zstd

Compress or decompress .zst files

TLDR

Compress a file into a new file with the .zst suffix

$ zstd [file]
copy

Decompress a file

$ zstd -d [file].zst
copy

Decompress to stdout

$ zstd -dc [file].zst
copy

Compress a file specifying the compression level, where 1=fastest, 19=slowest and 3=default

$ zstd -[level] [file]
copy

Unlock higher compression levels (up to 22) using more memory (both for compression and decompression)

$ zstd --ultra -[level] [file]
copy

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 core, 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 :

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:

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).

-l, --list

Display information related to a zstd compressed file, such as size, ratio, and checksum. Some of these fields may not be available. This command can be augmented with the -v modifier.

Operation modifiers

Restricted usage of Environment Variables

Using environment variables to set parameters has security implications. Therefore, this avenue is intentionally restricted. Only ZSTD_CLEVEL and ZSTD_NBTHREADS are currently supported. They set the compression level and number of threads to use during compression, respectively.

ZSTD_CLEVEL can be used to set the level between 1 and 19 (the "normal" range). If the value of ZSTD_CLEVEL is not a valid integer, it will be ignored with a warning message. ZSTD_CLEVEL just replaces the default compression level (3).

ZSTD_NBTHREADS can be used to set the number of threads zstd will attempt to use during compression. If the value of ZSTD_NBTHREADS is not a valid unsigned integer, it will be ignored with a warning message. 'ZSTD_NBTHREADShas a default value of (1), and is capped at ZSTDMT_NBWORKERS_MAX==200.zstd` must be compiled with multithread support for this to have any effect.

They can both be overridden by corresponding command line arguments: -# for compression level and -T# for number of compression threads.

DICTIONARY BUILDER

zstd offers dictionary compression, which greatly improves efficiency on small files and messages. It's possible to train zstd with a set of samples, the result of which is saved into a file called a dictionary. Then during compression and decompression, reference the same dictionary, using command -D dictionaryFileName. Compression of small files similar to the sample set will be greatly improved.

--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-fastcover. The legacy dictionary builder can be accessed with --train-legacy. The cover dictionary builder can be accessed with --train-cover. Equivalent to --train-fastcover=d=8,steps=4.

-o file

Dictionary saved into file (default name: dictionary).

--maxdict=#

Limit dictionary to specified size (default: 112640).

-#

Use # compression level during training (optional). Will generate statistics more tuned for selected compression level, resulting in a small compression ratio improvement for this level.

-B#

Split input files in blocks of size # (default: no split)

--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=#,split=#,shrink[=#]]

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. If split is not specified or split <= 0, then the default value of 100 is used. Requires that d <= k. If shrink flag is not used, then the default value for shrinkDict of 0 is used. If shrink is not specified, then the default value for shrinkDictMaxRegression of 1 is used.

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]. If split is 100, all input samples are used for both training and testing to find optimal d and k to build dictionary. Supports multithreading if zstd is compiled with threading support. Having shrink enabled takes a truncated dictionary of minimum size and doubles in size until compression ratio of the truncated dictionary is at most shrinkDictMaxRegression% worse than the compression ratio of the largest dictionary.

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

zstd --train-cover=k=50,split=60 FILEs

zstd --train-cover=shrink FILEs

zstd --train-cover=shrink=2 FILEs

--train-fastcover[=k#,d=#,f=#,steps=#,split=#,accel=#]

Same as cover but with extra parameters f and accel and different default value of split If split is not specified, then it tries split = 75. If f is not specified, then it tries f = 20. Requires that 0 < f < 32. If accel is not specified, then it tries accel = 1. Requires that 0 < accel <= 10. Requires that d = 6 or d = 8.

f is log of size of array that keeps track of frequency of subsegments of size d. The subsegment is hashed to an index in the range [0,2^f - 1]. It is possible that 2 different subsegments are hashed to the same index, and they are considered as the same subsegment when computing frequency. Using a higher f reduces collision but takes longer.

Examples:

zstd --train-fastcover FILEs

zstd --train-fastcover=d=8,f=15,accel=2 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#, --block-size=#

cut file(s) into independent blocks of size # (default: no block)

--priority=rt

set process priority to real-time

Output Format: CompressionLevel#Filename : IntputSize -> OutputSize (CompressionRatio), CompressionSpeed, DecompressionSpeed

Methodology: For both compression and decompression speed, the entire input is compressed/decompressed in-memory to measure speed. A run lasts at least 1 sec, so when files are small, they are compressed/decompressed several times per run, in order to improve measurement accuracy.

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 9 strategies numbered from 1 to 9, from faster to stronger: 1=ZSTD_fast, 2=ZSTD_dfast, 3=ZSTD_greedy, 4=ZSTD_lazy, 5=ZSTD_lazy2, 6=ZSTD_btlazy2, 7=ZSTD_btopt, 8=ZSTD_btultra, 9=ZSTD_btultra2.

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 30 (1 GiB) on 32-bit platforms and 31 (2 GiB) on 64-bit platforms.

Note: If windowLog is set to larger than 27, --long=windowLog or --memory=windowSize needs to be passed to the decompressor.

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 30 (1 GiB).

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 29 (524 Mib) on 32-bit platforms and 30 (1 Gib) on 64-bit platforms.

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 'windowLog' - 1.

minMatch=mml, mml=mml

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 mml is 3 and the maximum is 7.

targetLength=tlen, tlen=tlen

The impact of this field vary depending on selected strategy.

For ZSTD_btopt, ZSTD_btultra and ZSTD_btultra2, it specifies the minimum match length that causes match finder to stop searching. A larger targetLength usually improves compression ratio but decreases compression speed. t For ZSTD_fast, it triggers ultra-fast mode when > 0. The value represents the amount of data skipped between match sampling. Impact is reversed : a larger targetLength increases compression speed but decreases compression ratio.

For all other strategies, this field has no impact.

The minimum tlen is 0 and the maximum is 128 Kib.

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. 1 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 reloaded amount by a factor 2. For example, 8 means "windowSize/2", and 6 means "windowSize/8". Value 0 is special and means "default" : ovlog is automatically determined by zstd. In which case, ovlog will range from 6 to 9, depending on selected strat.

ldmHashLog=lhlog, lhlog=lhlog

Specify the maximum size for a hash table used for long distance matching.

This option is ignored unless long distance matching is enabled.

Bigger hash tables usually improve compression ratio at the expense of more memory during compression and a decrease in compression speed.

The minimum lhlog is 6 and the maximum is 30 (default: 20).

ldmMinMatch=lmml, lmml=lmml

Specify the minimum searched length of a match for long distance matching.

This option is ignored unless long distance matching is enabled.

Larger/very small values usually decrease compression ratio.

The minimum lmml is 4 and the maximum is 4096 (default: 64).

ldmBucketSizeLog=lblog, lblog=lblog

Specify the size of each bucket for the hash table used for long distance matching.

This option is ignored unless long distance matching is enabled.

Larger bucket sizes improve collision resolution but decrease compression speed.

The minimum lblog is 1 and the maximum is 8 (default: 3).

ldmHashRateLog=lhrlog, lhrlog=lhrlog

Specify the frequency of inserting entries into the long distance matching hash table.

This option is ignored unless long distance matching is enabled.

Larger values will improve compression speed. Deviating far from the default value will likely result in a decrease in compression ratio.

The default value is wlog - lhlog.

Example

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

--zstd=wlog=23,clog=23,hlog=22,slog=6,mml=3,tlen=48,strat=6

-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.

BUGS

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

AUTHOR

Yann Collet

Copied to clipboard
Coindodo