zstd
ZSTD(1) User Commands ZSTD(1)
NAME
zstd - 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 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 :
o Source files are preserved by default. It's possible to remove them
automatically by using the --rm command.
o When compressing a single file, zstd displays progress notifica-
tions and result summary by default. Use -q to turn them off.
o zstd does not accept input from console, but it properly accepts
stdin when it's not the console.
o 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 op-
eration mode. If no files are given or file is -, zstd reads from stan-
dard 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 ter-
minal.
Unless --stdout or -o is specified, files are written to a new file
whose name is derived from the source file name:
o When compressing, the suffix .zst is appended to the source file-
name to get the target filename.
o 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 suf-
fix 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 ac-
cepted as synonyms for MiB.
Operation mode
If multiple operation mode options are given, the last one takes ef-
fect.
-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 --decom-
press).
-d, --decompress, --uncompress
Decompress.
-t, --test
Test the integrity of compressed files. This option is equiva-
lent 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
o -#: # compression level [1-19] (default: 3)
o --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.
o --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 set-
ting overwrites compression level if one was set previously. Simi-
larly, if a compression level is set after --fast, it overrides it.
o -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_NBWORK-
ERS_MAX==200. This modifier does nothing if zstd is compiled with-
out multithread support.
o --single-thread: Does not spawn a thread for compression, use a
single thread for both I/O and compression. In this mode, compres-
sion is serialized with I/O, which is slightly slower. (This is
different from -T1, which spawns 1 compression thread in parallel
of I/O). This mode is the only one available when multithread sup-
port is disabled. Single-thread mode features lower memory usage.
Final compressed result is slightly different from -T1.
o --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 com-
bined with multi-threading and --long mode. It does not work with
--single-thread. It sets window size to 8 MB 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).
o --long[=#]: enables long distance matching with # windowLog, if not
# is not present it defaults to 27. This increases the window size
(windowLog) and memory usage for both the compressor and decompres-
sor. 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.
o -D DICT: use DICT as Dictionary to compress or decompress FILE(s)
o --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. Node: for all levels, you can use --patch-from in --sin-
gle-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=
o --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 milage may vary.
o -C, --[no-]check: add integrity check computed from uncompressed
data (default: enabled)
o --[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).
o --no-dictID: do not store dictionary ID within frame header (dic-
tionary 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 -M#, --memory=#: Set a memory usage limit. By default, Zstandard
uses 128 MB 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 (ie. 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 MB).
o --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 param-
eters, resulting in better and potentially faster compression, es-
pecially for smaller source sizes.
o --size-hint=#: When handling input from a stream, zstd must guess
how large the source size will be when optimizing compression pa-
rameters. 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 -o FILE: save result into FILE
o -f, --force: overwrite output without prompting, and (de)compress
symbolic links
o -c, --stdout: force write to standard output, even if it is the
console
o --[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.
o --rm: remove source file(s) after successful compression or decom-
pression. If used in combination with -o, will trigger a confirma-
tion prompt (which can be silenced with -f), as this is a destruc-
tive operation.
o -k, --keep: keep source file(s) after successful compression or de-
compression. This is the default behavior.
o -r: operate recursively on directories
o --filelist FILE read a list of files to process as content from
FILE. Format is compatible with ls output, with one file per line.
o --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.
o --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 "out-
put-dir/var/tmp/abc". If there are multiple input files or directo-
ries, name collision resolution will follow the same rules as
--output-dir-flat.
o --format=FORMAT: compress and decompress in other formats. If com-
piled 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.
o -h/-H, --help: display help/long help and exit
o -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.
o -v, --verbose: verbose mode, display more information
o -q, --quiet: suppress warnings, interactivity, and notifications.
specify twice to suppress errors too.
o --no-progress: do not display the progress bar, but keep all other
messages.
o --show-default-cparams: Shows the default compression parameters
that will be used for a particular src file. If the provided src
file is not a regular file (eg. named pipe), the cli will just out-
put the default parameters. That is, the parameters that are used
when the src size is unknown.
o --: All arguments after -- are treated as files
Restricted usage of Environment Variables
Using environment variables to set parameters has security implica-
tions. 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, re-
spectively.
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 at-
tempt 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_NB-
WORKERS_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 dictio-
nary, 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 typ-
ically 100x the target dictionary size (for example, 10 MB for a
100 KB dictionary).
Supports multithreading if zstd is compiled with threading sup-
port. Additional parameters can be specified with --train-fast-
cover. 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 gener-
ate statistics more tuned for selected compression level, re-
sulting 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 pre-
cise 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 dic-
tionary. 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 multi-
threading 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 dictio-
nary is at most shrinkDictMaxRegression% worse than the compres-
sion 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 differ-
ent 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 sub-
segments 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 re-
duces 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 dictio-
nary 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/de-
compressed several times per run, in order to improve measurement accu-
racy.
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 re-
quirements 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 compres-
sion. 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 bi-
nary 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 im-
prove 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 de-
creases compression speed. t For ZSTD_fast, it triggers ul-
tra-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 compres-
sion 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 en-
abled. Reloading more data improves compression ratio, but de-
creases speed.
The minimum ovlog is 0, and the maximum is 9. 1 means "no over-
lap", 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 exam-
ple, 8 means "windowSize/2", and 6 means "windowSize/8". Value 0
is special and means "default" : ovlog is automatically deter-
mined by zstd. In which case, ovlog will range from 6 to 9, de-
pending 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 ex-
pense of more memory during compression and a decrease in com-
pression 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 dis-
tance 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 compres-
sion 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 pos-
sible to select a custom value. Note that job size must respect a mini-
mum 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
zstd 1.4.8 December 2020 ZSTD(1)
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