Ubuntu Manpage: zstd - zstd, zstdmt, unzstd, zstdcat - Compress or decompress .zst files

NAME

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

SYNOPSIS

       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, from fast modes
at > 200 MB/s per core, to strong modes with excellent 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 displays a short help page when command line is an error. Use -q to turn it off.

○ zstd does not accept input from console, though it does accept stdin when it´s not the
console.

○ zstd does not store the input´s filename or attributes, only its contents.

zstd processes 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 multiple .zst files. zstd will decompress such agglomerated
file as if it was 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 > /dev/null, decompressed data is discarded and checksummed for errors. No
files are created or removed.

-b# Benchmark file(s) using compression level #. See BENCHMARK below for a description
of this operation.

--train FILES
Use FILES as a training set to create a dictionary. The training set should contain
a lot of small files (> 100). See DICTIONARY BUILDER below for a description of
this operation.

-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´s output can be
augmented with the -v modifier.

Operation Modifiers
○ -#: selects # 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.

○ --fast[=#]: switch to ultra-fast compression levels. If =# is not present, it defaults
to 1. The higher the value, the faster the compression speed, at the cost of some
compression ratio. This setting overwrites compression level if one was set
previously. Similarly, if a compression level is set after --fast, it overrides it.

○ -T#, --threads=#: Compress using # working threads (default: 1). If # is 0, attempt to
detect and use the number of physical CPU cores. In all cases, the nb of threads is
capped to ZSTDMT_NBWORKERS_MAX, which is either 64 in 32-bit mode, or 256 for 64-bit
environments. This modifier does nothing if zstd is compiled without multithread
support.

○ --single-thread: Use a single thread for both I/O and compression. As compression is
serialized with I/O, this can be slightly slower. Single-thread mode features
significantly lower memory usage, which can be useful for systems with limited amount
of memory, such as 32-bit systems.

Note 1: this mode is the only available one when multithread support is disabled.

Note 2: this mode is different from -T1, which spawns 1 compression thread in parallel
with I/O. Final compressed result is also slightly different from -T1.

○ --auto-threads={physical,logical} (default: physical): When using a default amount of
threads via -T0, choose the default based on the number of detected physical or
logical cores.

○ --adapt[=min=#,max=#]: zstd will dynamically adapt compression level to perceived I/O
conditions. Compression level adaptation can be observed live by using command -v.
Adaptation can be constrained between supplied min and max levels. The feature works
when combined with multi-threading and --long mode. It does not work with
--single-thread. It sets window size to 8 MiB by default (can be changed manually, see
wlog). Due to the chaotic nature of dynamic adaptation, compressed result is not
reproducible.

Note: at the time of this writing, --adapt can remain stuck at low speed when combined
with multiple worker threads (>=2).

○ --long[=#]: enables long distance matching with # windowLog, if # is not present it
defaults to 27. This increases the window size (windowLog) and memory usage for both
the compressor and decompressor. This setting is designed to improve the compression
ratio for files with long matches at a large distance.

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

○ -D DICT: use DICT as Dictionary to compress or decompress FILE(s)

○ --patch-from FILE: Specify the file to be used as a reference point for zstd´s diff
engine. This is effectively dictionary compression with some convenient parameter
selection, namely that windowSize > srcSize.

Note: cannot use both this and -D together.

Note: --long mode will be automatically activated if chainLog < fileLog (fileLog being
the windowLog required to cover the whole file). You can also manually force it.

Note: for all levels, you can use --patch-from in --single-thread mode to improve
compression ratio at the cost of speed.

Note: for level 19, you can get increased compression ratio at the cost of speed by
specifying --zstd=targetLength= to be something large (i.e. 4096), and by setting a
large --zstd=chainLog=.

○ --rsyncable: zstd will periodically synchronize the compression state to make the
compressed file more rsync-friendly. There is a negligible impact to compression
ratio, and the faster compression levels will see a small compression speed hit. This
feature does not work with --single-thread. You probably don´t want to use it with
long range mode, since it will decrease the effectiveness of the synchronization
points, but your mileage may vary.

○ -C, --[no-]check: add integrity check computed from uncompressed data (default:
enabled)

○ --[no-]content-size: enable / disable whether or not the original size of the file is
placed in the header of the compressed file. The default option is --content-size
(meaning that the original size will be placed in the header).

○ --no-dictID: 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.

○ -M#, --memory=#: Set a memory usage limit. By default, zstd uses 128 MiB for
decompression as the maximum amount of memory the decompressor is allowed to use, but
you can override this manually if need be in either direction (i.e. you can increase
or decrease it).

This is also used during compression when using with --patch-from=. In this case, this
parameter overrides that maximum size allowed for a dictionary. (128 MiB).

Additionally, this can be used to limit memory for dictionary training. This parameter
overrides the default limit of 2 GiB. zstd will load training samples up to the memory
limit and ignore the rest.

○ --stream-size=#: Sets the pledged source size of input coming from a stream. This
value must be exact, as it will be included in the produced frame header. Incorrect
stream sizes will cause an error. This information will be used to better optimize
compression parameters, resulting in better and potentially faster compression,
especially for smaller source sizes.

○ --size-hint=#: When handling input from a stream, zstd must guess how large the source
size will be when optimizing compression parameters. If the stream size is relatively
small, this guess may be a poor one, resulting in a higher compression ratio than
expected. This feature allows for controlling the guess when needed. Exact guesses
result in better compression ratios. Overestimates result in slightly degraded
compression ratios, while underestimates may result in significant degradation.

○ -o FILE: save result into FILE.

○ -f, --force: disable input and output checks. Allows overwriting existing files, input
from console, output to stdout, operating on links, block devices, etc. During
decompression and when the output destination is stdout, pass-through unrecognized
formats as-is.

○ -c, --stdout: write to standard output (even if it is the console); keep original
files unchanged.

○ --[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.

○ --[no-]pass-through enable / disable passing through uncompressed files as-is. During
decompression when pass-through is enabled, unrecognized formats will be copied as-is
from the input to the output. By default, pass-through will occur when the output
destination is stdout and the force (-f) option is set.

○ --rm: remove source file(s) after successful compression or decompression. This
command is silently ignored if output is stdout. If used in combination with -o,
triggers a confirmation prompt (which can be silenced with -f), as this is a
destructive operation.

○ -k, --keep: keep source file(s) after successful compression or decompression. This is
the default behavior.

○ -r: operate recursively on directories. It selects all files in the named directory
and all its subdirectories. This can be useful both to reduce command line typing, and
to circumvent shell expansion limitations, when there are a lot of files and naming
breaks the maximum size of a command line.

○ --filelist FILE read a list of files to process as content from FILE. Format is
compatible with ls output, with one file per line.

○ --output-dir-flat DIR: resulting files are stored into target DIR directory, instead
of same directory as origin file. Be aware that this command can introduce name
collision issues, if multiple files, from different directories, end up having the
same name. Collision resolution ensures first file with a given name will be present
in DIR, while in combination with -f, the last file will be present instead.

○ --output-dir-mirror DIR: similar to --output-dir-flat, the output files are stored
underneath target DIR directory, but this option will replicate input directory
hierarchy into output DIR.

If input directory contains "..", the files in this directory will be ignored. If
input directory is an absolute directory (i.e. "/var/tmp/abc"), it will be stored into
the "output-dir/var/tmp/abc". If there are multiple input files or directories, name
collision resolution will follow the same rules as --output-dir-flat.

○ --format=FORMAT: compress and decompress in other formats. If compiled with support,
zstd can compress to or decompress from other compression algorithm formats. Possibly
available options are zstd, gzip, xz, lzma, and lz4. If no such format is provided,
zstd is the default.

○ -h/-H, --help: display help/long help and exit

○ -V, --version: display version number and exit. Advanced: -vV also displays supported
formats. -vvV also displays POSIX support. -q will only display the version number,
suitable for machine reading.

○ -v, --verbose: verbose mode, display more information

○ -q, --quiet: suppress warnings, interactivity, and notifications. specify twice to
suppress errors too.

○ --no-progress: do not display the progress bar, but keep all other messages.

○ --show-default-cparams: shows the default compression parameters that will be used for
a particular input file, based on the provided compression level and the input size.
If the provided file is not a regular file (e.g. a pipe), this flag will output the
parameters used for inputs of unknown size.

○ --: All arguments after -- are treated as files

gzip Operation Modifiers
When invoked via a gzip symlink, zstd will support further options that intend to mimic
the gzip behavior:

-n, --no-name
do not store the original filename and timestamps when compressing a file. This is
the default behavior and hence a no-op.

--best alias to the option -9.

Environment Variables
Employing environment variables to set parameters has security implications. Therefore,
this avenue is intentionally limited. 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_NBTHREADS has 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 ideally
contain a lot of samples (> 100), and weight typically 100x the target dictionary
size (for example, ~10 MB for a 100 KB dictionary). --train can be combined with -r
to indicate a directory rather than listing all the files, which can be useful to
circumvent shell expansion limits.

Since dictionary compression is mostly effective for small files, the expectation
is that the training set will only contain small files. In the case where some
samples happen to be large, only the first 128 KiB of these samples will be used
for training.

--train supports multithreading if zstd is compiled with threading support
(default). Additional advanced parameters can be specified with --train-fastcover.
The legacy dictionary builder can be accessed with --train-legacy. The slower cover
dictionary builder can be accessed with --train-cover. Default --train is
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 bytes). As usual, quantities
are expressed in bytes by default, and it´s possible to employ suffixes (like KB or
MB) to specify larger values.

-# 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 into blocks of size # (default: no split)

-M#, --memory=#
Limit the amount of sample data loaded for training (default: 2 GB). Note that the
default (2 GB) is also the maximum. This parameter can be useful in situations
where the training set size is not well controlled and could be potentially very
large. Since speed of the training process is directly correlated to the size of
the training sample set, a smaller sample set leads to faster training.

In situations where the training set is larger than maximum memory, the CLI will
randomly select samples among the available ones, up to the maximum allowed memory
budget. This is meant to improve dictionary relevance by mitigating the potential
impact of clustering, such as selecting only files from the beginning of a list
sorted by modification date, or sorted by alphabetical order. The randomization
process is deterministic, so training of the same list of files with the same
parameters will lead to the creation of the same dictionary.

--dictID=#
A dictionary ID is a locally unique ID. The decoder will use this value to verify
it is using the right dictionary. By default, zstd will create a 4-bytes random
number ID. It´s possible to provide an explicit number ID instead. It´s up to the
dictionary manager to not assign twice the same ID to 2 different dictionaries.
Note that 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.

Note that RFC8878 reserves IDs less than 32768 and greater than or equal to 2\^31,
so they should not be used in public.

--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 achievable 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 chunks of size # (default: no chunking)

       --priority=rt
              set process priority to real-time

       Output  Format:  CompressionLevel#Filename:  InputSize  ->  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

### -B#: Specify the size of each compression job. This parameter is only available when
multi-threading is enabled. Each compression job is run in parallel, so this value
indirectly impacts the nb of active threads. Default job size varies depending on
compression level (generally 4 * windowSize). -B# makes it possible to manually select a
custom size. Note that job size must respect a minimum value which is enforced
transparently. This minimum is either 512 KB, or overlapSize, whichever is largest.
Different job sizes will lead to non-identical compressed frames.

--zstd[=options]:
zstd provides 22 predefined regular compression levels plus the fast levels. This
compression level is translated internally into a number of specific parameters that
actually control the behavior of the compressor. (You can see the result of this
translation with --show-default-cparams.) These specific parameters can be overridden 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 fastest to strongest:
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 fewer collisions which usually makes compression faster,
but requires more memory during compression.

The minimum hlog is 6 (64 entries / 256 B) and the maximum is 30 (1B entries / 4
GiB).

chainLog=clog, clog=clog
Specify the maximum number of bits for the secondary search structure, whose form
depends on the selected strategy.

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,
which only has the primary hash table.

The minimum clog is 6 (64 entries / 256 B) and the maximum is 29 (512M entries / 2
GiB) on 32-bit platforms and 30 (1B entries / 4 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.

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

BUGS

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

AUTHOR

       Yann Collet

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

DICTIONARY BUILDER

BENCHMARK

ADVANCED COMPRESSION OPTIONS

SEE ALSO

BUGS

AUTHOR