the amount of reload by a factor 2. Default ovlog is 6, which means "reload windowSize / 8". Exception : the maximum compression level (22) has a default ovlog of 9.

-B#:

Select the size of each compression job. This parameter is available only when multi-threading is enabled. Default value is 4 * windowSize, which means it varies depending on compression level. -B# makes it possible to select a custom value. Note that job size must respect a minimum value which is enforced transparently. This minimum is either 1 MB, or overlapSize, whichever is largest.

Example

The following parameters sets advanced compression options to those of predefined level 19 for files bigger than 256 KB:

--zstd=windowLog=23,chainLog=23,hashLog=22,searchLog=6,searchLength=3,targetLength=48,strategy=6

BUGS

Report bugs at: https://github.com/facebook/zstd/issues

AUTHOR

Yann Collet

zstdr documentation built on June 7, 2017, 1:02 a.m.